man

How to Use Linux’s man Command: Hidden Secrets and Basics
How to Easily Read a Linux Man Page
- Underlined or Italicized Text: It means you need to replace it with an appropriate argument.
- Ellipses: It means that argument or expression is repeatable.

Navigation

Gentoo and fossbytes

Enter – Move down one line
Space – Move down one page
g – Move to the top of the page
G – Move to the bottom of the page
q – Quit

Search within a man page

Use / and type your search pattern.

Use 'n' for forward search and 'N' for reverse search.

The matched line will be moved to the top of the screen.

By default, the search is case insensitive or we can use man -i COMMAND.

Regular expression is supported. For example to find all of the long arguments with: /(--)[a-Z]

Colored man pages

This is a cool tip!

By default, the man program normally uses a terminal pager program such as less to format its output.

Add the following to "~/.bashrc" file. For example, LESS_TERMCAP_mb customizes the appearance of blink text in the less pager (or man command) and LESS_TERMCAP_us customizes the appearance of underlined text in the less pager.

# Customize less colors
export LESS_TERMCAP_mb=$'\e[1;32m'  # Blinking text: bold green
export LESS_TERMCAP_md=$'\e[1;34m'  # Bold text: bold blue
export LESS_TERMCAP_me=$'\e[0m'     # End mode
export LESS_TERMCAP_so=$'\e[01;47;34m'  # Standout: bold white on blue
export LESS_TERMCAP_se=$'\e[0m'     # End standout mode
export LESS_TERMCAP_us=$'\e[1;4;31m'  # Underlined text: bold underlined red
export LESS_TERMCAP_ue=$'\e[0m'     # End underline

(New way, The most Pager) How To Display Color Man Pages in Linux and Unix or How to Display man Pages in Color on Linux.

Navigate to another man page within a man page

See here.

When inside the man page, press ! followed by a valid shell command.

For example : !man cat

View a specific "Section"

$ whatis printf
printf (1)           - format and print data
printf (3)           - formatted output conversion
Printf (3o)          - Formatted output functions.
$ man 3 printf

Read man pages in vi without using temporary files

What is a way to read man pages in vim without using temporary files

man find | vi -

Search man page referenced by

man -f KEYWORD

This command is equivalent to whatis -r KEYWORD

Search from all man pages

man -k KEYWORD will give you a list of all man pages which relate to 'KEYWORD'.

TLDR pages/cheat sheet: alternative to Man

https://tldr.sh/
TLDR pages: Simplified Alternative To Linux Man Pages. Be sure to install the latest versions of nodejs and npm. Tested on Ubuntu 16.04, 20.04. Example:
```
$ npm install -g tldr
$ tldr tar
```
Display Linux Commands Cheatsheets With Tealdeer In Terminal A full list of commands is on TLDR Pages.

Cheat.sh (better than TLDR)

Cheat.sh Shows Cheat Sheets On The Command Line Or In Your Code Editor. There are different ways to use it. One way does not require to install anything as long as we have the curl command.

curl cheat.sh/tar

curl cht.sh/python/random+list # Python programming language cheat sheet for random list

My test shows cheat.sh can find more commands and it gives colored output.

Some books

UNIX in a nutshell
sed & awk
lex & yacc
bash
Linux Pocket Guide
bash cookbook: pdf is online
Classic Shell Scripting
GNU EMACS
Learning the vi and Vim editors 7th
UNIX POWER TOOLS
Bash Pocket Reference
System Administration 3rd
TCP/IP 3rd: pdf is online
DNS and BIND 5th
Network Troubleshooting Tools
25 Free Books To Learn Linux For Free
17 books for Linux and open source fans

Beautiful desktop

.desktop file

This is not related to beautiful desktop. It is used to launch applications in Linux. Without the .desktop file, your application won’t show up in the Applications menu and you can’t launch it with third-party launchers such as Synapse and Albert Launcher.

The .desktop files are commonly saved in

~/local/share/applications
/usr/share/applications

List of installed desktop environment

ls -l /usr/share/xsessions/

Themes

5 of the Best Linux Dark Themes that Are Easy on the Eyes

Virtual consoles/virtual terminals

Linux allows virtual consoles (aka virtual terminals) to be opened while an X Window System is executing.

Use Ctrl + Alt + FX to open a virtual console-- there are six virtual text-based consoles (F1 to F6). Use Alt + F7 (or possibly other keybinds) to return to the X Window System.

Managing devices in Linux -> Fun with device files.

Change/increase console fonts

Desktops/Workspaces

Ctrl + Alt + -> or Ctrl + Alt + <- to switch workspaces.

Ctrl + Alt + down can list the open applications on the current workspace.

Ctrl + Alt + up can show all workspaces and the open applications. We can use mouse to move an app to any workspace.

SuperKey + left tile a window to left. SuperKey + right tile a window to right.

Complete List of Linux Mint 18 Keyboard Shortcuts for Cinnamon for more examples.

Mouse

How To Bind Mouse Buttons To Keyboard Keys Or Commands (Linux Using X11)

Virtual memory

vmstat

hcache

A tool fork from pcstat, with a feature that showing top X biggest cache files globally

Memory: free command

Is Linux Eating Your RAM? How to Manage Your Memory
How to Use the free Command on Linux (*detailed and comprehensive)
How to Check Memory Usage From the Linux Terminal

Three types of memory reported by the free command.

Used: RAM that is currently in use by an application.
Available: RAM that may be in use for disk caching but can be freed up for applications. What is 'available' vs 'free' memory in free command?. Available = Estimation of how much memory is available for starting new applications, without swapping.
Shared: Amount of memory used by the tmpfs file systems.
Free: RAM that is not in use by an application or disk caching.
Total = Used + Free + Buffers/Cache

How to Clear RAM Memory Cache and Buffer

# To clear pagecache, enter the following command:
echo 1 > /proc/sys/vm/drop_caches

# To clear dentries and inodes, change the number to 2:
echo 2 > /proc/sys/vm/drop_caches

# To clear pagecache, dentries, and inodes all together, change the number to 3:
echo 3 > /proc/sys/vm/drop_caches

How do I determine the number of RAM slots in use

sudo dmidecode -t memory

Logging memory

Linux system monitor tools where Sysstat package can be used.
How Much Memory Is Installed and Being Used on Your Linux Systems?
How to log the memory consumption on Linux?
8 Commands to Check Memory Usage on Linux
5 commands to check memory usage on Linux
- free -m, free -h, watch -n 10 free -m (free -s 10 gives an error 'seconds argument `10' failed)
- head -3 /proc/meminfo
- vmstat -s
- top
- htop

Shows Per-Program Memory Usage On Linux

atop command. atop -m and press p (per program). Look at the 'RSIZE' column.
How to view summaric memory usage of groups of commands (instead of processes). ps -C firefox --no-headers -o pmem. The result is one column. It shows the percentage not the actual memory. And it cannot catch some programs like "Web Content" or "WebExtensions" associated with Firefox.
ps_mem: https://www.linuxuprising.com/2019/04/psmem-shows-per-program-memory-usage-on.html.

$ pip install ps_mem
$ ps_mem -p $(pgrep -d, -u $USER)
 Private  +   Shared  =  RAM used	Program
...
249.9 MiB +  43.9 MiB = 293.9 MiB	firefox
549.7 MiB +  82.2 MiB = 631.9 MiB	Web Content (6)
  1.0 GiB + 149.6 MiB =   1.2 GiB	chrome (16)

                          3.2 GiB

Check RAM information

sudo dmidecode -t memory

sudo dmidecode -t 17

Free up memory

Monitor Memory Utilization And Send an Email

Bash Script to Monitor Memory Usage on Linux
A Shell Script to Send Email Alert When Memory Gets Low. This uses $(free -mt | grep Total | awk '{print $4}') to get the available memory.
Getting Started with Monit for Server Monitoring

zram

rop swap for zram on Linux

Things to do after a fresh install of GNU/Linux

Run upgrade such as apt-get update; apt-get upgrade. It helps to resolve the unmet dependencies issue too.
Increase audio quality
Make sure firewall is enabled.
Disable any unnecessary services
Install Timeshift
Install ClamAV / Clamtk antivirus

Query whether the OS is 64-bit or 32-bit

SYSTEM_ARCH=getconf LONG_BIT
echo $SYSTEM_ARCH

Command line improved

https://remysharp.com/2018/08/23/cli-improved

Directory permission / attribute

See http://unix.stackexchange.com/questions/21251/how-do-directory-permissions-in-linux-work

When applying permissions to directories on Linux, the permission bits have different meanings than on regular files.

The write bit allows the affected user to create, rename, or delete files within the directory, and modify the directory's attributes
The read bit allows the affected user to list the files within the directory
The execute bit allows the affected user to enter the directory, and access files and directories inside

When we create a new directory, the attribute is 775. Some pre-created directories (Desktop, Documents, Music, Pictures, Public) have an attribute 755.

Making a new temporary directory

https://www.howtoforge.com/linux-mktemp-command/

mktemp  # temp directory is under /tmp
mktemp -d tempdirXXX # temp directory is under the current directory
mktemp tempfileXXX # temp file under the current directory

Shell

Login shell and non-login shell

/etc/profile
/etc/profile.d/*.sh
~/.bash_profile or ~/.profile (for example, environment variable like PATH)
~/.bashrc
/etc/bashrc
~/.bashrc

Non-Login shell

~/.bashrc
/etc/bash.bashrc or /etc/bashrc
~/.bashrc (bash-related settings, for example, prompt string, aliases)

Note: Bash only reads the first of the files in ~/ that it finds (and ignore the rest). rc means run commands for example, .nanorc.

Aliases and Functions for Individual Users

/etc/profile (systemwide environment and shell variables)
/etc/profile.d/*.sh (systemwide environment and shell variables)
~/.bash_profile (user environment and shell variables)
~/.bashrc (executes /etc/bashrc)
/etc/bashrc (systemwide aliases and shell functions)
~/.bashrc (user aliases and shell functions)

Why does it take tens of seconds to get a shell prompt?

https://serverfault.com/a/722496 If your profile or bashrc have expensive things, consider trimming them back.

On raspbian commenting out some lines does help. Interestingly, the same lines does not make any difference on x86 server.

the source command

The . is a shorthand for the source command in bash. source ~/.bashrc and . ~/.bashrc are equivalent.

login shell (.bash_profile) vs interactive shell (.bashrc)

login shell - non desktop environment. ~/.bash_profile is sourced for the bash shell. It is the shell you get when logging in or opening a new terminal session.
- You do not usually have .bash_profile on Ubuntu, nor should you usually create that file.
- On new Ubuntu, there is no .bash_profile. It has .profile.
- you should not put aliases in .profile at all, nor is .bash_profile a good place for them because you will want your aliases to work in interactive shells whether or not they are login shells.
- Login shells are interactive shells.
interactive shell - Ctrl+Alt+t to open a terminal from a graphical mode (desktop environment) and also the ssh connection. ~/.bashrc is source. We usually edit ~/.bashrc to set up the environment to include fancy prompt, set aliases, set history options, or define custom shell functions. Bash Check If Shell Is Interactive or Not Under Linux / Unix Oses
Non-interactive shell - instances of the shell you can't use interactively. Shells that are started to run a command or script.

To determine the shell type: echo $-

export environment variables

Both a login shell and an interactive one. SSH (Putty) to connect to a remote machine.
When a shell runs a script or a command passed on its command line, it's a non-interactive, non-login shell.

/root/.bashrc

If we use "sudo SOME_COMMAND", ~/.bashrc won't work. In this case, we have to

run "sudo su"
Edit /root/.bashrc

Login banner

How to configure login banners in Linux (RedHat, Ubuntu, CentOS, Fedora). /etc/motd and /etc/login.warn

Change to root shell

The following command will switch to an environment similar to what the user would expect had the user logged in directly.

sudo su -
# OR
sudo su
# OR
sudo -s

This can be useful when running 'su' or 'su -' failed because of an authentication failure error (note Ubuntu locked the root account).

pinky: find out about the people logged on to your Linux computer

How to Use the pinky Command on Linux

su: Switch to another user and run a command

su means 'substitute user'.

Use

su newusername   # keeps the environment variables of the original user
su - newusername # clears all the environment variables

Linux Run Command As Another User runuser -l command
```
runuser -l  userNameHere -c 'command'
```

Keyboard shortcut to move cursor in the terminal

Alt + b: go left (back) one word
Alt + f: go forward on word

Example: cd ~/bitbucket/gbmpdx/annovar_biowulf (Now press Alt+b to see the cursor moves)

Record terminal session to a text file

script history_log.txt
# recording begins
exit # stop recording

This will include everything showing on your screen.

Tools To Record Your Terminal And Generate Animated Gif or SVG Images

Asciinema & agg

It works on Ubuntu 23.04
Asciinema
- Asciinema – Record and Share your terminal sessions on the fly
3 Ways to Solve Pip Install Error on Ubuntu 23.04
https://github.com/asciinema/agg

$ # sudo pip3 install asciinema  # Error
$ # sudo apt install python3-asciinema # Error
$ sudo apt install pipx
$ pipx ensurepath
Success! Added /home/brb/.local/bin to the PATH environment variable.

Consider adding shell completions for pipx. Run 'pipx completions' for
instructions.

You will need to open a new terminal or re-login for the PATH changes to take
effect.

Otherwise pipx is ready to go!

Open another tab

pipx install asciinema
asciinema rec 2g-test
asciinema play 2g-test
chmod +x Downloads/agg-x86_64-unknown-linux-gnu 
Downloads/agg-x86_64-unknown-linux-gnu 2g-test 2g-test.gif
open 2g-test.gif

terminalizer

https://github.com/faressoft/terminalizer

sudo apt update
sudo apt install nodejs npm
sudo npm install -g npm
sudo npm install -g terminalizer
terminalizer record demo
terminalizer play demo
terminalizer render demo 
# https://github.com/faressoft/terminalizer/issues/29
# https://github.com/faressoft/terminalizer/issues/211

Termtosvg

(archived) Termtosvg – Record Your Terminal Sessions As SVG Animations In Linux

Record and Replay Linux Terminal Sessions Activity: script

Learn how to Record and Replay Linux Terminal Sessions Activity

Recording your terminal: asciinema

https://asciinema.org/

Dictionary/translation

Dictionary. Lookup word-meaning using DICT protocol with curl command
```
$ curl dict://dict.org/d:word
```
The output of the above method is too complicated. The 'trans' utility provides a better output (even still no colors)
```
$ trans -d word
```

Translation shell

$ wget git.io/trans
$ chmod +x ./trans
$ sudo apt update; sudo apt install gawk
$ sudo mv trans /usr/local/bin/
$ trans :zh-TW "interrogate"
interrogate
/inˈterəˌɡāt/

審問
(Shěnwèn)p

Clear screen

ctrl + l

Clear text

ctrl + u: "deletes" all the entered text to the left of the cursor. Does not work in macOS shell but R console from RStudio in macOS still works.

Redirect standard error

http://bash.cyberciti.biz/guide/Standard_error. Use 2> operator.

command 2> errors.txt

Redirect standard output

This can be used in the cron job or displaying a clock on the desktop.

$ cat ~/bin/clock
dclock -date "Today is %A %B %Y" -led_off black -bg black -fg yellow -geometry 577x194+119+139   &>/dev/null &

Quotes and asterisk

Combining these two will not work. For example

brb@T3600 ~ $ ls -l ~/GSE48215/*.fastq
-rw-r--r-- 1 brb brb 16226673016 Jun 14 14:13 /home/brb/GSE48215/SRR925751_1.fastq
-rw-r--r-- 1 brb brb 16226673016 Jun 14 14:13 /home/brb/GSE48215/SRR925751_2.fastq
brb@T3600 ~ $ ls -l '~/GSE48215/*.fastq'
ls: cannot access ~/GSE48215/*.fastq: No such file or directory
brb@T3600 ~ $ ls -l "~/GSE48215/*.fastq"
ls: cannot access ~/GSE48215/*.fastq: No such file or directory

cat command alternatives

bat
Bat – Linux cat command with syntax highlighting and Git integration
Direct installing bat using 'apt install' does not work:( Grab the binary from github works.

$ curl -s  https://api.github.com/repos/sharkdp/bat/releases/latest |grep browser_download_url |  cut -d '"' -f 4 | grep 'amd64.deb' | grep -v musl | wget -i -
$ sudo dpkg -i bat_*_amd64.deb

ls command

List Files With Detailed Information from How to Use the ls Command in Linux

To use UID/GID instead of the user name and group name in ls -l, use the -n option.

ls -n

To make a pretty output by showing selected columns (col 9 is the file name and col 5 is the file size)

$ ls -nt bad |  grep -v ^total | awk '{ printf  "%-20s %15i\n", $9, $5}'
recal.bai                    8069704
recal.bam                12275091222
recal_data.table             1012453
realigned_reads.bai          8065496

Follow the symbolic link

Use -H option

ls -lH myDir

List only directories

ls -d */               # current directory
ls -ld ~/Downloads/*/  # ~/Downloads
ls -l -d */

List only files

ls -l | egrep -v '^d'

Find and Delete Broken Symbolic Links

find /path/to/directory -xtype l -delete

Special characters, escape

List of characters which needs to be escaped in a linux shell command

|  &  ;  <  >  (  )  $  `  \  "  '  <space>  <tab>  <newline>

Opened MS-Office documents' filenames start with "~$". The dollar sign character has to be escaped; eg ls -l ~\$* to list these kind of files or rm ~\$* to delete these files.

Check non-English characters

How to Find Non-ASCII Characters in Text Files in Linux

perl -ne 'print if /[^[:ascii:]]/' sample.txt

ls | more without lose color

$ ls --color=auto
$ ls --color | more

Most likely your ls is aliased to ls --color=auto. If you do ls --color (which is morally equivalent to ls --color=always), that will force it to turn on colors.

ls directories color

How to Change the Colors of Directories and Files in the ls Command. No need to use the export command when we want to add it to .bashrc file.

# orange color
export LS_COLORS=$LS_COLORS:'di=0;33:'
# OR yellow color if your terminal supports 256 colors
export LS_COLORS=$LS_COLORS:'di=38;5;226:'

ls output with color background

stackexchange or askubuntu.

In my case, after I apply chmod 755 -R XXXX, the weird green background color goes away.

ls output without user/group columns

https://unix.stackexchange.com/a/205165

ls -lhog

To further skip the first 2 columns (the permissions and link count) we can use

ls -lhog | sed 's/^[^ ][^ ]*  *[^ ][^ ]* //'
# total 95G
#  51G May  7 11:37 SRR10156301_3.fastq  --> file
#  23G May  7 11:35 SRR10156301_2.fastq  --> file
#  18G May  7 11:35 SRR10156301_1.fastq  --> file
# 4.0K May  7 11:24 SRR10156301          --> directory

ls output selected columns

The following example will sort the output by file size and output only the file size and file name columns.

ls -lS  . | awk '{print $5 "\t" $9}'

ls -lhS  . | awk '{print $5 "\t" $9}'

# Mac
ls -lS  *.Rmd |head | awk '{print $6 "\t" $10}'
# a range of columns:
ls -lS  *.Rmd |head | awk -v f=6 -v t=10 \
    '{for(i=f;i<=t;i++) printf("%s%s",$i,(i==t)?"\n":OFS)}'

ls suddenly wrapping items with spaces in single quotes

Use ls -N to remove single quotes for files containing spaces. See Why is 'ls' suddenly wrapping items with spaces in single quotes?

Better to add export QUOTING_STYLE=literal to .bashrc

ls on BSD/macOS

Use the -G option to get a color output

$ ls -G

realpath

realpath FILENAME to get the full path of a file.

tree command

tree -d: show directories only

tre command

github. Binary files for x86 and arm linux are available. To use it, run "tre DIRECTORYNAME" or "tre" to list files recursively from the current directory.

cp command

Linux cp command tutorial for beginners (8 examples)

copy a directory

cp -avr Dir1 Dir2

where -a will preserve the attributes of files/directories, -v means verbally and -r means copy the directory recursively.

Copy a file with progress bar with pv (plus how to eject the USB drive)

http://www.tecmint.com/monitor-copy-backup-tar-progress-in-linux-using-pv-command/

sudo apt-get install pv
pv file1 > file2    # don't forget the ">" operator and the destination is a file, not a directory

After that, instead of clicking the reject icon from the file manager to eject it, it is better to use a command line to do that because there is no expect time for users to know when it will take for finish writing the data to a USB drive.

sudo apt-get install udisks

sudo udisks --unmount /dev/sdb1   # /dev/sdb1 is the partition
sudo udisks --detach /dev/sdb     # /dev/sdb is the device

My testing shows this procedure works (tested by running md5sum after eject/plug-in) when I need to copy a 9GB file.

Reliable way: Split the large file and copy smaller chunks

# Use 'sudo iotop -o' to monitor the I/O
split -b 4G inputFile  # create xaa, xab, ... files
cat x* > outputFile    # merge them. md5sum check succeeds

type  x* > outputFile  # Windows OS. # https://stackoverflow.com/a/60254

# Use Prefix, and use numeric suffixes starting from 0
split -b 4M -d inputFile inputFile.part
md5sum inputFile
cat inputFile.part* > inputFile2
ech "LONG_MD5_SUM_From_inputFile inputFile2" | md5sum -c

It is interesting copying smaller files (eg 4GB) to USB drives is quite stable (just use the cp command). Even for a not-too large file (6.7GB), pv step looks OK but the unmount/detach step failed.

For a 6.7GB file, it will split it into a 4GB and 2.7GB files. Merge takes longer time if it is done on the USB drive. That is, it is best to do merge in the final destination (internal disk/storage).

split in the internal hdd: 1min 38sec
merge in the internal hdd: 37sec
merge in the USB 3.0 drive: 2min 17sec

Remember: Use a reliable USB drives.

The operation could not be completed because the volume is dirty

On a USB 2.0 drive, I can copy files to there but the drive cannot be rejected (Ubuntu has a pop-up showing it is still writing data to it).

When I forcibly rejects the drive and plug it in a Windows PC, Windows shows the message The operation could not be completed because the volume is dirty. This gives a way to run chkdsk (check and repair a file system).

Open a Windows File Manager
Right click the USB drive
Properties
Tools -> Check now... Start

Done. Now I can use the drive again.

The Linux equivalent to chkdsk is fsck. fsck is a front end that calls the appropriate tool (fsck.ex2, fsck.ex3, e2fsck, ...) for the filesystem in question.

umount /dev/sdb1        # thumb drive
sudo fsck /dev/sdb1

sudo fsck -a /dev/sdb1  # auto repair

For the root disk, you have to use a live CD. Otherwise, you will see a message like

$ fsck /dev/sdb1
fsck from util-linux 2.20.1
e2fsck 1.42.9 (4-Feb-2014)
/dev/sdb1 is mounted.

WARNING!!!  The filesystem is mounted.   If you continue you ***WILL***
cause ***SEVERE*** filesystem damage.

Do you really want to continue<n>? no

Files

List files by sorting according to the file size

Use the -S option in ls.

List files using wildcard without showing files under subdirectories

Use the -d option in ls. For example, the following command will not showing files under R-3.4.4 and R-3.5.3

$ ls -d R*
drwxr-xr-x 15 brb brb     4096 Mar 14 09:48 R-3.4.4
-rw-rw-r--  1 brb brb 30474612 Mar 15  2018 R-3.4.4.tar.gz
drwxr-xr-x 15 brb brb     4096 Mar 14 09:31 R-3.5.3
-rw-rw-r--  1 brb brb 30205979 Mar 11 04:04 R-3.5.3.tar.gz

Delete multiple files

How to Remove Multiple Subdirectories with One Linux Command

rm -r ~/Documents/htg/{done,ideas,notes}

Delete a certain type of files recursively under a directory

For example to delete *.o files under the current directory,

find . -type f -name '*.o' -delete

Remove all files/directories except for one file/some file type

https://unix.stackexchange.com/a/153863

# recursively, including hidden ones
find . ! -name 'file.txt' -type f -exec rm -f {} +

# Non-recursively
find . -maxdepth 1 ! -name 'file.txt' -type f -exec rm -f {} +

find: warning: you have specified the -maxdepth option after a non-option argument !, but options are not positional (-maxdepth affects tests specified before it as well as those specified after it). Please specify options before other arguments.

Remove all hidden files/directories

Bash Find And Delete All Hidden Files Directories

rm -rf .??*  # tested, http://labtestproject.com/linuxcmd/rm.html

# list all hidden files/directories
find . -name ".*" -print

# delete all hidden files
find . -name ".*" -type f -delete

find . -name ".DS_Store" -delete

Create a new directory and cd to it

How to Make a New Directory and Change to It with a Single Command in Linux

Set permissions for new files or directories

umask: user file-creation mode mask
With umask 007, newly created files and directories will have all permissions (read, write, and execute) for the user and the group, but no permissions for others. This is a good practice when you want to share data with other users in the same group but want to completely exclude users who are not group members.
Set up default umask on Linux: adding the command umask 007 to the .bashrc file sets the default umask value for your shell. This means that every time you start a new shell session, the umask will be set to 007.
How it works:
- The default permissions for files are 666 (read and write for owner, group, and others) and for directories are 777 (read, write, and execute for owner, group, and others).
- The default permissions for files are 666 and for directories are 777. In binary, these are 110 110 110 and 111 111 111 respectively. The umask value 007 is 000 000 111 in binary.
- The AND operation is performed between the binary representations of the default permissions and the bitwise NOT of the umask value. That is, the mask is negated (its bitwise compliment is taken) and this value is then applied to the default permissions using a logical AND operation.
- The result of the AND operation is 110 110 000 for files and 111 111 000 for directories. In decimal, these are 660 and 770 respectively.
See What is Umask and How To Setup Default umask Under Linux?, Linux umask command.

chown and chmod recursively (-R)

Use -R (capital R).

chmod -R u=rwx,go=rx /var/www/html

Note it is better not to remove 'x' on folders. Otherwise we will lose the permission to change file attributes for any file under the folder.

Note "chmod -R ugo+rwx XXX" is the same as "chmod -R 777 XXX" b/c "u" stands for user, "g" stands for group and "o" stands for others.

Get the chmod numerical value for a file/directory

https://unix.stackexchange.com/a/46921

# BSD/OS X: 
stat -f "%OLp" <file>

# Linux: 
stat --format '%a' <file>
stat -c %a <file>

After that we can use ls -lhog to verify.

Files under a directory have question mark attribute

Use sudo chmod -R a+x /some/directory to fix. This happened when I unzip a zip file compressed in a Windows OS.

Files have an integer owner in attributes

What does the “number” in the owner field of files signify in linux?

You probably did a copy that preserved the original group and owner of these files. Within linux internally the owner and group is basically just an id.

You can change the ower and group to an existing owner and group with the commands chown and chgrp respectively.

Uppercase S in permissions of a folder and setGID

I happen to create this case by chmod -R 760 ShareFolder. To make 'S' to become 's', I just need to use chmod -R 2770 ShareFolder.

immutable files

Manage file and directory attributes using chattr and lsattr command

Display files sorted by modified date in a directory recursively

stackoverflow.com

stat --printf="%y %n\n" $(ls -tr $(find DIRNAME -type f))

superuser.com

find -type f -printf '%T+\t%p\n' | sort -n

Both of methods give the same output. Note the latest changed file is shown at the bottom of the output.

Sort files by their size

use the '-S' option.

ls -lS

Files starting with a dash (meta-characters)

Move File Starting With A Dash

$ > '-foo.txt'
$ rm "-foo.txt"
rm: invalid option -- 'o'
Try 'rm ./-foo.txt' to remove the file '-foo.txt'.
Try 'rm --help' for more information.
$ rm -- -foo.txt

Inodes

Recover Deleted Files

How to Recover Deleted Files on Linux

alias

https://www.cyberciti.biz/faq/how-to-turn-on-or-off-colors-in-bash/

$ alias # list all aliases
$ alias | grep ls
$ unalias ls
$ alias ls='ls --color=auto' # save it in ~/.bash_profile or ~/.bashrc
$ alias server_name="ssh -v -l john 192.168.1.11" # or modify /etc/hosts
$ alias open='xdg-open'
$ alias sshnocheck='ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no'
$ alias scpnocheck='scp -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no'
$ alias sshserver='ssh [email protected]'
$ alias checkport='sudo lsof -i -P -n | grep LISTEN'

NOTE the last column of the output from "checkport" will show who can connection to the port. For example, if it shows "127.0.0.1:10999 (LISTEN)", it means only the server can connect to port 10999. If it shows "*:22 (LISTEN)", it means any machines can connect to the server through port 22.

To avoid using the alias, use one of the following ways (eg use the command's full path)

$ \ls
$ /bin/ls
$ command ls
$ 'ls'

Useful aliases (Added to ~/.bashrc)

alias nano="nano -c --softwrap"

We can use the backslash to escape the double quote (or others like dollar sign).

If a command needs the sudo right, include the command inside alias. In the following example, if we skip "sudo" then running the command "sudo lsof2" will result in an error: sudo: lsof2: command not found.

alias lsof2="sudo lsof -i -P | egrep \"PID|LISTEN\""

Use unalias [alias name] to remove an alias.

Find file defining an alias

how to find file defining an alias. It works on macOS. Many of them are defined in ~/.oh-my-zsh/lib/directories.zsh. Another way (without saving) is How to find out where alias (in the bash sense) is defined when running Terminal in Mac OS X.

lolcat - bring color to text

sudo apt-get install ruby	
sudo gem install lolcat
lolcat -h
lolcat --version
fortune | lolcat

ps | lolcat
man ls | lolcat

lolcat test.R

sudo apt install figlet
figlet Merry Christmas | lolcat

alias lolls="ls -l | lolcat"
lolls

File manager

Cloud commander

https://cloudcmd.io/
https://www.ostechnix.com/cloud-commander-a-web-file-manager-with-console-and-editor/. You can upload files from the Cloud services like Google drive, Dropbox, Amazon cloud drive, Facebook, Twitter, Gmail, GtiHub, Picasa, Instagram and many.

diff

Run diff with large files

diff (or even better the cmp command) works fine with 8G fastq files. Note cmp compare files byte by byte so it probably won't run out of memory. How to diff large files on Linux.

Another strategy is to split a large file into small pieces. For example,

split -b 500MB FILEname # into 500MB files each
split -l 200 FILEname   # into smaller files with 200 lines each

Meld freezes When I tested it with two large files (800k & 936k lines coming from human gtf files). Actually the whole linux system became unresponsive.

Actually Meld is sluggish when it is used in small files in Odroid XU4 running Ubuntu 16.04 MATE. I have used Meld 3.14.2 and the latest 3.16.2.

Kompare

https://apps.kde.org/en/kompare

Meld cannot change theme. So if my desktop has a dark theme, meld is hard to read.

Kompare still has a light them.

diff & colordiff-color on terminal: compare side by side

PS. For a GUI version of diff, Meld works fine. Need to install first. apt-get install colordiff. http://www.cyberciti.biz/programming/color-terminal-highlighter-for-diff-files/

sudo apt-get install colordiff
diff -y file1 file2 | colordiff
# Ignore same rows (two ways):
# diff -C0 file1 file2 | colordiff
# diff -U0 file1 file2 | colordiff

# On systems that I have no root right, I need to install it from the source 
# (just need to run 'make')
$ diff file1 file2 | ~/bin/colordiff-1.0.18/colordiff.pl

where -y option means to show the output in two columns.

Interpretation of the diff output:

The first line of the diff output will contain:

line numbers corresponding to the first file,
a letter (a for add, c for change, or d for delete), and
line numbers corresponding to the second file.

In our output above, 2,4c2,4 means: "Lines 2 through 4 in the first file need to be changed in order to match lines 2 through 4 in the second file." It then tells us what those lines are in each file:

Lines preceded by a < are lines from the first file (color in red);
lines preceded by > are lines from the second file (color in green).
The three dashes ("---") merely separate the lines of file 1 and file 2.

2,4c2,4
< I need to run the laundry.
< I need to wash the dog.
< I need to get the car detailed.
---
> I need to do the laundry.
> I need to wash the car.
> I need to get the dog detailed.

colordiff -ur path1 path2

If you change -ur to -urN then that will also show the contents of files that are only present in one of the paths.

The meaning of colors can be found in /etc/colordiffrc (man colordiff or colordiff web site)

plain=off
newtext=darkgreen
oldtext=darkred
diffstuff=darkcyan
cvsstuff=cyan

git diff --no-index -- file.a file.b

Any visual diff in Linux console?

gnome-terminal

Remember the session

gnome-terminal --help-all --tab-with-profile
- https://stackoverflow.com/questions/17422810/opening-terminal-with-multiple-tabs-using-shell-script
- https://stackoverflow.com/questions/1700995/start-gnome-terminal-with-multiple-virtualenv-tabs-and-services-pre-titled
Can no longer set terminal title in Ubuntu 16 (gnome-terminal)
Opening multiple tabs with gnome-terminal: use --tab and profile options
Save multiple gnome-terminal layout?: --load-config and --save-config options. NOTE gnome 3.18 option "--save-config" is no longer supported. But strangely enough, "--load-config" is still there.
How to remember multiple tabs' session in terminal? (Alike FireFox session manager): --profile= and --save-config options. --working-directory and --tab options.

The following is proved working on Ubuntu 18.04

gnome-terminal --tab --working-directory=$HOME/Downloads \
               --tab --working-directory=$HOME/Documents

Fun: piano

Let Us Play Piano In Terminal Using Our PC Keyboard

Terminals in grids

See Terminal_multiplexer.

Tilix

Terminator

GNU screen

tmux*

Byobu

Guake / Yakuake / Tilda

Drop down terminals for the GNOME / KDE / GTK Environments. Great for quick access to a terminal!

System date/time, ntpd

$ timedatectl
      Local time: Mon 2019-06-10 08:37:09 EDT
  Universal time: Mon 2019-06-10 12:37:09 UTC
        RTC time: Mon 2019-06-10 12:37:09
       Time zone: America/New_York (EDT, -0400)
 Network time on: yes
NTP synchronized: yes
 RTC in local TZ: no

Linux file timestamps

Linux File Timestamps Explained: atime, mtime, and ctime

Change the date/timestamp of a file - touch

Modify the file relative to its existing modification time

filename=MyFileName
touch -d "$(date -R -r $filename) - 2 hours" $filename # 2 hours before
touch -d "$(date -R -r $filename) + 2 hours" $filename # 2 hours later

See How can I change the date modified/created of a file?

Find binary file location, type

which - Display the full path of shell commands. See examples from cyberciti.biz.

$ which ls
/bin/ls

whereis - locate the binary, source, and manual page files for a command. See examples from cyberciti.biz.

$ whereis ls
ls: /bin/ls /usr/share/man/man1p/ls.1p.gz /usr/share/man/man1/ls.1.gz

type, How to find a path of a Linux command like a pro

$ type ls
ls is aliased to `ls --color=tty'
ls is /bin/ls

locate. Use locate command mindfully. It is used to find the location of files and directories. Note that locate does not search the files on disk rather it searches for file paths in a database. For example, the following command will search .png files over the system (not only the personal directory).

locate "*.png"

See How to use updatedb command as an ordinary user?, Use updatedb and locate to index files.

find: Find a file

find operates recursively into sub folders.

-iname

Search by a file name

$ find . -type f -name "abc*" # find a file starting with abc
$ find . -iname '*.txt'  # -iname or -name is necessary

It also works for searching files on subdirectories.

$ find . -name transcripts.gtf
./RH_bio/transcripts.gtf
./dT_ori/transcripts.gtf
./dT_tech/transcripts.gtf
./dT_bio/transcripts.gtf
./RH_ori/transcripts.gtf
./RH_tech/transcripts.gtf

-mtime, -ctime, -atime argument

Find files modified in 10 days.

$ find . -ctime -10 # include subdirectories
.
./mediawiki-1.32.0.tar.gz
./d
./d/deepSurv.pdf

Find files modified in one day and contain string 'est'

$ find . -mtime -1 -exec grep --with-filename est {} \;

If the search directory is not the current directory, we need to add a forward slash to the directory name.

$ find ~/Desktop -iname '*.txt'  # Not working
$ find ~/Desktop/ -iname '*.txt' # Working

The following example shows we can list multiple search criteria. The “‑r” option in tar appends files to an archive. xargs is a handy utility that converts a stream of input (in this case the output of find) into command line arguments for the supplied command (in this case tar, used to create a backup archive).

find / -type f -mtime -7 | xargs tar -rf weekly_incremental.tar
gzip weekly_incremental.tar

What is the difference between mtime, atime and ctime?

mtime (modification time) indicates the time the contents of the file has been changed. Mind you, only the contents. Not the attributes.
ctime (change time) is the timestamp of a file that indicates the time that it was changed. Now, the modification can be in terms of its content or in terms of its attributes.
atime (access time) is the timestamp that indicates the time that a file has been accessed.

xargs

See Linux Programming

-exec COMMAND {} \;

Search and list files with details
```
find . -type f -name "*street*" -exec ls -lh {} \;
```
-exec ls -lh {} \; executes the ls -lh command on each found file. The {} is a placeholder for the current file, and \; indicates the end of the -exec command.
How to run find -exec?, Using semicolon (;) vs plus (+) with exec in find
```
# This command will run grep chrome on each file individually.
find . -exec grep chrome {} \;

# This command will run grep chrome on batches of files, reducing the number of times the command is executed.
# Using + can significantly improve performance
find . -exec grep chrome {} +
```
Note:
- The backslash before ; is to escape ; so linux won't interpret it directly.
- Command Compatibility: Not all commands support being executed with multiple arguments at once. For example, grep can handle multiple files, but some other commands might not. In other words, while -exec COMMAND {} + can be more efficient, it’s important to ensure the command you’re using supports this usage and behaves as expected.
Find files and execute something (google: find --exec)
```
$ find ./ -name "*.tar.gz" -exec tar zxvf {} \;
```

Find and move files to a new directory

find OLDDIR -type f -exec mv -t NEWDIR {} +

Find the total file size of a list of files.
What is meaning of {} + in find's -exec command?
Why does 'find -exec cmd {} +' need to end in '{} +'?
How to run find -exec? The following will find out the total file size of the 'accepted_hits.bam' file under all sub-directories.
```
find ./ -iname "accepted_hits*" -exec du -ch {} + | grep total$
```
where '-c' produces a grand total, and will substitute {} with the filename(s) found in -exec.

How to find and delete directory recursively

How to find and delete directory recursively on Linux or Unix-like system. Application: recursively remove backups older than 30 days.

Find all soft link files

find /tmp -type l

Recursive statistics on file types in directory?

You could use find and uniq for this. This is fast!

$ find . -type f | sed 's/.*\.//' | sort | uniq -c

Exclude or Ignore Files

Find command Exclude or Ignore Files (e.g. Ignore All Hidden .dot Files )

Avoid Permission Denied Messages

How to fix find command permission denied messages

Redirecting ALL standard error (not only permission denied error): 2>/dev/null.

find . -iname "data*.txt" -print 2>/dev/null

Focus on the 'permission denied' message: grep -v "Permission denied"

find / -name foo 2>&1 | grep -v "Permission denied"

Find Files That Have Been Modified Recently in Linux

fd: The Find Command Alternative

Fd: The Find Command Alternative For Mastering File Search In Linux

grep: Find a file by searching contents

grep -r -i "Entering" ~/Downloads/R-3.0.0/

where -r means recursively searching the directory and -i means case insensitive.

Sometimes using -R is more effective because of the symbolic links issue.

$ grep -r -i phpmyadmin /etc/apache2/  # nothing returned
$ grep -R -i phpmyadmin /etc/apache2/

We can also display the row numbers for matches by using the -n parameter in grep.

# What variants appear in dbsnp
grep -n 'rs[0-9]' XXX.vcf

To exclude lines with a pattern, using the -v parameter.

# How many variant were called
grep -v "^#" XXX.vcf | head

To exclude binary files, use -I parameter.

To show only matched filenames, using the -l parameter.

grep -l "iterator" *.cpp
# if we add '-n', the '-n' option won't work.

To search with certain file extensions, use --include argument; see this post.

grep -r -i --include \*.h --include \*.cpp KEYWORD ~/path[12345]  
# escape with \ just in case you have a directory with asterisks in the filenames

How To Find All Files Containing Specific Text On Linux From The Command Line

grep -Rni --exclude-dir={Private,Personal} --include={*.txt,*.js} 'text' ~/Documents
# Exclude hidden directories
grep -R --exclude-dir=".*" 'text' ~/Documents

The only issue with using the -f argument is that grep is going to attempt to interpret the keywords as if they are patterns, which can slow it down when parsing against an extremely large file. So you can also specify the -F parameter, which tells grep to only do exact matches against the strings.

grep -f searchstringsFile filetosearch > output.txt

# -F, --fixed-strings
grep -F searchstring filetosearch > output.txt

If the pattern is saved in a file, use the -f parameter

grep -f PATTERNFILE INPUTFILE

If there are two keywords, use the following

$ grep "begin\|completed" --color swarm_58606147_0.o  # needs an escape
begin 2018-01-12 14:46:05
alignment is completed 2018-01-12 16:45:24
marking duplication is completed 2018-01-12 17:52:01
assign read group is completed 2018-01-12 18:22:49
indel re-alignment is completed 2018-01-12 19:29:32
BQSR is completed 2018-01-12 22:26:22
GATK is completed 2018-01-12 23:43:3
$ egrep "begin|completed" --color swarm_58606147_0.o 
# no need an escape if we use extended regular expressions

We can use R to compute the time spent in each step; see Dealing with dates.

Check https://www.howtoforge.com/tutorial/linux-grep-command/ for more examples

Using grep to search only for words / exact match ("-w" option)
Using grep to search two different words (egrep -w 'word1|word2' /path/to/file)
Count line for matched words ("-c" option)
Grep invert match ("-v" option)
How to list only the names of matching files ("-l" option)

Preserve colouring after piping grep to grep

https://stackoverflow.com/a/2327216

Use grep --color=always .For example, grep --color=always KEYWORD Myfile | more.

Include/follow soft/symbolic link files

grep grep -Rl instead of "grep -rl". Recall "-l" is to list filename only. See "man grep".

Compressed files

zgrep or zipgrep command

GUI

A GUI version of a tool to search files is searchmonkey (open source, Linux, Windows). On Ubuntu, we install it by

sudo apt-get install searchmonkey

It is also useful to change the settings so we can click a filename and open it in the desired text editor. To do that, go to Settings -> Preferences -> System Call -> Text Editor. I enter 'geany' since I want to use geany to open my C programs. Note. the v2.0 source code needs to be built using i386 gcc library and Qt 4.8.x. Still, I cannot get rid of some errors coming from the source code.

-- option

How To Use grep To Search The --help Output To Find Out What CLI Arguments That Begin With A Dash Do

rm --help | grep -w -- -r

rm --help | grep -w -- '-[rf]'  # multiple one letter

Summary of find and grep commands

Command	Examples
find	find [DIRECTORY] -iname '.txt' find [DIRECTORY] -maxdepth 2 -iname .php find -name '.php' -o -name '.txt' # OR operator
grep	grep -r -i "check_samtools" DIRECTORY/ dpkg -l libgtk* \| grep '^i'

Format the output: column

This command will make the output of some command easy to read; see 18 Commands That Will Change The Way You Use Linux Forever.

For example: mount | column -t

Count number of columns: awk

The following command shows the number of columns for the first few rows of a text file.

head MYFILE | awk '{ print NF}'

head MYFILE | awk -F '\t'  '{ print NF}'

Count number of rows in a file: wc

wc -l MYFILE

The source code of wc (or any Linux command) can be found by using this method

brb@brb-T3500:~/Downloads$ which wc
/usr/bin/wc
brb@brb-T3500:~/Downloads$ dpkg -S /usr/bin/wc
coreutils: /usr/bin/wc
brb@brb-T3500:~/Downloads$ sudo apt-get source coreutils

As we can see from the coreutils-8.21/src directory, there are over 100 C programs including <cat.c>, <chmod.c>, <cp.c>, ...<wc.c>.

Print certain rows/lines of a text file

The following example will print out lines 10 to 60 of FILENAME.

sed -n '10,60p' FILENAME

Or to print out line 60,

sed -n '60p' FILENAME

It seems this method is not as fast as I expected. For example, the tail command will immediately print out the result without waiting!

Print a text file with line number: less

How to Use the less Command on Linux

less -N myfile

output colored console to html

Use ansi2html.sh. It only requires gawk.

Use wget to download it
sudo apt-get install gawk
chmod +x ansi2html.sh
colordiff file1 file2 | ./ansi2html.sh > diff.html

using a the result of a diff in a if statement

ls -lR $dir > a
ls -lR $dir > b

DIFF=$(diff a b) 
if [ "$DIFF" != "" ] 
then
    echo "The directory was modified"
fi

Another example

if [ "$(diff file1.html file2.html)" == "" ]; then echo Same; else echo Different; fi

Prompt

Colored prompt

For example, the following code will change the prompt to a light blue color. NOTE that we need ∖[ and ∖] in order to avoid a problem of miscalculating the cursor's starting position.

# blue   
export PS1='\[\e[1;34m\]\u@\h:\w\$ \[\e[0m\]'     # bright blue (good)
export PS1='\[\e[0;34m\]\u@\h:\w\$ \[\e[0m\]'     # darker blue

# yellow
export PS1='\[\e[1;33m\]\u@\h:\w\$ \[\e[0m\]'     # bright yellow
export PS1='\[\e[0;33m\]\u@\h:\w\$ \[\e[0m\]'     # dark yellow (good)

# red
export PS1='\[\e[1;31m\]\u@\h:\w\$ \[\e[0m\]'     # bright red
export PS1='\[\e[0;31m\]\u@\h:\w\$ \[\e[0m\]'     # dark red (good)

# green
export PS1='\[\e[1;32m\]\u@\h:\w\$ \[\e[0m\]'     # bright green
export PS1='\[\e[0;32m\]\u@\h:\w\$ \[\e[0m\]'     # dark green

# cyan
export PS1='\[\e[1;36m\]\u@\h:\w\$ \[\e[0m\]'     # bright cyan
export PS1='\[\e[0;36m\]\u@\h:\w\$ \[\e[0m\]'     # dark cyan (good)

# purple
export PS1='\[\e[1;35m\]\u@\h:\w\$ \[\e[0m\]'     # bright purple (good)
export PS1='\[\e[0;35m\]\u@\h:\w\$ \[\e[0m\]'     # dark purple

To make a permanent change, we can add the line to ~/.bashrc file and (is it necessary) un-comment the following line

force_color_prompt=yes

Some of my settings in .bashrc file

# Office Linux Mint, bright blue, display current time
# Example: 12:45PM ~/Downloads$
PS1="\[\033[1;34m\]\$(date +%H:%M%p) \w$\[\033[0m\] "

# Biowulf and Helix, dark yellow
# Example: biowulf:~/R$
PS1='\[\e[0;33m\]\h:\w\$ \[\e[0m\]'
alias nano="nano -c --softwrap"

# Mac, light green
export PS1="\[\e[0;32m\]mac$\[\e[0m\] "

Shorten prompt

How can I shorten my command line (bash) prompt? It is useful especially in VM.

Add a timestamp to your Bash prompt

See man 3 strftime for the date, time format. In Linux Mint, the date applets uses the %A %B %e, %H:%M which gives a format like 'Friday July 15, 10:23'.
For a Good Strftime - Online date/time formatting tool
http://bneijt.nl/blog/post/add-a-timestamp-to-your-bash-prompt/. Set

PS1	Prompt
default	brb@p45t:~/Downloads$
PS1='[\D{%F %T}] \u@\h \W\$ '	[2016-07-08 16:56:48] brb@brb-P45T-A ~/Downloads$
PS1="\[\033[1;34m\]\$(date +%H:%M%p) \w$\[\033[0m\] "	10:54AM ~/Downloads$

From here, we can skip %F (not showing the date), \W (not showing the current directory) and change %T to %H:%M (not showing seconds).

Another solution: using PROMPT_COMMAND variable. http://askubuntu.com/questions/193416/adding-timestamps-to-terminal-prompts. Add this line to the ~/.bashrc file:

export PROMPT_COMMAND="echo -n \[\$(date +%H:%M%p)\]\ "

and the output will be something like:

[07:03AM] user@hostname:~$

zsh: display time to the right hand side

add a line RPROMPT='%*' to ~/.zshrc

Proxy

The 15 Best Web Proxies for Geo-Blocked Content and Online Privacy

Listen to pandora in Europe: install squid proxy

http://www.cyberciti.biz/faq/access-pandora-radio-using-proxy-server-outside-usa/

Interestingly, the firefox connection settings should choose HTTP Proxy instead of 'SOCKS host'.

SSH, scp

See ssh.

Graphical way to display disk usage

http://www.makeuseof.com/tag/how-to-analyze-your-disk-usage-pattern-in-linux/

For example, to use xdiskusage, we run apt-get install xdiskusage and launch it by xdiskusage ~/.

Ubuntu has a built-in program called "Disk Usage Analyzer". Just search it from Dash. Looks useful!

df : Display disk space

df -h
df -h -T  # show the 't'ype of the file system like tmpfs, ext4, squashfs (snap), vfat
df -h -t ext4 # show file systems of given type (ext4 in this example)
df -a     # show all file system (include ones that have a size of zero blocks)

df -h | grep -v snap # ignore snap partitions
df -h | grep -v loop

Note for the NTFS type, it will be reported as fuseblk by mount or df command.

duf

Terminal Tip ‘duf’ is Prettier Alternative to the ‘df’

Disk encryption

LUKS

Encrypt files

croc Is A Tool For Resumable, Encrypted File And Folder Transfers Between Computers (Command Line)

rm command and trash can

Make “rm” Command To Move The Files To “Trash Can” Instead Of Removing Them Completely

du/ncdu and block size: Display directory size with sorting and human readable

Use ncdu program (more interactive). Although it is a command line program, we can use the mouse to move through each directory to see its sub-directories.

ncdu can show the hidden directory size. This is useful. For example, ~/.local/share/Trash and ~/.singularity/docker can take a lot of space.

sudo apt-get install ncdu
ncdu

http://www.cyberciti.biz/faq/linux-check-disk-space-command/

And the du method.

du -csh *.jpg           # total is at the bottom
du -sh ~/*              # won't include hidden directories, Fast
du -h ~/ --max-depth=1  # include hidden directories, SLOW
du -h ~/ --max-depth=1 --exclude ".*" | sort -nr | cut -f2 | xargs -d '\n' du -sh
du -a -h ~/  # kilobytes will be used, '-a' is to see all files, not just directories.
du -a ~/ | sort -nr | head -n 10   # sort from the largest file size first (in bytes)
                                   # this includes directories and any files under any directories
du -sh * | sort -hr | head -n 10   # this does not go to subdir; only show files and top directories

The --exclude is to hide hidden directories, '-n' is to compare according to string numerical value, and '-r' is to reverse the result.

Note that the 'du' commands may be cheating. See the following screenshot.

The discrepancy is explained by 'sector'. See http://askubuntu.com/questions/122091/difference-between-filesize-and-size-on-disk. Note: it seems 4096 is what I see from all devices.

$ sudo dumpe2fs /dev/sda1 | grep -i "block size"
dumpe2fs 1.41.14 (22-Dec-2010)
Block size:               4096

$ sudo tune2fs -l /dev/sda1 | grep -i "block size"

$ sudo fdisk -l | tail
$ sudo tune2fs -l /dev/mmcblk0p2 | grep -i "block size"

To show a file size in terms of blocks, we can use

ls -s

So for example, if a file takes 150 blocks, and if a block takes 4096 bytes, then the file takes 150*4096/1024 KB on disk.

gdu

gdu Is A Fast Console Disk Usage Analyzer (Alternative To ncdu, du, Etc.)

Find the total size of certain files within a directory

Find the total size of certain files within a directory branch

du -ch ./photos/*.jpg | grep total
find ./photos -type f -name '*.jpg' -exec du -ch {} + | grep total$

Apache benchmark (ab) testing

ab -n 100 -c 10 http://taichimd.us/

Monitor progress of copying/transferring files: pv

How to monitor progress of Linux commands using PV and Progress utilities

# Method 1: rsync
rsync --progress -a sourceDirectory destinationDirectory
rsync --info=progress2 source dest

# Method 2: pv
sudo apt-get install pv
## copy a single file
pv inputfile > outputfile

## multiple files or directories 
tar c sourceDirectory | pv | tar x -C destinationDirectory

## https://stackoverflow.com/a/26226261
docker save <image> | bzip2 | pv | \
     ssh user@host 'bunzip2 | docker load'

rsync

See Backup.

Wireless File Transfer Apps on Linux

The 7 Best Wireless File Transfer Apps on Linux

sudo

How to Control sudo Access on Linux

https://www.howtogeek.com/447906/how-to-control-sudo-access-on-linux/

sudo adduser NEWUSER
sudo usermod -a -G sudo NEWUSER

sudo vs su

The Difference Between sudo and su Explained: password and shell.

How to Keep ‘sudo’ Password Timeout Session Longer in Linux

http://www.tecmint.com/set-sudo-password-timeout-session-longer-linux/

How to run multiple commands in sudo

https://www.cyberciti.biz/faq/how-to-run-multiple-commands-in-sudo-under-linux-or-unix/

Run sudo commands without a password?

Alternative browsers, text browsers

See Browser.

Filezilla

Keyboard shortcut. Especially, Alt+Down=Transfers the currently selected item to an item of the same name in the other pane.

The device is busy

http://oletange.blogspot.com/2012/04/umount-device-is-busy-why.html

brb@brb-P45T-A:~$ sudo umount /media/brb/TOSHIBA 
[sudo] password for brb: 
umount: /media/brb/TOSHIBA: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
brb@brb-P45T-A:~$ sudo umount /dev/sdc1
umount: /media/brb/TOSHIBA: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
brb@brb-P45T-A:~$ lsof /media/brb/TOSHIBA/
COMMAND  PID USER   FD   TYPE DEVICE   SIZE/OFF NODE NAME
VBoxSVC 5600  brb   18w   REG   8,33 4294967295    3 /media/brb/TOSHIBA/Windows 10.ova (deleted)
brb@brb-P45T-A:~$ kill -9 5600
brb@brb-P45T-A:~$ lsof /media/brb/TOSHIBA/
brb@brb-P45T-A:~$ sudo umount /dev/sdc1
brb@brb-P45T-A:~$

See http://ocaoimh.ie/2008/02/13/how-to-umount-when-the-device-is-busy/

# fuser -m /dev/sdc1
/dev/sdc1: 538
# ps auxw|grep 538
donncha 538 0.4 2.7 219212 56792 ? SLl Feb11 11:25 rhythmbox

Another handy one is:

umount -l /dev/sdwhatever

mkfs command

dd and mkfs

How to Use the mkfs Command on Linux, How to Format Storage Drives Using the Linux Terminal

dd if=/dev/zero of=~/howtogeek.img bs=1M count=250
mkfs.ext2 ~/howtogeek.img
sudo mkdir /mnt/geek
sudo mount ~/howtogeek.img /mnt/geek
sudo chown dave:users /mnt/geek/

cd /mnt/geek
cp ~/Documents/Code/*.? .

sudo umount /mnt/geek
cd /mnt
sudo rmdir geek

Format a USB drive: exfat

Easily Format A USB Flash Drive On Ubuntu 18.04 Using USB Stick Formatter (mintStick deb & source)

This is a GUI application. After the installation, search "USB Stick Formatter".

For some reason, it doesn't have the 'exFAT' option. My system has installed exFAT drivers. This post said installing exFAT related drivers only helps reading/writing but not formatting.

If I want exFAT format, I need to use the USB Stick Formatter to format the drive first (for example fat32), plug it and then using the following command to format it to exFAT.

sudo apt install exfat-fuse exfat-utils # ubuntu 20.04 and lower
sudo apt install exfat-fuse exfatprogs # ubuntu 22.04 and higher

sudo fdisk /dev/sdc  # g,p,n,p,1,ENTER,ENTER,w
sudo fdisk -l /dev/sdc # the new partition has Id 83 and Type Linux. 

sudo mkfs.exfat -n Staples /dev/sdc1
sudo fdisk -l /dev/sdc # still Linux? but Gparted shows exfat

sudo mount /dev/sdc1 /mnt # to use

Note that fdisk or sfdisk cannot differentiate NTFS/exFAT. But cfdisk or GParted can.

sudo cfdisk /dev/sdX

Create an ext3/ext4 file system on a USB flash drive

https://en.wikipedia.org/wiki/Smartmontools. My ubuntu 20.04 has smartctl 7.1 while the latest version is 7.3 (Fedora has it)
How to Format Disk Partitions in Linux
GUI: using Ubuntu's Disks utility.

umount /dev/sdb1   (depending on the device of course)
lsblk     # check the drive's partition name
sudo mkfs -t ext4 /dev/sdb1
lsblk -f  # Verify

sudo mkfs.ext3 /dev/sdb1
sudo e2label /dev/sdb1 usbdrive   (change the label)

sudo mkfs.vfat /dev/sdb1   # MS-DOS

exFat - cross platform partition format

Mac
Gparted cannot create exFAT partition (it is greyed out)
This Trick Makes a USB Drive Work Perfectly With Windows, Mac, Linux, and Anything Else
How to Mount and Use an exFAT Drive on Linux or How to get a drive formatted with exfat working?
How to format an SD Card to ExFat in Ubuntu

sudo apt-get install exfat-utils exfat-fuse
# Still need to create a partition (ex. FAT32) first using gparted in order to get it mounted
sudo fdisk -l 
sudo mkfs.exfat -n LABEL /dev/sd**  #  LABEL with whatever you want to label your drive

lsblk -f  # verify the partitions, no sudo is needed

This should delivery a working exfat file system (read and write support, but not formatting the drives with exfat via Gnome Disks and GParted).

Add a new user with home directory; list all users

adduser xxx

adduser is better than useradd since useradd does not create home directory and it does not even ask the password for new user. adduser will interactively ask user information.

To delete the user and home directory, use

deluser --remove-home xxx

To view the user information, type id USERNAME or cat /etc/passwd.

How to List Users in Linux cat /etc/passwd | wc -l

gzip with multi cores: pigz

Use pigz utility. It makes a lot of difference. For example for a 21GB file, gzip can't finish the job after 30 minutes. But pigz only took 7 minutes on a 12-core machine.

According to 'pigz --help', the default threads is the number of online processors, or 8 if unknown.

sudo apt-get install pigz
pigz -9 FILENAME   # best compression method & convert the file to FILENAME.gz

tar cf - paths-to-archive | pigz -9 -p 12 > archive.tar.gz

There is no need to use pigz to un-compress the file. gunzip is fast enough and only takes 4 minutes to decompress.

The '-9' (best compression) option does not make difference (6.6G vs 6.5G).

Note that we have to be careful when we use md5sum to compare compressed files.

Compress a folder without full path name

Suppose we want to compress the folder ~/Documents and its subfolders. We want to include Documents folder name but not /home/brb/Documents name.

# Method 1. Include 'Documents' as the top folder name
cd ~/
tar -czvf tmp.tar.gz Documents
# Method 2. Mind the last dot. Not include 'Documents' as the top folder.
tar -czvf tmp.tar.gz -C /home/brb/Documents .

# Double check the tarball
tar -tzvf tmp.tar.gz

If we want to strip the upper directories when we uncompress a tar file, use --strip-components. For example, we can use --strip-components=1 to remove the Documents folder.

Fix mess created by accidentally untarred files in the current dir

Suppose I accidentally untar a tarball in /var/www/html/ directory instead of /home/projects/www/current. It created mess in /var/www/html/. The easiest way to fix this mess:

cd /var/www/html/
/bin/rm -f "$(tar ztf /path/to/file.tar.gz)"
## or better ##
tar ztf /path/to/file.tar.gz | xargs -d'\n' rm -v

lzma

https://en.wikipedia.org/wiki/Lempel–Ziv–Markov_chain_algorithm
nomadBSD. Since NomadBSD is designed to be a persistent system, we do not provide ISO files, as ISO-9660 is a read-only filesystem.

squashfs

List contents of tar.gz or tar.bz2

tar -tzvf myfile.tar.gz

tar -tjvf myfile.tar.bz2  # replace z with j

gzip: stdin: not in gzip format

I got the following message when I try to run tar -xzvf or tar -tzvf command.

$ tar -tzvf filename.tar.gz 
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
$ file filename.tar.gz 
filename.tar.gz: POSIX tar archive

The answer is How to solve: stdin: not in gzip format.

Solution: Since it was not a gzipped file, a simple tar is able to extract the file: tar xvf MyFile.tar.gz

Extract files, AVFS

See Extract files.

Show folder size for one level only

du --max-depth=1 -h

The graphical tool is called Disk Usage Analyze which is already available on Ubuntu.

Soft link and hard link

Soft link

ln -s /full/path/of/original/file /full/path/of/soft/link/file

Understanding Linux Links Part 1 & Part 2

The order of original and linked above in ln -s is similar to the mount command where we put the original volume first and the system's directory second. See an example here.

But when we issue "ls -l" we see it list the the original file at the end; e.g. /full/path/of/soft/link/file -> /full/path/of/original/file.

Hard link

What's the difference between a hard links and copied files?

Why are hard links not allowed for directories?
Hard linked file looks the same as the source file when viewed by "ls"
Change in either of hard linked file and the source file will affect the other
Hard linked file and source file can be deleted separately without affecting the other

echo "abcd" > foo
ln foo foo2
ls -l foo*
echo "efgh" >> foo2
cat foo
rm foo
cat foo2

Self-hosted servers

https://github.com/Kickball/awesome-selfhosted This is a list of Free Software network services and web applications which can be hosted locally. Selfhosting is the process of locally hosting and managing applications instead of renting from SaaS providers.
Sovereign: A set of Ansible playbooks to build and maintain your own private cloud: email, calendar, contacts, file sync, IRC bouncer, VPN, and more.

Cockpit: manage and monitor my servers using just a web browser

https://cockpit-project.org/
https://localhost:9090 is the default interface
Cockpit: an overview
Cockpit and the evolution of the Web User Interface
How I use Cockpit for my home's Linux server management
How to add public SSH keys for users in Cockpit
How to Manage Linux Servers with the Cockpit Web Interface including Monitoring Multiple Computers.

DNS

Port number is 53. An example is Pi-hole.

Email server

See Mail_server.

Backup

See Backup.

at command: Schedule a task

Schedule a task with the Linux at command
Schedule Jobs in Linux With 'at' Command
4 alternatives to cron in Linux
On Debian, I need to run sudo apt install at.
Using the at command to schedule tasks on Linux

$ echo "rsync -av /home/tux/ me@myserver:/home/tux/" | at 1:30 AM

$ echo "command_to_be_run" | at 09:00

$ atq  # list of jobs

$ atrm 6   # delete the 6th job

Cron job by root

Note that there is a "user" field for cron jobs defined in /etc/crontab or /etc/cron.d.

Place one: /etc/crontab and /etc/cron.daily, /etc/cron.hourly, /etc/cron.monthly, /etc/cron.weekly.
```
cat /etc/crontab
# How to List Daily Cron Jobs
ls -la /etc/cron.daily
```
Place two: /etc/cron.d. It contains "anacron" and "e2scrub_all" on my Debian 11. See How to run a cron job as a specific user?

Running a cron job as a user

Some examples

http://www.thegeekstuff.com/2009/06/15-practical-crontab-examples/
https://crontab.guru/examples.html and tips
Can I break a line cron command into multiple lines? No. Creating another script file in this case.
A specific time

MIN HOUR DOM MON DOW CMD
30 08 10 06 * $HOME/full-backup >> $HOME/myscript.log 2>&1; echo "Executed at $(date)\n----------" >> $HOME
/myscript.log
#   30 – 30th Minute
#   08 – 08 AM
#   10 – 10th Day
#   06 – 6th Month (June)
#   * – Every day of the week

Twice a day

00 11,16 * * * /home/ramesh/bin/incremental-backup 
#    00 – 0th Minute (Top of the hour)
#    11,16 – 11 AM and 4 PM
#    * – Every day
#    * – Every month
#    * – Every day of the week

Every 10 minutes

*/10 * * * * /home/ramesh/check-disk-space

Every odd day or even day

# Will only run on odd days:
0 0 1-31/2 * * command

# Will only run on even days:
0 0 2-30/2 * * command

crontab

How to Use Crontab to Automate Repetitive Tasks in Linux. Some examples: play sound, backup files, check if sites are online.
50 Amazing Linux Crontab Commands For The SysAdmins

crontab SOME-CRON-FILE; crontab -l

Make sure the .sh file gives a complete path. For example,

#!/bin/sh
R --vanilla  < arraytoolsip.R

does not work in cron job although it works perfect when we manually run it from the right path. The sh file should be

#!/bin/sh
R --vanilla  < $HOME/Dropbox/scripts/arraytoolsip.R

To disable everything on crontab -l, use one of the following methods:

run crontab -e then comment out each line you don't want to run with #. OR
run crontab -r to empty the current crontab. OR
run crontab with no arguments, and then type Ctrl+D. It will create an empty crontab, overwriting your previous crontab.

GUI

Schedule Commands And Scripts In Linux With Zeit (GUI For Cron And At)

PATH and Shell

Cron knows nothing about your shell; it is started by the system, so it has a minimal environment. If you want anything, you need to have that brought in yourself. For example, to use 'ifconfig' command, I need to give it a complete path in my script file.

$ cat syncIP 
/sbin/ifconfig eth0 | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}'

and the cron job

06 15 * * * /home/MYUSERNAME/Ubuntu\ One/syncIP > $HOME/Ubuntu\ One/ip.txt 2>&1

See here on how to add environment variable into cron environment.

Disable mail alert

If something went wrong with executing a cron job, cron will output a message "You have new mail in /var/mail/$USER". You can open this file using a text editor. To disable this alert, run 'crontab -e (see this post)

0 1 5 10 * /path/to/script.sh >/dev/null 2>&1
# OR
0 1 5 10 * /path/to/script.sh > /dev/null

Another way is to add MAILTO="" at the top of the crontab file.

Run a command at boot

How to Execute a Command or Script at Reboot or Startup
1. Run Script or Command at Reboot or Startup Using /etc/rc.d/rc.local File
2. Execute a Command or Script at Reboot or Startup Using the crontab
3. Run a Command or Script at Reboot or Startup Using the systemd service unit

Sending an email on boot

@reboot sleep 300 && python /home/pi/startup_mailer.py

pausing for 10 seconds before running the python script

@reboot /bin/sleep 10 ; /usr/bin/python /home/pi/PumpkinPi/src/pumpkinpi.py &

rc.local

How to enable rc.local shell script on systemd while booting Linux system

Running crontab as root

Use sudo crontab -e to edit. After saving it, no need to initialize it. Use sudo crontab -l to list the cron job.

Display and back up cron jobs

Linux List / Display and view all cron jobs

Check log

sudo grep CRON /var/log/syslog --color

Anacron

Anacron keeps track of the last time a task was run, and if it was missed, it runs it.

Anacron typically runs daily, while cron runs every minute.

cat /etc/anacrontab

GUI cron

md5sum

Linux md5sum Command Explained For Beginners (5 Examples)

How to verify files?

md5sum file1.txt file2.txt file3.txt > hashes
md5sum --check hashes

fsck

Fsck error on boot

fsck error on boot: /dev/sda6: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY

This happened when I resize an Ubuntu partition.

fsck -fy /dev/sda1

Force fsck on the Next Reboot or Boot Sequence

https://www.linuxuprising.com/2019/05/how-to-force-fsck-filesystem.html

https://www.cyberciti.biz/faq/linux-force-fsck-on-the-next-reboot-or-boot-sequence/ Create a blank file /forcefsck and fsck will check your drive next time your reboot.

sudo touch /forcefsck

The fsck was used to fix a journal checksum error on a USB drive which has been formatted as Ext4 was used on a security camera application; see motionEyeOS.

Can I run fsck or e2fsck when Linux file system is mounted?

https://www.cyberciti.biz/faq/can-i-run-fsck-or-e2fsck-when-linux-file-system-is-mounted/

What is the difference between fsck and e2fsck?

https://superuser.com/a/19984

Swap partition

Swap partition and swap file
```
# mkswap /dev/sdxy
# swapon /dev/sdxy
# nano /etc/fstab
```
UUID=device_UUID none swap defaults 0 0
How to Create a Swap File on Linux
How to Permanently Disable Swap in Linux

Swap file vs swap partition

Jetson Nano – Run on USB Drive
If "htop" or "free -h" shows swap but "Disks" does not show a swap partition, it is likely the system is using a swap file instead of a dedicated swap partition. Use sudo swapon --show to check for the presence of a swap file or check /etc/fstab to see the a line referencing a file rather than a partition.

Mount drive, add a new hard drive

/etc/fstab and blkid

See Fstab.

autofs, /etc/auto.master

Building a network attached storage device with a Raspberry Pi

Fix a malfunctioning USB device or port

5 Ways to Fix a Malfunctioning USB Device or Port on Linux

Check the physical health of a USB stick

Linux check the physical health of a USB stick

USB drive

Rename USB drive partition label. It seems if a device does not have a label, Ubuntu will use its 32-digit UUID as the mount point (eg. /media/$USER/$Long_UUID). This is very cumbersome. To fix that, we can open the Disks utility and select the partition of the device. Click the two-gear icon and pick Edit Filesystem... where we can change the filesystem label. After that, we can reject the USB and re-plug it to see the new mount directory based on the new label we specified (/media/$USER/$Label). We can also use the command lsblk (no sudo needed) to check.
If I use "GParted" utility to check the "partition name ", it is not the same as the name I just specified through the "Disks" utility. But the "Information" window give a complete data. It is a little confusing that the partition label becomes the filsystem label and the Partition name shown on GParted was different & seems not to be used.

How do I correctly mount a NTFS partition in /etc/fstab?

Run the following to confirm the USB device is detected.

sudo fdisk -l
# OR
dmesg | grep -i "SCSI"

Now suppose the usb device is found in dev/sdb1.

sudo mkdir /mnt/usb
sudo mount -t vfat -o rw,users /dev/sdb1 /mnt/usb

The above mount command assumes the usb drive has Windows vfat partition and users give non-root users the ability to unmount the drive. If the USB drive is partitioned linux ext2/3, we can merely run mount command as

sudo mount /dev/sdb1 /mnt/usb

At the end, run umount command like

sudo umount /mnt/usb

To make the mounting automatically, edit the file /etc/fstab.

/dev/sdb1       /mnt/usb           vfat    defaults        0       0 
/dev/sdb2       /mnt/usb2          ext3    defaults        0       0
UUID=XXXXXXXXXX /mnt/usb3       ntfs-3g    rw              0       0

and run

sudo mount -a

Mount an iso file

http://www.shellhacks.com/en/Mounting-an-ISO-Image-in-Linux

sudo mkdir -p /mnt/mount_point # create a mount point
sudo mount -o loop /home/user/disk.iso /mnt/mount_point
mount  # verify

Mount remote Windows share

http://www.shellhacks.com/en/HowTo-Mount-Remote-Windows-Partition-Share-under-Linux

Sharing files with Windows by using NitroShare

Simple way of Sharing files between Ubuntu 16.04 and Windows 10 by using open-source NitroShare which is based on Qt framework.

NTFS usb drive in xubuntu

http://xflinux.blogspot.com/2011/01/mount-ntfs-volumes-automatically-in.html

sudo apt-get install ntfs-config

Now go to Applications>> System>> Ntfs Configuration Tool

Expand the "Advanced Configuration" and select all those partitions you want to be auto mounted and writable( The tool will detect all partitions at its startup).

Make sure the " Enable write support for internal devices" option is selected. Now click Close.

Many drives, one folder

mhddfs program.

Partition tables

Partition Tables and the Dangers of Editing Them

parted command

How to partition a disk in Linux

Recommended partition schemes

HOME /home directory

How to Move Your Linux home Directory to Another Drive

/var directory filled up

How to move /var directory to another partition

blkid | grep sdc1  # get UUID 
mkdir /mnt/newvar
mount /dev/sdc1 /mnt/newvar
df -h /mnt/newvar
rsync -aqxP /var/* /mnt/newvar  # q=quiet,x=one-file-system, P=partial,progress
umount /mnt/newvar/  /mnt/var/
nano /etc/fstab
# UUID=XXXX   /var    ext4  defaults  0   2

Reboot

Why put things other than /home to a separate partition?

The /var partition is used by Docker and Apache.

/home, /boot and /var partitions can be separated.

Process/job

ps and top commands

How to Use the ps Command to Monitor Linux Processes, Linux how long a process has been running?

ps -C shutter # Listing only a Process by Command
              # Adding the 'watch' command to show the process in real-time
ps -C dd --format pid,cmd,%cpu # Show PID, CMD and %CPU

sudo ps -p {PID} -o pid,cmd,lstart,etimes,etime

ps -e | less # Listing Process for All Users

ps -eH --forest | less # hierarchy

ps -e | grep firefox # Listing Processes by Name

ps -p 3403 # Listing Processes by Process ID

ps -Flww -p THE_PID # Listing a process detail like "htop" gives

ps -u mary  # Listing Processes Owned by a User

sudo pkill top # Killing Processes by Name

sudo killall top # Killing Multiple Processes by Name

Kill a process and the pstree command

https://en.m.wikipedia.org/wiki/Pstree
On Ubuntu docker container, we can need to run apt install psmisc to get the pstree command.

Killing a process and all of its descendants. This covers a PPID, PID and more importantly PGID, SID. Also ps j -A command can show these IDs for the running processes.
```
$ tail -f /var/log/syslog | grep "CRON" &
$ ps j
$ kill -SIGTERM -- -($Some_PGID)
```

pgrep & kill

# find the PID
pgrep ProgramName
# Kill the ProgramName process
kill -9 PID

killall. For example, if Firefox is acting up (as Firefox will do from time to time) simply type killall firefox and it should kill the application completely. In the rare circumstances that this doesn’t work you can always type xkill and then click on the window that won’t close; this will completely close a given window immediately. See this. To kill a privileges process, use for example sudo killall crond.
```
sudo killall -u USERNAME
```

How To kill An Inactive OR Idle SSH Sessions. The pstree -p command can show a tree diagram of all the processes.
pkill command. For example, pkill gedit.

Kill a process running on a specific port

Kill a Process Running on a Specific Port in Linux (via 4 Methods)

How to Kill Zombie Processes on Linux

Simulate/produce high cpu load

How can I produce high CPU load on a Linux server?

# method 1:
sudo apt install stress
stress --cpu 3

# method 2:
for i in 1 2 3 ; do while : ; do : ; done & done
jobs   # list background jobs
for i in 1 2 3 4; do kill %$i; done  # kill "job" (not "PID") 1,2,3,4

ps, pgrep and pidof: How much resource is used by a process

Find the process ID first by ps -ef | grep APPLICATIONAME where "-e" is to show the running processes and "-f" is for a full listing. Then

ps -p <pid> -o %cpu,%mem,cmd

For example,

$ ps -ef | grep akregator
brb      15013  1942  1 10:41 ?        00:00:05 akregator --icon akregator -caption Akregator
brb      15186 24045  0 10:50 pts/11   00:00:00 grep --color=auto akregator
$ ps -p 15013 -o %cpu,%mem,cmd
%CPU %MEM CMD
 1.0  0.8 akregator --icon akregator -caption Akregator

pgrep

08:49AM ~$ ps -ef | grep firefox
brb       7798  7778  0 08:49 pts/2    00:00:00 grep --color=auto firefox
brb      25486 24869  0 Sep10 ?        00:42:48 /usr/lib/firefox/firefox
brb      25612 25486  0 Sep10 ?        00:19:49 /usr/lib/firefox/firefox .....
08:49AM ~$ pgrep firefox
25486

pidof

08:49AM ~$ pidof firefox
27951 25961 25612 25486
08:51AM ~$ pidof /usr/lib/firefox/firefox
27951 25961 25612 25486

$ kill $(pidof firefox)

How to keep resource usage low

Habits to Keep Linux Resource Usage Low

Avoid concurrency

Avoid These Problems By Limiting Bash Scripts to Run Once At A Time. pgrep, lsof and flock commands.

All You Need To Know About Processes in Linux

http://www.tecmint.com/linux-process-management/

wait command and background jobs

The wait command in Linux is a shell built-in command that pauses the execution of a shell script until all background jobs or specified JobID/PIDs terminate and return their exit status.

# Example 1: Wait for all background processes to finish
command1 &
command2 &
wait
echo "All background processes have finished."

# Example 2: Wait for a specific process to finish
command1 &
PID=$!
command2 &
wait $PID
echo "Command1 has finished."

run commands in a background and allow log off

nohup /path/to/script >output 2>&1 &

Or to disable output and be more safe. It also explains the concept of file descriptor/fd in Unix.

nohup command </dev/null >/dev/null 2>&1 &

job: How do I send an already-running process into the background

Steps:

'Ctrl+Z' to stop (pause) the program and get back to the shell. It sends SIGTSTP to a foreground application.
bg to run it in the background.
jobs -l to get the jobID and process ID
disown -h [job-spec] where [job-spec] is the job number (like %1 for the first running job; find about your number with the jobs command) so that the job isn't killed when the terminal closes.

Stopped job

A stopped job is one that has been temporarily put into the background and is no longer running, but is still using resources (i.e. system memory). Because that job is not attached to the current terminal, it cannot produce output and is not receiving input from the user.

jobs -s showing stopped jobs
jobs -l showing the job PID

Send kill to a stopped job, it will do nothing but queue than bring it in in foreground, it will terminate. So don't repeatly sending a 'kill' command.

fg %1 move the stopped job ID #1 to the foreground (works)
kill %1 # kill job ID #1
kill 12345 # kill job PID 12345
kill -9 `jobs -ps` may not work
kill -9 $(jobs -p)

nice

How to Set Process Priorities With nice and renice on Linux

watch command

Watching activity on Linux with watch and tail commands. Both the watch -n and tail -f commands can provide auto-updating views of information/

We can use the watch command to monitor a specific process such as the progress of the dd command.

Terminal 1

watch -n 10 who
watch ps -C dd --format pid,cmd,%cpu

Terminal 2

dd if=/dev/zero of=/dev/null

Use --color for color output.

while + sleep

How To Repeat A Command Every X Seconds On Linux: watch, while + sleep,

ulimit

Notepadqq - Notepad++-like editor

Notepadqq. It is written using Qt. It does not have printing function:(

Note apps that can sync

Top 8 Notepad Apps for Linux That You Can Sync. Some are compatible with Evernote.

Evernote

Evernote alternative

How to Install Turtl Server - Evernote Alternative - on Ubuntu 16.04

Backup/restore Evernote

https://www.makeuseof.com/tag/backup-restore-evernote/

Markdown

Preview markdown/view markdown offline

Atom text editor has a built-in function to preview HTML or markdown files. Menu -> Packages -> Markdown Preview -> Toggle Preview.
http://stackoverflow.com/questions/9843609/view-markdown-files-offline. Grip works fine.

sudo pip install grip
grip readme.md

For image, see http://stackoverflow.com/questions/13051428/how-to-display-images-in-markdown-files-of-github. The trick is adding ?raw=true after the image name.

# title 1
![screenshot](myfile.png?raw=true)

Chrome markdown preview plus extension does not show images from github.

Markdown editor

Text editor with navigation

nano/pico editor

The nano editor is also called pico in R. See ?edit in R.

nano editor

vi editor

Cloud

Boot

U-boot

http://www.denx.de/wiki/U-Boot

Pandora linux client

Pithos (GUI)
Pianobar (Command line)

COW (copy on write) file system

filesystem

tmpfs and /dev/shm

Apache redirection

http://cran.r-project.org/mirror-howto.html

Redirect a Website URL from One Server to Different Server in Apache

Important linux directories

Filesystem Hierarchy Standard, Wikipedia

/bin - executables used by the base system
/boot
/dev
/etc - configuration files
/media
/mnt
/opt - optional application packages
/proc - process information only. Access Kubernetes Objects Data From /Proc Directory
/sbin - critical executables for running the system, but should be used by superuser
/usr - user programs and data. For example /usr/bin contains most of the libraries used by apps. /usr/share/ contains Architecture-independent data (eg some pretty images are located in /usr/share/backgrounds)
/var - variable data such as databases, mails spools and system logs.

Difference of /bin, /sbin, /usr/local/bin, ...

/bin : For essential binaries; e.g. bash, cat, ls.
/sbin : is similar to /bin but for scripts with superuser (root) privileges required; e.g. shutdown command is located here. Local users have to use sudo to run binaries here.
/usr/bin : Same as first, but for general system-wide & non-essential binaries; e.g. grep, zip, docker, etc.
/usr/sbin : Same as above, but for scripts with superuser (root) privileges required.
/usr/local/bin or /usr/local/sbin for system-wide available (personal) scripts. For example, install docker-compose is merely to download the binary and place it under /usr/local/bin/ directory.

If you want to create your own scripts and make them available to all users, you’re pretty safe adding them to /usr/local/bin. Or to add my scripts to my local bin (~/bin) and then I create a symbolic link in /usr/local/bin to the commands I want to make public. As a result, I can manage all my scripts from the same directory but still make some of them publicly available since /usr/local/bin is added to $PATH. See this post.

DHCP

DHCP server IP

Linux find DHCP server IP address using CLI

Get a New IP Address

dhclient -r   #  release your IP Address

dhclient   #  get your DHCP to issue you a new IP Address based on how it’s been configured.

Open a file/URL using the default application from the command line

gnome-open (works on Mint)
kde-open (KDE users)
xdg-open (window-manager independent). XDG stands for X Desktop Group; see https://en.wikipedia.org/wiki/Freedesktop.org

Check a file's encoding

file -bi myfile

For example,

file -bi Downloads/hmv_.rc 
# text/x-c++; charset=utf-16le

Know you system using the command line

https://www.howtoforge.com/tutorial/get-to-know-your-system/

Alerting and visualization tools

5 alerting and visualization tools for sysadmins

System monitor tools (TUI)

glances command: more than htop

https://nicolargo.github.io/glances/ and its Documentation. It seems to be more stable to install via apt command instead of the bash script. There is no need to use 'sudo' to run the command.
Glances includes Disk I/O, Network I/O, internal and external IPs, current time, et al. Its official website at http://glances.readthedocs.io/en/latest/.
- Glances uses online services to grab the external IP address. https://github.com/nicolargo/glances/issues/961
- To hide the internal and external IPs, type "I" (capital).
- To disable the IP, type "--disable-ip" (not available in v3.x.x ?)
- It shows a warning or critical alert (e.g. memory high usage) at the bottom of the screen.
- My command: glances -t 5 # update every 5 seconds
https://www.tecmint.com/glances-an-advanced-real-time-system-monitoring-tool-for-linux/
Monitoring & Troubleshooting Basics with Glances (video)

Glances is similar to htop but it provides network stats and disk usage too. It also supports web UI. Install it by sudo apt-get install glances.

conky and autostart

Ubuntu > Conky (internal link)
Raspberry Pi case (internal link)

For auto start on Lubuntu, see How can I add new autostart programs in Lubuntu?

On Lubuntu 18.04, add the path to the application to ~/.config/lxsession/Lubuntu/autostart

top and htop command

15 simple TOP command examples on Linux to monitor processes
- M (capital): sorted by memory
- P: sorted by CPU
- T: sorted by running time
- c: full command path
- u: filter by user
- V: forest/tree mode; parent child hierarchy. This will disable 'M', 'P' or 'T'.
- f: show/hide columns
- top -d 5 change the refresh interval from 3 seconds to 5 seconds
Why are some processes highlighted in top?
Htop command has a screen help. htop command in Linux
- Tree view. Press 't'. Press '+' or '-' to expand or collapse.
- ">" to change to sort view.
Shortcut character keys and function keys
Understand Linux CPU Load. The maximum number should be the number of cores.
In Armbian, htop shows CPU temperature and frequencies; see a screenshot.

nmon

https://en.wikipedia.org/wiki/Nmon. The stats are saved to a comma-separated values (CSV) data file for later graphing and analysis
HomePage
Monitoring with Nmon

CPU frequency

How To Optimize Performance And Battery Life With Auto-cpufreq In Linux

What is the correct way to view your CPU speed on Linux? (x86)

watch -n.1 "cat /proc/cpuinfo | grep \"^[c]pu MHz\""

How can I get the current CPU frequency of an ARM processor on Ubuntu?

ls /sys/devices/system/cpu/cpu0/cpufreq/

scout_realtimep

This is used by Dataplicity

gtop command (100% Javascript)

https://www.cyberciti.biz/howto/gtop-awesome-system-monitoring-dashboard-for-terminal/.

Pros:

CPU history graph in time
Memory history graph in time (not useful)
Network bandwidth usage is real-time. It is accurate as what nload gives.
Percentage usage of memory, swap, disk usage
Top processes

$ docker run --rm -it \
    --name gtop \
    --net="host" \
    --pid="host" \
    aksakalli/gtop

$ sudo apt install npm nodejs
$ npm install gtop -g
$ gtop

Press p to sort by process ID (PID).
Press c to sort by CPU usage.
Press m to sort by memory usage.

It can be installed on Linux Mint 18.2 but not in Ubuntu 14.04 or raspbian (9 stretch).

$ npm install gtop -g
npm http GET https://registry.npmjs.org/gtop
npm http GET https://registry.npmjs.org/gtop

npm http GET https://registry.npmjs.org/gtop
npm ERR! Error: CERT_UNTRUSTED
npm ERR!     at SecurePair.<anonymous> (tls.js:1370:32)
npm ERR!     at SecurePair.EventEmitter.emit (events.js:92:17)
npm ERR!     at SecurePair.maybeInitFinished (tls.js:982:10)
npm ERR!     at CleartextStream.read [as _read] (tls.js:469:13)
npm ERR!     at CleartextStream.Readable.read (_stream_readable.js:320:10)
npm ERR!     at EncryptedStream.write [as _write] (tls.js:366:25)
npm ERR!     at doWrite (_stream_writable.js:223:10)
npm ERR!     at writeOrBuffer (_stream_writable.js:213:5)
npm ERR!     at EncryptedStream.Writable.write (_stream_writable.js:180:11)
npm ERR!     at write (_stream_readable.js:583:24)
npm ERR! If you need help, you may report this log at:
npm ERR!     <http://github.com/isaacs/npm/issues>
npm ERR! or email it to:
npm ERR!     <[email protected]>

npm ERR! System Linux 4.4.0-119-generic
npm ERR! command "/usr/bin/nodejs" "/usr/bin/npm" "install" "gtop" "-g"
npm ERR! node -v v0.10.25
npm ERR! npm -v 1.3.10

gotop

A terminal based graphical activity monitor inspired by gtop and vtop. It is quite beautiful.

Gotop – Yet Another TUI Graphical Activity Monitor, Written In Go

Compared to gtop, it has a temperature monitor. However, it can only show the average CPU usage (one line) on my Xeon computer.

git clone --depth 1 https://github.com/cjbassi/gotop /tmp/gotop
/tmp/gotop/scripts/download.sh
sudo cp gotop /usr/local/bin; rm gotop
gotop

Note the temperatures do not show up in Raspbian (raspberry pi 3 b+).

termui: Golang terminal dashboard

https://github.com/gizak/termui

Bashtop and btop

Bashtop Is A Cool Linux Resource Monitor Written In Bash
*btop 20k star. C++ version and continuation of bashtop and bpytop. This is very similar to bashtop but it also shows the IP. Mac, Linux x86, armv7, aarch64, arm64, armv5. Install btop++ on Raspberry Pi, Ubuntu 22.04.

bottom

https://github.com/ClementTsang/bottom 10k star

S-tui command

Monitor Linux CPU temperature, frequency, power in a graphical way

below

below: a time traveling resource monitor

System monitor tools (GUI)

Comparisons:

Some lists:

Install Ganglia on Ubuntu 16.04 Server (Xenial Xerus)
Linux Dash Web based monitoring tool. Source code is on github.
Monitorix and on Ubuntu16.04

Linux-Dash

https://github.com/afaqurk/linux-dash. Not working when I tested on RPi and Ubuntu.

Nagios

Zabbix

Munin and Monit

Server Monitoring with Munin and Monit on Ubuntu 16.04 LTS

Cacti

sysstat, sar

# CPU
sar 2 10 # every two seconds, 10 times
# Memory
sar -r  # look at the kbcommit and commit columns

sar -r -f /var/log/sysstat/sa02

https://www.maketecheasier.com/monitor-linux-performance-with-sysstat/
https://www.tecmint.com/install-sysstat-in-linux/
http://www.thegeekstuff.com/2011/03/sar-examples/
https://www.blackmoreops.com/2014/06/18/sysstat-sar-examples-usage/
Visualize sar data with kSar.
1. export LC_ALL=C. This will convert date/time. For example, 12:00:01 AM will become 00:00:01 and 12/09/2017 will become 12/09/17.
2. sar -A -f /var/log/sysstat/saXX > ~/Downloads/sardata.txt.
3. Click on Data -> Load from text file. Select ~/Downloads/sardata.txt file. Note that nothing will happen in the kSar GUI.
4. Click 'kSar' to show the tree.
kSar (depends on JDK) for graphics (instead of usinsg the sadf command).
1. Download and unzip it to ~/bin.
2. Execute bash ~/bin/kSar-5.0.6/run.sh.
3. On the GUI, click Data -> Run local command.. -> sar 2 10, for example. This will start to record the cpu usage 10 times with a 2 seconds interval.
4. You can view the real-time plot (shown on the right panel) by clicking kSar -> CPU -> CPU all (left panel).

Stacer

Linux System Optimizer and Monitoring

Prometheus

Curses and ncurses: TUI library

Position text on your screen in Linux with ncurses

Calcurse

Keep up with your calendar and to-do list with Calcurse

Bitbucket (free for 5 users)

Bitbucket 101

Unlimited private repos
Code reviews
JIRA integration
REST API
Custom domains

See this post to know how to fix the problem of unknown author. In short, when I uncheck "Use global user setting" from Repository-> Repository Settings -> Advanced does the commit author change as expected.

See here for a list of Android apps related to bitbucket.

Image

See Images.

GIMP

Reload/Refresh .profile file

https://askubuntu.com/a/59127

. ~/.profile

. is a bash builtin and a synonym for source, see man bash.

After changing the .profile file, you have to logout from your account and login, then it will be sourced once automatically.

History of commands

history command with date and time

http://www.cyberciti.biz/faq/unix-linux-bash-history-display-date-time/

Running the following code once and history will give date and time the next time you issue the history command.

echo 'export HISTTIMEFORMAT="%Y-%m-%d %T "' >> ~/.bashrc

Note that the original post asks to write the line to ~/.bash_profile but this is not working in the desktop environment.

Note that on zsh, the above method is not supported. We can use history -i instead. Type man zshoptions or man zshbuiltins for more information.

Bang bang - Run a command/Fetch parameters from previous history

^P: Move up through the command history list one command at a time.
^N: Move down through the command history list one command at a time.
!!: Run the previous command. For example, we can run sudo !! in order to run the previous command with sudo.
!n: Run command number n (useful)
!string: Run most recent command starting with characters in string (useful). For example, !ls
!?string: Run most recent command containing characters that match string
!*: Fetch parameters from last command (useful). For example, if we run "ls /var" first. Then when we run stat !*, it would run stat /var. Or we can run cd !* and it will cd to /var directory.
!_: Fetch the last parameter from last command. For example, if we run "ls /var/ /etc" first. Then when we run stat $_, it would run stat /etc. (zsh shell only)
Ctrl + r and type a keyword (most useful). Press ctrl + r to scroll the match. This is called reverse i search.

For example,

!-1
!4
!tail

Recall commands with reverse-i-search

Ctrl + r. See Bash bang commands: A must-know trick for the Linux command line

To continue with the search, just hit Ctrl + r keys again.

To run a forward search, hit Ctrl + s. How to cycle through reverse-i-search in BASH?

Increase history limit

http://unix.stackexchange.com/questions/17574/is-there-a-maximum-size-to-the-bash-history-file Unlimited Bash History

$ echo $HISTSIZE  # $HISTSIZE variable controls how much history is displayed 
500
$ export HISTSIZE=1000
$ echo “HISTSIZE=1000” >> ~/.bashrc

# $HISTFILESIZE variable controls how many commands are retained in your .bash_history file.
$ echo $HISTFILESIZE
2000
$ wc -l .bash_history
2000 .bash_history

Not to add to bash history

Add a space after the command.

It is useful if there is a password in the command.

For example,

$ ls ~/             # this won't be recorded in history
$  ls ~/Downloads/  # this will be recorded in history
$ history

Delete a single command from history

To stop adding history entries, you can place a space before the command, as long as you have ignorespace in your HISTCONTROL environment variable.

You can force Bash to exclude commands starting with empty space by placing this in your .bashrc file:

export HISTCONTROL=$HISTCONTROL:ignorespace

How to Clear Bash History on Linux

$ cat /dev/null > ~/.bash_history && history -c && exit

multiple terminals

Use ‘’’history -a’’’ to write the history to the file if we do not plan to close the terminal. See How to use the history command on Linux.

Listen to HiChannel internet radio

Use Radio Tray

I use it to listen m3u file (VLC also supports it too).

Web Analytics Reporting Tools

Painting software

Krita - professional painting program made by artists that want to see affordable art tools for everyone. Krita Is the Free GIMP Alternative You Should Be Using.

Pinta. It can be install by apt-get command. It works just line Window's paint. Ctr + v to paste an image and save to a file. To crop an image, click the selection tool on the most left hand side (it's a black color on v1.6 but a gray color on v1.7), then select a rectangle. Now click 'Image' > 'Crop to Selection' to finish. Pinta New Release After 5 Years. Here’s How to Get it!
```
sudo add-apt-repository ppa:pinta-maintainers/pinta-stable
sudo apt update
sudo apt install pinta
```

mtPaint. It is included in Odroid - xu4 - Lubuntu 14.04. To crop an image, just select an area and click Image > Crop. It can be used to take a screenshot from the desktop by using either the application or through the command line (mtpaint -s). It will then display the screenshot in the application if you use the command line.
MyPaint

Take a screenshot (and edit them)

See Take screenshots.

Cozy - audiobook player

ebook readers

7 Best eBook Readers for Linux: Calibre, FBReader, Okular, Lucidor, Bookworm, Easy Ebook Viewer and Buka.

Calibre - Read ebook in epub format

See Calibre

RSS reader

Some references:

5 Best Feed Reader Apps for Linux
14 Best RSS Feed Readers for Linux in 2018. It contains nice screenshots.

Some examples:

Fluent Reader. Open source. Linux, Windows, macOS.
Akregator. KDE based. This is preinstalled in CentOS-KDE under the Internet category. It is also called 'Feed Reader'.
QuiteRSS. It works on Linux, Windows and MacOS.
Liferea. GTK based. It is considered one of the best RSS feed readers on Ubuntu Linux. It can synchronize with several online feed managers such as InoReader among others.
FeedReader. Looks nice. Works with several online feed managers.
Newsbeuter: RSS feed in terminal
Newsboat: terminal. Newsboat: The Best Terminal-Based RSS Feed Reader for Linux. Not for general use since it assumes the articles are all text-based.
RSSOwl. Depends on Java. Cross platform.
Firefox and Thunderbird have built-in support for RSS.

Clear gibberish all over the screen

Just type “reset”. See BASH Fix Display and Console Garbage and Gibberish on a Linux / Unix / macOS. It is useful, for example, accidentally I run cat command over binary file.

Display/screen

Turn off/on your monitor via command line

xset dpms force off # Press any key to turn it on 
xset dpms force on
xset -q # check the status of the X server settings

If we want to turn off/on the screen via ssh, add

export DISPLAY=:0.0

first before calling the xset command, or use '-display' argument

xset -display :0.0 dpms force off 
xset -display :0.0 dpms force on

autoxrandr

Plug your laptop into different monitor setups. https://www.donarmstrong.com/posts/autorandr/

Move a window without clicking the titlebar

Hold down the Alt key and then click in the window anywhere, and move your mouse.

Add new screen/display resolutions

http://ubuntuforums.org/showthread.php?t=1112186 (tested on UDOObuntu 2 beta 2running Ubuntu 14.04 + SainSmart 9" LCD display. The commands (not persistent) are

xrandr | grep maximum
gtf 800 480 59.9  # give some output used in the following line
xrandr --newmode "800x480_59.90" 29.53 800 816 896 992 480 481 484 497 -HSync +Vsync
xrandr --addmode "DISP3 BG" 800x480_59.90
xrandr --output "DISP3 BG" --mode 800x480_59.90

I cannot find the file /etc/X11/xorg.conf in my UDOObuntu 2 beta 2. It seems this file does not exist anymore. See this post about how to re-create it.

Wayland

echo $XDG_SESSION_TYPE to check whether X11 or Wayland is in use
To change from wayland to X11:
- Method 1: log out, click the username, choose Xorg and type the password
- Method 2:

sudo nano /etc/gdm3/custom.conf

Change the line WaylandEnable=true to WaylandEnable=false And restart the system1.

Ubuntu 22.04 LTS with GNOME 42 and Wayland as the default
Linux on the GPD Pocket 2 (Ubuntu, Debian, and Fedora)
- “xrandr -o right” command can be used to rotate the screen
- But it does not work in Fedora because Fedora uses the Wayland display server rather than xserver.
How To Automate Repetitive Typing (Text Expansion) With AutoKey On Linux. AutoKey is an X11 application, it won't work properly if you use Wayland (e.g. Fedora uses Wayland by default; if you want to use Xorg you must logout, and select "GNOME on Xorg" from the cog icon next to the Sign In button).
How to Configure Xorg as Default GNOME Session in Fedora
How to know whether Wayland or X11 is being used

loginctl show-session $(awk '/tty/ {print $1}' <(loginctl)) -p Type | awk -F= '{print $2}'

export DISPLAY

What is the $DISPLAY environment variable? The value of the display environment variable is:
```
hostname:displaynumber.screennumber
```
If we want to run a GUI app on a remote computer (such as Raspberry Pi/Beaglebone Black) and show the GUI app on the remote computer's screen using ssh, we can issue the following command before running the app.
```
export DISPLAY=:0.0
```

See which groups you belong to, id & group commands

id <username>
groups 
groups <username>

Main Types of User Accounts on Linux

The 4 Main Types of User Accounts on Linux:

Superuser Account
Regular Accounts
System Accounts (Core OS functionality, eg root, bin, sys, daemon) awk -F: '$3 <= 100 {print $1, $3, $7}' /etc/passwd Note that the third field is the user ID (UID). System accounts generally have UIDs below 100 (though this threshold may vary slightly depending on your distribution).
Service Accounts (eg SQL, apache).

List all user groups

How to List All User Groups on Linux To see a list of groups

cat /etc/group
# OR
getent group

Note that "getenv group" Lists groups from all configured sources, including both local and networked sources, depending on your system’s NSS (Name Service Switch) configuration (in /etc/nsswitch.conf).

finger: show user information

finger USERNAME

groupadd, chgrp, usermod, ACL (access control lists)

sudo mkdir -p /var/www/reports/
sudo groupadd project 

sudo usermod -a -G project tecmint 
sudo chgrp -R project /var/www/reports/
sudo chmod -R 2775 /var/www/reports/

create more system users and add them to the directory group as follows:

sudo useradd -m -c "Aaron" -s/bin/bash -G project aaron
sudo useradd -m -c "John" -s/bin/bash -G project john
sudo useradd -m -c "Ravi" -s/bin/bash -G project ravi

sudo mkdir -p /var/www/reports/aaron_reports
sudo mkdir -p /var/www/reports/john_reports
sudo mkdir -p /var/www/reports/ravi_reports

See who's in a group

grep '^group_name_here:' /etc/group

finger USERNAME # See more detail about a user

Add a standard user to sudo group

This is useful on Debian distribution where a new user does not have the sudo power. First log in as root,

# usermod -aG sudo username

Shared library management

http://www.cyberciti.biz/tips/linux-shared-library-management.html

ldconfig : Updates the necessary links for the run time link bindings.
ldd : Tells what libraries a given program needs to run.
ltrace : A library call tracer.
ld.so/ld-linux.so: Dynamic linker/loader.

Log files

$ ls -lt /var/log

ssh log files: /var/log/auth.log

/var/log/syslog: it is useful to use tail -f /var/log/syslog to show the log in real time
/var/log/auth.log: it includes ssh log in information and lots of CRON sessions opened and closed every minutes.

And

lnav (The Logfile Navigator)

How to Monitor Log Files in Real Time in Linux (Desktop and Server) . sudo apt install lnav; sudo lnav

Apache log

/var/log/apache2/error.log (small 83K). Useful to troubleshoot errors/crashes of Apache.

grep "May 08" /var/log/apache2/error.log

/var/log/apache2/access.log (large 10M)

mail

/var/log/maillog

Logrotate

How to Setup Logrotate on Linux (to Keep Your Server from Running Out of Space)

uprecords command

uptime command

uptime
watch -n 60 uptime

Windows

Find out from the logs what caused system shutdown?

How to find out from the logs what caused system shutdown?

Get notified when a system is rebooted

Get Notified When Your Raspberry Pi is Booted with Pushbullet. This makes use of /etc/rc.local file. If it does not work, we can use cron to run a command at startup; see sending an email on boot.

timeout command

Linux command similar to top to show hard disk activity

Use iotop. On ubuntu, we can use sudo apt-get install to install it. Use sudo iotop to launch it. Use -o to show processes that are actually doing IO.

sudo apt-get install iotop

sudo iotop -o -u $USER

Another program is iostat and the -d (disk) option. The -x option will display extension I/O status.

sudo apt-get install sysstat
iostat -dx 5 # every 5 seconds

24 iostat, vmstat and mpstat Examples for Linux Performance Monitoring

curl and wget

See Curl.

Torrent

Torrench

Torrench: How To Search And Download Torrent Files Using Terminal (Linux, Mac, Windows)

aria2 - command line downloader supports torrents and multi-connection

aria2 command examples

The -x argument helps a little bit. Download a file 112MB; see https://www.archlinux.org/download/

$ time aria2c  http://mirror.jmu.edu/pub/archlinux/iso/2016.11.01/archlinux-bootstrap-2016.11.01-i686.tar.gz # 16 seconds
$ time aria2c -x10 http://mirror.jmu.edu/pub/archlinux/iso/2016.11.01/archlinux-bootstrap-2016.11.01-i686.tar.gz # 11 seconds

Axel

It can create an unlimited number of worker threads to download any kind of data. See https://www.beginnersheap.com/top-5-command-line-download-accelerators-linux/

lftp

It supports FXP (site-to-site transfers) and dropping to background
How to use lftp to accelerate ftp/https download speed on Linux/UNIX. It can launch several commands in parallel in the background.

Apply a patch to source code

For example Tophat 2.0.12 compatibility with Samtools 1.0,

$ ls
support_for_tophat_1.patch  tophat-2.0.12  tophat-2.0.12.tar.gz

$ grep -r -i "check_samtools" tophat-2.0.12/
tophat-2.0.12/src/tophat.py:def check_samtools():
tophat-2.0.12/src/tophat.py:        check_samtools()

$ cp support_for_tophat_1.patch tophat-2.0.12/src/
$ cd tophat-2.0.12/src/
$ patch tophat.py < support_for_tophat_1.patch 
patching file tophat.py
Hunk #1 succeeded at 1540 (offset 3 lines).
Hunk #2 succeeded at 1563 (offset 3 lines).

IP address fundamental

http://www.howtogeek.com/133943/geek-school-learning-windows-7-ip-addressing-fundamentals/.

There are three classes for private IP ranges.

1-126.0.0.0 from Class A
128-191.0.0.0 from Class B
192-223.0.0.0 from Class C

Subnet

Gateway IP address

How To Find Default Gateway IP Address In Linux And Unix From Commandline

Get internal IP address

$ hostname -I

$ ifconfig  # also works on Android through Termux

https://www.raspberrypi.org/documentation/remote-access/ip-address.md

Private/internal/local IP/network

https://en.wikipedia.org/wiki/IPv4#Private_networks

10.0.0.0 – 10.255.255.255
172.16.0.0 – 172.31.255.255
192.168.0.0 – 192.168.255.255

There are other special-use addresses for private networks

100.64.0.0 to 100.127.255.255 (similar to private IP addresses like 192.168.x.x or 10.x.x.x)

Get external IP address

It seems there is no way to get the external IP address without not using external services.

host myip.opendns.com resolver1.opendns.com
# Look for the bottom line output  myip.opendns.com has address XXX.XX.XX.XXX

# https://unix.stackexchange.com/a/81699
sudo apt-get install dnsutils
dig +short myip.opendns.com @resolver1.opendns.com
# OR
curl ifconfig.me   # by google
                # also works on Android through Termux
# 
curl http://ipecho.net/plain; echo
# OR
curl ipv4.ipogre.com

To store my IP in a shell variable

myip="$(dig +short myip.opendns.com @resolver1.opendns.com)"
echo "My WAN/Public IP address: ${myip}"

The above only gives the IP. The following method gives geo information too.

curl ipinfo.io # ifconfig.me, icanhazip.com, ipecho.net/plain, ifconfig.co
# OR give a specific IP (domain name does not work)
curl ipinfo.io/216.58.194.46

{
  "ip": "216.58.194.46",
  "hostname": "dfw25s12-in-f14.1e100.net",
  "city": "Mountain View",
  "region": "California",
  "country": "US",
  "loc": "37.4192,-122.0574",
  "org": "AS15169 Google Inc.",
  "postal": "94043"
}

IP geolocation

Test url: ubuntu.mirrors.pair.com

https://iplocation.net
command line - input is an IP address

curl https://ipapi.co/8.8.8.8/json/

curl https://ipinfo.io/8.8.8.8

command line - input is a web address

IP_ADDRESS=$(dig +short example.com)
curl https://ipapi.co/$IP_ADDRESS/json/

https://www.ipligence.com/geolocation
http://www.ipfingerprints.com/
http://ip-api.com/ (it shows your IP, internal IP, OS, browser/user-agent, DNS server from outside?). The final query URL is simple; for example http://ip-api.com/#ubuntu.mirrors.pair.com
ipstack. How to Use the IPStack API for IP Geolocation Lookups. 10,000 searches per month for free.
The Free IP Geolocation API Is a Valuable Webmaster Tool

Domain, WHOIS

WHOIS LOOKUP. For example, consider "r-pkg.org" domain,
- The 'Sponsoring Registrar' shows who is the sponsoring registrar (eg GoDaddy.com).
- The 'Registrant Name' shows who registered this domain.
- Command Line Interface.
```
sudo apt-get install whois
whois r-pkg.org 
```

The 9 Best Websites to Look Up WHOIS Information for Free
IP-Lookup. For example, consider "r-pkg.org" domain.
- The linux command line tool 'ping' shows the ip address.
- When we use the ip address to search in the IP-Lookup, the IP owner info > 'Organization' or 'OrgName' field shows the owner of this IP (eg Digital Ocean, Inc).
- The Domain owner info there gives the same (or less) information as WHOIS LOOKUP.
WhoIsHostingThis or webhostinghero (the returned result will be like Amazon, GoDaddy, CloudFlare, Github, Verizon, etc).

Subnet

IPv4

Class A: 255.0.0.0 or /8;
Class B, 255.255.0.0 or /16;
Class C, 255.255.255.0 or /24.

For example, in the subnet 192.168.5.0/255.255.255.0 (192.168.5.0/24) the identifier 192.168.5.0 commonly is used to refer to the entire subnet.

In the /16 subnet 192.168.0.0/255.255.0.0, which is equivalent to the address range 192.168.0.0–192.168.255.255

IPv6

ping command

ping uses the ICMP Echo Message to force a remote host to echo a packet back to the local host. If packets can travel to and from a remote host, it indicates that the two hosts can successfully communicate.

Check if a server is up/down if pining to that machine is not allowed

Hint: use nmap to scan open ports

nmap -PS22 -p22 <hostname> # port 22 only
# OR
nc -z -v <hostname> 22

Shell script to check whether a server is reachable?. I found if a server is up and I am trying a closed port, it will take about 2 minutes before it returns.

if nc -z $server 22 2>/dev/null; then
    echo "$server ✓"
else
    echo "$server ✗"
fi

6 Methods to Quickly Check if a Website is up or down from the Linux Terminal

Check if Sites are Online using a PHP script

How to Use Crontab to Automate Repetitive Tasks in Linux

Build a home network

Virtual router using vSphere. (Good ! It includes an introduction to vSphere installation)
http://rbgeek.wordpress.com/2012/05/14/ubuntu-as-a-firewallgateway-router/

IP Subnet Calculator

https://www.dan.me.uk/ipsubnets?ip=10.0.0.0

CIDR block IP range (network - broadcast) Subnet Mask IP Quantity
10.0.0.0/24 10.0.0.0 - 10.0.0.255 255.255.255.0 256
10.0.0.0/16 10.0.0.0 - 10.0.255.255 255.255.0.0 65536=256^2
10.0.0.0/8 10.0.0.0 - 10.255.255.255 255.0.0.0 16777216=256^3

How Use Your Router and ISP’s Modem/Router Combo In Tandem

http://www.howtogeek.com/255206/how-use-your-router-and-isps-modemrouter-combo-in-tandem/

Troubleshoot and repair network problems

http://www.linuxuser.co.uk/features/troubleshot-repair-linux-networks

Computer 1 can ping Computer 2 but not reverse

Use the route command to solve. See also this post.

Simple Network Management Protocol (snmp)

https://en.wikipedia.org/wiki/Simple_Network_Management_Protocol
Default port number is 161
What Is SNMP? How To Install & Configure SNMP in Linux
Install and configure SNMP on Ubuntu
Change port number on Dell iDrac 8
DDoS attack

Monitor network by Cacti (GUI)

Monitor network by command line

3 Simple, Excellent Linux Network Monitors: iftop, nethogs and vnstat.

bandwidth

bandwhich Shows What`s Taking Up Your Network Bandwidth On Linux And macOS

iftop

Use the interface top iftop command. On ubuntu, we need to use sudo apt-get install iftop and then run it by sudo iftop -i eth0. After that, we can press some keys to toggle options.

p: port
s: source
d: destination

See thegeekstuff.

It is strange that the output shows other devices names in my network.

$ dig A pandora.com
$ ipcalc -b 208.85.40.20
$ sudo iftop -F 208.85.40.20/24 -i wlan0

nethogs

$ sudo nethogs wlan0

Why does the snapd service use so much data?

nload

nload -m

nload – Monitor Linux Network Bandwidth Usage in Real Time

The result is the same as gtop (gtop is cooler) gives.

bmon

https://www.tecmint.com/bmon-network-bandwidth-monitoring-debugging-linux/

vnstat for network traffic monitor

# 1. Install vnStat
sudo apt-get install vnstat

# 2. Pick a Interface to Monitor using vnStat
vnstat -u -i eth0
vnstat --iflist
vnstatd -d # start the daemon
ps -ef | grep vnst

# 3. vnStat Basic Usage
vnstat

# 4. vnStat hours, days, months, weeks Network Data
vnstat -d
vnstat -m

# 5. Export the data to Excel or other DB
vnstat --dumpdb

# 6. Display Live Network Statistics
vnstat -l

# 7. Change the default vnstat output format
vnstat -s (--short)
vnstat --style 0

# 8. Display Top 10 Traffic Days
vnstat --top10

Network related linux commands

package includes net-tools programs arp, hostname, ifconfig, ipmaddr, iptunnel, mii-tool, nameif, netstat, plipconfig, rarp, route and slattach. Many of them are obsoleted by the ip program.
The 7 Best Networking Tools That Replace the Deprecated net-tools on Linux 2022

nmcli and nmtui (useful for getting DNS IPs)

Connect to WiFi in Linux Using NMCLI command

nmcli radio wifi on
nmcli dev status

nmcli dev wifi list # Show signal/bar/rate
sudo nmcli dev wifi rescan

sudo nmcli --ask dev wifi connect MySSID

How to Connect Wi-Fi from Linux Terminal Using Nmcli Command

# Find the up network adaptors
$ nmcli dev status
DEVICE           TYPE      STATE        CONNECTION         
docker0          bridge    connected    docker0            
eno1             ethernet  connected    Wired connection 1 

# Find the DNS 
$ nmcli dev show | grep 'IP4.DNS'

iwconfig

8 Linux Commands: To Find Out Wireless Network Speed, Signal Strength And Other Information

Getting information (Link Quality, Signal level) about the wireless.

sudo iwconfig

See link quality continuously on screen

watch -n 1 cat /proc/net/wireless

(GUI) NetworkManager

ps -ef | grep NetworkManager

ifconfig - spoof the hardware address at the software level

To change the MAC address temporarily on a NIC (network interface controller),

sudo ifconfig eth0 down
sudo ifconfig eth0 hw ether 00:11:22:33:44:55
sudo ifconfig eth0 up

And it seems there is no need to modify /etc/network/interfaces.

For wlan

sudo ifconfig wlan0 down
sudo ifconfig wlan0 hw ether 00:11:22:33:44:55
sudo ifconfig wlan0 up

See

ip command

It is said ip is replacing the old ifconfig command on modern Linux distributions.

Linux ip Command Examples

http://www.makeuseof.com/tag/networking-commands-linux-terminal/

ip a
ip addr
ip address show

ip link set DEVICE down # eg ip link set eth0 down

ip link set DEVICE up

iptables

See the ufw command which provides an easy way to configure iptables.
How to disable iptables firewall temporarily

route

7 Linux Route Command Examples

Display Existing Routes (route -n)
Adding a Default Gateway (route add default gw 192.168.1.1)
List Kernel’s Routing Cache Information (route -Cn)
Reject Routing to a Particular Host or Network (route add -host 192.168.1.51 reject)
Make 192.168.3.* Accessible from 192.168.1.* (route add -net 192.168.3.0 netmask 255.255.255.0 gw 192.168.3.10)
Make 192.168.1.* Accessible from 192.168.3.* (route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.10)
Allow Internet Access/External World (route add default gw 125.250.60.59)

On Ubuntu 16.04, it shows

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         FIOS_Quantum_Ga 0.0.0.0         UG    600    0        0 wlp3s0
link-local      *               255.255.0.0     U     1000   0        0 wlp3s0
192.168.1.0     *               255.255.255.0   U     600    0        0 wlp3s0
$ route -n   # showing numerical IP address instead of host name.
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    600    0        0 wlp3s0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 wlp3s0
192.168.1.0     0.0.0.0         255.255.255.0   U     600    0        0 wlp3s0

Flag value 'U' means up and 'G' means gateway'.

Connect two networks

The trick is explained in this post or the above route command.

For example, my network structure is

Modem/router: LAN IP 192.168.1.*/24
PC1: connect to Modem/router
Second router (ASUS) connect to Modem/router: its WAN IP is 192.168.1.ASUS. It's LAN IP 192.168.2.*/24
PC2 (raspberry pi): connect to the second router (ASUS): its IP is 192.168.1.212

By default, PC2 can ssh to PC1 but PC1 cannot access PC2.

The following command will solve the problem that PC1 cannot access PC2:

# From PC1
$ sudo route add -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.1.ASUS
$ ssh [email protected]    # 192.168.2.212 is the IP address for the Raspberry Pi

$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0          0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0
192.168.2.0     192.168.1.ASUS   255.255.255.0   UG        0 0          0 eth0

One article from linux.com using the ip command.

traceroute

sudo apt-get install traceroute
traceroute 8.8.8.8

On Windows, we can use the tracert command. For example, tracert www.microsoft.com.

nslookup (convert between hostname and ip) and host

$ host google.com
google.com has address 172.217.5.238
google.com has IPv6 address 2607:f8b0:4004:802::200e
google.com mail is handled by 30 alt2.aspmx.l.google.com.
google.com mail is handled by 10 aspmx.l.google.com.
google.com mail is handled by 40 alt3.aspmx.l.google.com.
google.com mail is handled by 50 alt4.aspmx.l.google.com.
google.com mail is handled by 20 alt1.aspmx.l.google.com.

$ nslookup google.com
Server:         127.0.1.1
Address:        127.0.1.1#53

Non-authoritative answer:
Name:   google.com
Address: 172.217.7.238

mac$ nslookup cran.r-project.org
Server:		156.40.70.10
Address:	156.40.70.10#53

Non-authoritative answer:
cran.r-project.org	canonical name = cran.wu-wien.ac.at.
Name:	cran.wu-wien.ac.at
Address: 137.208.57.37

mac$ nslookup 137.208.57.37
Server:		156.40.70.10
Address:	156.40.70.10#53

Non-authoritative answer:
37.57.208.137.in-addr.arpa	name = cran.wu-wien.ac.at.

The first two lines show the IP address of my DNS. If we run the command inside a Docker container where DNS was specified manually, the IP address we specified will be shown here.

dig

Dig provided by Google G Suite Toolbox. If the office internet is interrupted, we can use a cell phone to search for the IP address of a website for trouble shooting.

$ sudo apt-get install dnsutils
$ dig world.std.com
; <<>> DiG 9.9.5-3ubuntu0.16-Ubuntu <<>> google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49227
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;google.com.                    IN      A

;; ANSWER SECTION:
google.com.             130     IN      A       172.217.5.238

;; Query time: 11 msec
;; SERVER: 127.0.1.1#53(127.0.1.1)
;; WHEN: Fri Dec 01 17:32:37 EST 2017
;; MSG SIZE  rcvd: 55

arp (Address Resolution Protocol)

The arp command can be used to show the MAC addresss of all hosts in LAN

arp -a

Check connectivity

Using bash tcp built-ins to test connectivity when nothing else is available:

echo >/dev/tcp/google.com/80; 
if [ $? -eq 0 ]; then echo "Connection Successful"; fi

Find open ports

command	example	comment
ss	sudo ss -tulpn \| grep LISTEN	contain pid, add "sudo" can show program/command names on the last column, *Even DietPi includes the ss command*
lsof	sudo lsof -i -P -n \| egrep "PID\|LISTEN"	sudo is required. Contain pid. Using egrep to include header in output. It includes the command/program names on the first column. lsof may not be available by default in many OS.
nmap (network mapper)	nmap localhost (OR IP)	shortest output and input. Need to install. No 'sudo' needed. Works on non local.
netstat (old 'net-tools' )	netstat -tlpn \| grep LISTEN \| awk '{print $4 "\t" $7}'	show pid and program name. Need to install netstat. Windows already has it.
nc	nc -zvw3 <hostname> <port>	need to specify a port number. More accurate than nmap. "-w3" means time out after 3 seconds.

What are network interface ports

Understanding Network Interface Ports in Linux. In networking, the term "port" can refer to different concepts depending on the context:

Hardware Ports:
Software Ports: These are virtual endpoints in an operating system used by network protocols to manage connections. Software ports are identified by a number, ranging from 0 to 65535. For example, port 80 is used for HTTP, and port 443 is used for HTTPS.

Socket

What Are Unix Sockets and How Do They Work

nmap/network mapper - port scanning & IPs in local network

nmap - Network exploration tool and security / port scanner

nmap does not show all open ports By default, Nmap scans the most common 1,000 ports for each protocol.
https://nmap.org/book/nmap-os-db.html. Local OS database is located at /usr/share/nmap/nmap-os-db. The 2nd line will show the revision number.
- Modifying the nmap-os-db Database Yourself
- Download the latest from https://svn.nmap.org/nmap/nmap-os-db. Note that the current revision number has to be found from the website. You can edit the file and insert the revision number on the 2nd line of your local copy.
- Even I update the database, it cannot detect my Ubuntu 14.04 OS (it only shows OS details: Linux 3.8 - 4.9). For the Raspberry Pi, it can show information from the network adapter; e.g. MAC Address: AA:BB:CC:DD:EE:FF (Raspberry Pi Foundation) but not the OS name (OS details: Linux 3.2 - 4.8).
```
sudo mv /usr/share/nmap/nmap-os-db /usr/share/nmap/nmap-os-db-old

cd /usr/share/nmap
sudo wget https://svn.nmap.org/nmap/nmap-os-db
```
http://www.cyberciti.biz/networking/nmap-command-examples-tutorials/
http://bencane.com/2013/02/25/10-nmap-commands-every-sysadmin-should-know/

http://www.tecmint.com/nmap-command-examples/

sudo apt-get install nmap

nmap 192.168.1.100   # does not require root privileges
                     # used to check open ports

nmap 192.168.1.*     # show IPs and ports in LAN

sudo nmap -sP 192.168.1.1/24 # show connected IPs (no hostnames?) and MAC addresses
                             # If you don't use 'sudo' only partial devices can be found
                             # The output may contains the hostname. For example,
                             # Nmap scan report for brb-P45T-A.fios-router.home (192.168.1.xxx)
nmap -sV 192.168.1.1 # show Daemon name (in VERSION column) together with port number

nmap -T4 -F 192.168.1.99-255 # show connected IPs and open ports
                             # -F means fast
nmap -F taichimd.us  # Note that domain name != server
nmap -v taichimd.us

nmap -A 192.168.1.1  # Aggressive scan (more output)

nmap -p http,ssh,mysql taichimd.us  # scan ports/services
                                    # note that mysql will be shown as closed
                                    # ssh port is not correct. 
                                    # Not sure how to get the correct ssh port using nmap
nmap --open taichimd.us   # scan open ports

sudo nmap -traceroute nih.gov

sudo nmap -sS -O 192.168.1.99 # -O shows operating system
                              # eth0 MAC

$ nmap localhost # showing the true ports from the server

Starting Nmap 7.01 ( https://nmap.org ) at 2017-10-09 15:01 EDT
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00016s latency).
Not shown: 996 closed ports
PORT    STATE SERVICE
22/tcp  open  ssh
25/tcp  open  smtp
80/tcp  open  http
631/tcp open  ipp

$ nmap localhost -p 3838-4000  # Shiny

A gui version of nmap is called Zenmap. How to install Zenmap Nmap GUI on Ubuntu 20.04 LTS

netstat: get a list of all open ports

How to use netstat in GNU/Linux

-l or --listening shows only the sockets currently listening for incoming connection.
-a or --all shows all sockets currently in use.
-e --show extended/additional information
-t or --tcp shows the tcp sockets.
-u or --udp shows the udp sockets.
-n or --numeric shows the hosts and ports as numbers, instead of resolving in dns and looking in /etc/services.
-s --Print network stats
-r --Print the network routing information
-p --Print PID and name of the program to which each socket belongs

netstat -l             # only listening ports
netstat -rn            # displays the system's routing table
netstat -at
netstat -ant           # For tcp
sudo netstat -pant     # show ports and programs (pant = 喘氣). Best of the best!!!
sudo netstat -peanut   # (output is too wide)
netstat -anp | grep 3306 | wc -l  # print # of connections for the port mysql port

Other commands

sudo lsof -i -P -n | grep LISTEN
sudo ss -tulpn
sudo netstat -tulpn | grep LISTEN
sudo netstat -tulpn   # include dhcpcd & avahi-daemon

ss command

ss is one of 4 commands to check open ports for Linux. The 4 commands are netstat, nmap, ss and lsof.

How to Use the ss Command on Linux

lsof command

How to Use lsof in Linux (With a Practical Example. How to discover what files are currently open and in use on your system?

sudo apt install lsof  # dietpi 

# Basic lsof Output
sudo su
lsof | head -n10

lsof -i   # this gives a more list than nmap command

Common ports

List of TCP and UDP port numbers
- Well-known ports: 0 to 1023
- Registered ports: 1024 to 49151
- Dynamic, private or ephemeral ports: 49152–65535
A List of Common Ports
Replace the default port (such as 22 for ssh) with anyone from 1024-65535 because ports numbers up to 1023 are “well known” ports & should be avoided
Some services:
- 81: Nginx Proxy Manager
- 1194: OpenVPN
- 3306: MYSQL
- 3838: Shiny
- 8000: portainer
- 9000: portainer (URL)
- 9090: cockpit
- 17500, 17600, 17603: Dropbox

Copy text to a clipboard to be used in other apps

Install the xclip program. See here or here.

sudo apt-get install xclip
# Examples
sort -n -k 3, -k 2 file.txt | xclip -selection clipboard

cat ~/.ssh/id_rsa.pub | xclip -sel clip

Works.

Start Emacs without X

Add -nw (no window) option.

emacs -nw

Audio

Record audio from mic

How to Save Important Voicemails for Both iPhone and Android. Hint: use Audacity.

mp3 codecs

https://help.ubuntu.com/community/RestrictedFormats

sudo apt-get install ubuntu-restricted-extras

Concatenate mp3 files

sudo apt-get install mp3wrap
mp3wrap output.mp3 *.mp3

Reduce the size of an mp3 file

Specify a new lower bitrate using the -b option in lame. For example if your starting mp3 has a quality of 256kbs you can lower its bitrate to 128kbps (or even lower like 64kbps) by:

lame --mp3input -b 128 input.mp3 output.mp3

Convert ogg to mp3

ffmpeg is not included in Ubuntu repository. Use the avconv command. http://superuser.com/questions/15327/how-to-convert-ogg-to-mp3

sudo apt-get install libav-tools
avconv -i input.ogg -c:a libmp3lame -q:a 2 output.mp3

Convert m4a/webm to mp3

avconv -i input.m4a output.mp3

Remove the vocals from any song using Audacity

https://www.makeuseof.com/tag/remove-vocals-song-audacity/

How to Remove Ambient Noise From Audio Files Using Audacity

Normalize the volume of an audio file

Can You Losslessly Increase the Volume of MP3 Files?
Use Audacity. To raise (Amplify) volume:
1. Edit > Select All.
2. Effect > Amplify. Increase db. Adjust the sound until the highest peaks and lowest valleys of the waveform reach the top and bottom of the window.
3. Check clip3. Export > MP3 or just start to listen.
Command line tool: avconv (replace ffmpeg program). See this post.

avconv -ss 00:00:10 -i OLD.mp3 -vol 2560 NEW.mp3

The anconv/ffmpeg -vol parameter amplifies the sound. The default value is 256 (no amplification), and you can adjust the number accordingly. Here it’s 2560, as it’s 10 times louder. Note that these are not decibel values or anything that sophisticated, but just an integer value. 512 equals to twice the volume, 768 three times, 1024 four times, etc. The -ss parameter specifies the start time offset. Here it will skip the first 10 seconds.

Command line tool: sox.

I found the converted file by sox has about one half file size compared to anconv/ffmpeg program (source file=47MB, anconv converted=135MB, sox converted file=54MB).

sudo apt-get install sox libsox-fmt-all
sox --norm OLD.mp3 NEW.mp3 trim 10
sox --norm OLD.mp3 NEW.mp3 silence 1 0.1 1%
sox -v 4.0 OLD.mp3 NEW.mp3             # increase volume

where '--norm' will normalize the audio and the 'trim' option set to skip the first 10 seconds. The silence parameter allows to trim silence at the beginning without a need to specify the number of seconds.

cut, delete or trim an audio

Open the audio file in audacity.
select a region in the waveform area. Do not select in the time interval area (above the waveform).
1. To precisely select a range from one position to the end. Click Zoom in several times. Click one position in the waveform and click Edit -> Select -> Cursor to the track end to select
2. Similarly, if we want to precisely select a range from the start to some position, we can click one position in the waveform and then click Edit -> Select -> Track start to cursor.
3. To move around the track, use the scrollbar (below the waveform and above the bottom toolbar, not quite clear in Ubuntu/Unity)
Click Edit -> Remove Audio or labels -> Cut/Delete/Trim Audio
play the new audio by clicking the green triangle.
File -> Export -> mp3 format.

Helpful resource for Audacity.

Different Toolbars
Tutorial - Editing an Existing Audio File

Fade out at the end of an audio

Select a region.
Effect -> Fade out

Wireshark

sudo apt-get install wireshark
sudo chmod 4711 `which dumpcap`

Wireshark & Rtmpdump - Sniffing RTMP Streams - Ubuntu 10.10

Track the Time a Command Takes

How To Find The Execution Time Of A Command Or Process In Linux

time command

time COMMAND
time (COMMAND1; COMMAND2)
time (COMMAND1 && COMMAND2)

help time

When I run a set of 7 jobs using parallel, time command gives an output

real  15m53.788s # the wall clock time the command took from execution till termination
user  95m20.238s # the time taken by the user space
sys   9m1.320s   # the time taken by kernel space

Here we see the real time is about 16m and the user time is about 6-7 times the real time. Indicating the parallel executing works.

/usr/bin/time command

/usr/bin/time provides more information then time command.

man time

Magazines

Latex

Editors

10 Best LaTeX Editors For Linux.
Texmaker. R's installr package has a function to install Texmaker. Cross platforms.
TEXworks. Cross platforms.
TexStudio. See a dark theme example.

Online editing

Latex Base. You can start to try it without registration. Free accounts cannot publish but still can download.
Overleaf. Free account for 1GB space.
ShareLatex

Missing cls

$ apt-cache search IEEEtran
texlive-publishers - TeX Live: Publisher styles, theses, etc.

sudo apt-get install texlive-publishers

Missing sty

$ apt-cache search pseudocode
gpt - G-Portugol is a portuguese structured programming language
libgportugol-dev - Development files for the G-Portugol library
libgportugol0 - G-Portugol library
texlive-science - TeX Live: Natural and computer sciences
$ sudo apt-get install texlive-science

PDF

See PDF.

Flow chart

LibreOffice Draw OR MS_PowerPoint (insert > shape). Check youtube.
yEd
Dia & wikipedia
(online) www.draw.io

Clock

xclock (analog)

oclock -geometry 500x500+100+0 &

oclock (analog)

oclock -bg blue -geometry 500x500+100+0 -bd purple -transparent &
oclock -bg blue -geometry 500x500+100+0 -bd purple -jewel green &

See oclock, X - a portable, network-transparent window system which includes an example of specifying the geometry parameter.

dclock (digital)

Digital clock for the X Window System with flexible display.

http://manpages.org/dclock
Note that we can send the command to the background using Ctrl +z and run bg. See run a command from the terminal without blocking it?

sudo apt-get install dclock
dclock -h
dclock -d
dclock -date "Today is %A %B %Y" -led_off black -bg black -fg yellow -geometry 577x194+119+139         # 'q' to quit
dclock -date "Today is %m/%d/%y" -led_off black -bg black -fg yellow -geometry 400x150+0+0 # width x hight + X + Y

In practice, I create a shell script file <bin/clock> with the following content. The first ampersand sign is to hide warnings messages and the 2nd ampersand sign is to put the process in the background.

dclock -date "Today is %A %B %Y" -led_off black -bg black -fg yellow -geometry 577x194+119+139 &>/dev/null &

Lubuntu digital clock format

http://netgator.blogspot.com/2012/09/change-edit-panel-digital-clock-format.html. My format is

%a, %x, %r
# Tue, 05/17/2016, 09:42:27 PM

%a %m-%d-%y, %I:%M %p
# Mon 05-30-16, 08:31 AM

xdaliclock (digital)

https://www.mankier.com/1/xdaliclock

Scaling is not good. Colors is changing with time.

xdaliclock -noseconds -cycle  # 'q' to quit

date command

sudo apt install toilet
sudo apt install figlet
watch -n 60 "date +'%m/%d/%y   %H:%M:%S' | toilet"

ls /usr/share/figlet # list of fonts
                     # looking for *.tlf
date +'%m/%d/%y%H:%M:%S' | toilet -f mono12
date +'%m/%d/%y%H:%M:%S' | toilet -f bigmono12   # good on 1024x600
date +'%m/%d/%y%H:%M:%S' | toilet -f bigascii12
date +'%m/%d/%y%H:%M:%S' | toilet -f ascii12

watch --color "date +'%m/%d/%y%H:%M:%S' | toilet -f bigmono12 -F metal"

while true; do echo "$(date '+%H:%M:%S' | toilet -f bigmono12 -F border --metal)"; sleep 1; done

Reminder take a break, relieve eye strain

Stretchly. It's open-source and cross-platform. Nodejs is required.

Workrave is another choice. The source code is available too.

Prevent Eye Strain While Working On Your Linux Desktop With Safe Eyes, Github source code

wine

Wine

Running Linux in the AWS/Amazon Web Services

http://www.linuxuser.co.uk/tutorials/run-linux-in-the-aws-cloud

Forum software

Simple Machines® Forum (SMF). For example http://pibot.org/forum/

RAID

http://www.maximumpc.com/what-every-pc-builder-should-know-about-raid-levels/

Timer

http://zeegaree.com/. Require 3 libraries that we need to install them using apt-get install. See the github page.

How to track you laptop using Prey

https://www.howtoforge.com/tutorial/how-to-track-your-linux-laptop/

last command

Linux last Command Tutorial for Beginners (8 Examples)

Display a list of system shutdown/reboot date/time

Linux Find Out Last System Reboot Time and Date Command

# Works on Linux and Mac
last shutdown
last reboot

Automatic reboot after power failure

It seems there is no reliable way to find out when the power failed.

The linux command 'last' can show some information about system reboot.

Another way is to modify the BIOS to select the option like 'Power off and Reboot'. This won't automatically boot your computer when it is shutdown normally.

How to restart/shutdown server safely

How to restart CentOS or RHEL server safely

Wake up and Shut Down Linux Automatically

Two best options

Bios: BIOS may have an easy-to-use wakeup scheduler
wakeonlan:
- Eanble it: Check if it is enabled by default. If not, we can 1) enable it through a command (ethtool -s eth0 wol g) or 2) using the Network Manager
- Send a wake up command: (from a second linux) /usr/bin/wakeonlan D0:50:99:82:E7:2B where D0:50:99:82:E7:2B is the IP on the machine you want to wake it up

BIOS

Find out BIOS version

Linux Find Out BIOS Version Using a Command Line Option

How to update Lenovo BIOS from Linux without using Windows

https://www.cyberciti.biz/faq/update-lenovo-bios-from-linux-usb-stick-pen/

Internet speed test

Web

https://fast.com/ (automatically run)
https://www.bing.com/search?q=internet+speed+test
http://www.speedtest.net/

Speedtest-cli

See Track_Internet_Dropouts.

sudo apt-get intall python-pip
sudo pip install speedtest-cli
# A slightly modified code that will create a one-line space/semi-colon 
# delimited result is 
git clone https://github.com/HenrikBengtsson/speedtest-cli-extras.git

speedtest-cli-extras/bin/speedtest-csv

works. But if I want to put it in cron, cron will issue an error speedtest-cli cannot be found. So I need to modify line 52 of the code in <speedtest-cli-extras/bin/speedtest-csv> to explicitly specify the location of speedtest-cli.

    /usr/local/bin/speedtest-cli --share > $log

NOTE: 1. the results differ from the network connection. For example, the speed is good when I test it on the machine directly connected to the router. 2. It is helpful to modify the last line of the bash script to output what I need. 3. The separator is ";" in the output.

curl and wget

How to Run Speed Test from the Command Line to Check Internet Connection Speed

uname - Print system information

uname -a will give you

OS (uname = uname -s if you are under a Linux environment)
OS (uname -s) eg Linux
node name (uname -n=hostname)
kernel release (uname -r) eg 3.16.0-38-generic
kernel version (uname -v)
machine architecture (uname -m) eg x86_64
processor (uname -p)
hardware platform (uname -i)
operating system (uname -o)

How to check if running in Cygwin, Mac or Linux?

Hardware information

How To Check System Hardware Manufacturer, Model And Serial Number In Linux
How to List Your Computer's Devices From the Linux Terminal: mount, lsblk, df, fdisk, /proc, lspci, lsusb, lsdev, lshw, lsscsi, dmidecode, hwinfo.

Command Line

Linux commands to display your hardware information
inxi: 3 ways to use the Linux inxi command
- inxi -b
- inxi -W taipei,taiwan (check the weather)
How To Find Linux System Details Using inxi

hwinfo

https://www.2daygeek.com/python-hwinfo-check-display-system-hardware-configuration-information-linux/

On Ubuntu, use sudo apt install -y hwinfo to install hwinfo. Install hwinfo on Ubuntu 20.04

dmesg command

How to Use the dmesg Command on Linux

sudo dmesg -L -T
sudo dmesg -L -T --follow   # Watching Live Events
sudo dmesg -L -T | grep -i usb    # Search for a specific term
sudo dmesg | grep -E "memory|tty|dma"  # Search for multiple terms

where -L to is force color output and -T is to make timestamp human-readable.

Linux Logo and the current system information

http://www.cyberciti.biz/hardware/howto-display-linux-logo-in-bash-terminal-using-screenfetch-linux_logo/

odroid@odroid:~$ sudo apt-get install screenfetch
odroid@odroid:~$ screenfetch
                          ./+o+-       odroid@odroid
                  yyyyy- -yyyyyy+      OS: Ubuntu 15.10 wily
               ://+//////-yyyyyyo      Kernel: armv7l Linux 3.10.96-77
           .++ .:/++++++/-.+sss/`      Uptime: 4d 23h 8m
         .:++o:  /++++++++/:--:/-      Packages: 2000
        o:+o+:++.`..```.-/oo+++++/     Shell: 2263
       .:+o:+o/.          `+sssoo+/    Resolution: 1920x1080
  .++/+:+oo+o:`             /sssooo.   DE: MATE 1.10.2
 /+++//+:`oo+o               /::--:.   WM: Metacity (Marco)
 \+/+o+++`o++o               ++////.   GTK Theme: 'Ambiant-MATE' [GTK2/3]
  .++.o+++oo+:`             /dddhhh.   Icon Theme: Ambiant-MATE
       .+.o+oo:.          `oddhhhh+    Font: Ubuntu 10
        \+.++o+o``-````.:ohdhhhhh+     CPU: ARMv7 rev 3 (v7l) @ 1.4GHz
         `:o+++ `ohhhhhhhhyo++os:      GPU: Gallium 0.4 on llvmpipe (LLVM 3.6, 128 bits)
           .o:`.syhhhhhhh/.oo++o`      RAM: 537MiB / 1990MiB
               /osyyyyyyo++ooo+++/    
                   ````` +oo+++o\:    
                          `oo++.      

odroid@odroid:~$ screenfetch -h

Neofetch, File:NeofetchMac.png, File:Neofetchpopos.png

Dictionary - Artha

Lifehacker. Once it is launched, it is sitting on the task bar. Press Ctrl+Alt+W after selecting a word to look it up in Artha (a balloon tip will pop up on the screen top-right). It also supports using regular expressions to search words.

sudo apt-get install artha

Translation

Translate Shell. No installation is needed. It is just a bash script (4990 lines) so it works on ODroid SOC. See also A Tool To Use Google Translate From Command Line In Linux

odroid@odroid:~/binary$ ./trans :zh-TW word
word
/wərd/

字
(Zì)

Definitions of word
[ English -> 正體中文 ]

noun
    字
        word, character, letter, calligraphy, symbol, style of writing
    詞
        word, term, speech, statement
    單詞
        word, individual word
    話
        words, word, dialect, saying, talk, speech
    言
        word, speech, character
    言辭
        words, word, what one says
    筆墨
        pen and ink, words, word, writings
    約言
        pledge, promise, word

verb
    為 ... 措辭
        word
odroid@odroid:~/binary$ time ./trans -brief :zh-TW word
字

real	0m4.249s
user	0m2.670s
sys	0m1.330s

ASCII art/ word art

figlet. How to Create ASCII Text Banners in Terminal, lolcat
toilet command. date | toilet . Color output. man toilet. TOIlet prints text using large characters made of smaller characters. It is similar in many ways to FIGlet with additional features such as Unicode handling, colour fonts, filters and various export formats.
http://patorjk.com/software/taag/#p=display&f=Ivrit&t=BRR-SeqTools

  ____  ____  ____       ____            _____           _     
 | __ )|  _ \| __ )     / ___|  ___  __ |_   _|__   ___ | |___ 
 |  _ \| |_) |  _ \ ____\___ \ / _ \/ _` || |/ _ \ / _ \| / __|
 | |_) |  _ <| |_) |_____|__) |  __/ (_| || | (_) | (_) | \__ \
 |____/|_| \_\____/     |____/ \___|\__, ||_|\___/ \___/|_|___/
                                       |_|

http://patorjk.com/software/taag/#p=display&f=Big&t=BRB-SeqTools

  ____  _____  ____        _____         _______          _     
 |  _ \|  __ \|  _ \      / ____|       |__   __|        | |    
 | |_) | |__) | |_) |____| (___   ___  __ _| | ___   ___ | |___ 
 |  _ <|  _  /|  _ <______\___ \ / _ \/ _` | |/ _ \ / _ \| / __|
 | |_) | | \ \| |_) |     ____) |  __/ (_| | | (_) | (_) | \__ \
 |____/|_|  \_\____/     |_____/ \___|\__, |_|\___/ \___/|_|___/
                                         | |                    
                                         |_|

http://patorjk.com/software/taag/#p=display&f=Small&t=BRB-SeqTools

  ___ ___ ___     ___          _____         _    
 | _ ) _ \ _ )___/ __| ___ __ |_   _|__  ___| |___
 | _ \   / _ \___\__ \/ -_) _` || |/ _ \/ _ \ (_-<
 |___/_|_\___/   |___/\___\__, ||_|\___/\___/_/__/
                             |_|

Install And Manage Software From Source

How To Install And Manage Software From Source Using GNU Stow In Linux

Software that scan Malware and rootkits

https://www.howtoforge.com/tutorial/how-to-scan-linux-for-malware-and-rootkits/

Text to speech

http://www.eguidedog.net/ekho.php. Compilation/build works on x86 Ubuntu 14 and Odroid Ubuntu 15.10. On Odroid I have to follow their instruction to use 'make CXXFLAGS=-DNO_SSE' instead of 'make'. However, sound feels shaky on Odroid xu4.
http://audiobookmaker.com/
http://project-modelino.com/online-resources-category.php?site_language=english&learn_language=chinese&category=tts

VPN

Ubuntu -> VPN

Mono Project

Mono is a software platform designed to allow developers to easily create cross platform applications part of the .NET Foundation

Mono is required for Repetier-Host software for 3D printing.

NAS server

OpenMediaVault

FreeNAS

ZFS system (FreeBSD-based).

10 Reasons Why You Should Store Your Data on a FreeNAS Box. Note With the current version of FreeNAS (FreeNAS 11) comes a hypervisor. See

Change detection

http://bhfsteve.blogspot.com/2013/03/monitoring-web-page-for-changes-using.html

3 command-line tools for feigning productivity

https://opensource.com/article/18/2/command-line-tools-productivity: Blessed-contrib (javascript), Genact, Hollywood.

Mind mapping

Freeplane

Diagram

Calligra Flow. Microsoft Visio alternative.

Open source surveillance

ZoneMinder

Systemctl, systemd

Linux 系统开机启动项清理中文 & English
Chkservice – An Easy Way to Manage Systemd Units in Terminal
Control an application to run when Raspberry pi boots up
Why Linux’s systemd Is Still Divisive After All These Years
How to Manage Systemd Services with Systemctl on Linux
- Manage Services with Systemd
- Manage Systemd Unit Files
- Manage Sockets with Systemctl
- Systemctl Additional Commands

Systemd vs SysVinit

Systemctl vs service commands

What is the difference between service and systemctl? service is an "high-level" command used for starting and stopping services in different unixes and linuxes. Depending on the "lower-level" service manager, service redirects on different binaries. For example, on CentOS 7 it redirects to systemctl.

$ service nginx start
# VS
$ systemctl start nginx

$ systemctl   # list all services
$ cat /lib/systemd/system/rsyslog.service
$ systemctl status rsyslog
$ cat /lib/systemd/system/ufw.service

How to Run a Linux Program at Startup with systemd

How to Run a Linux Program at Startup with systemd
- Creating the Service Program for systemd to Start sudo nano /usr/local/bin/htg.sh, sudo chmod +x /usr/local/bin/htg.sh
- Creating the Service Unit File sudo nano /etc/systemd/system/htg.service, sudo chmod 640 /etc/systemd/system/htg.service
- Starting the Service Automatically with the systemd Command sudo systemctl daemon-reload, sudo systemctl enable htg, sudo systemctl start htg
- Verifying the Service sudo systemctl status htg.service
- Stopping and Disabling the Service - sudo systemctl stop htg.service
How to Create a New systemd Service on Linux
Automatically mount a storage device with a systemd service

How Long Does it Take To Boot Your Linux System

Find Out How Long Does it Take To Boot Your Linux System

$ systemd-analyze       # total boot time along with the time taken by 
                        # firmware, boot loader, kernel and the userspace
$ systemd-analyze blame # breakdown the boot time into each unit

Check if Your Linux System Uses systemd

How to Check if Your Linux System Uses systemd

chkservice

chkservice Is A systemd Unit Manager With A Terminal User Interface

Kernel

Firmware update

Fwupd 1.9.9 Released with Support for Lenovo X1 Yoga Gen7 530E 2-in-1 Laptops

Game

See Game.

Best Linux Adobe Alternatives You Need to Know

https://linux.cn/article-8928-1.html and https://www.maketecheasier.com/adobe-alternatives-for-linux/

Linux distributions

Linux Distribution

chroot

Chroot

Linux: Difference between revisions

Latest revision as of 22:13, 7 December 2024

man

Navigation

Search within a man page

Colored man pages

Navigate to another man page within a man page

View a specific "Section"

Read man pages in vi without using temporary files

Search man page referenced by

Search from all man pages

TLDR pages/cheat sheet: alternative to Man

Cheat.sh (better than TLDR)

Some books

Beautiful desktop

.desktop file

List of installed desktop environment

Themes

Virtual consoles/virtual terminals

Change/increase console fonts

Desktops/Workspaces

Mouse

Virtual memory

vmstat

hcache

Memory: free command

How do I determine the number of RAM slots in use

Logging memory

Shows Per-Program Memory Usage On Linux

Check RAM information

Free up memory

Monitor Memory Utilization And Send an Email

zram

Things to do after a fresh install of GNU/Linux

Query whether the OS is 64-bit or 32-bit

Command line improved

Directory permission / attribute

Making a new temporary directory

Shell

Login shell and non-login shell

Aliases and Functions for Individual Users

Why does it take tens of seconds to get a shell prompt?

the source command

login shell (.bash_profile) vs interactive shell (.bashrc)

/root/.bashrc

Login banner

Change to root shell

pinky: find out about the people logged on to your Linux computer

su: Switch to another user and run a command

Keyboard shortcut to move cursor in the terminal

Record terminal session to a text file

Tools To Record Your Terminal And Generate Animated Gif or SVG Images

Asciinema & agg

terminalizer

Termtosvg

Record and Replay Linux Terminal Sessions Activity: script

Recording your terminal: asciinema

Dictionary/translation

Clear screen

Clear text

Redirect standard error

Redirect standard output

Quotes and asterisk

cat command alternatives

ls command

Follow the symbolic link

List only directories

List only files

Find and Delete Broken Symbolic Links

Special characters, escape

Check non-English characters

ls | more without lose color

ls directories color

ls output with color background

ls output without user/group columns

ls output selected columns

ls suddenly wrapping items with spaces in single quotes

ls on BSD/macOS

realpath

tree command