Linux Programming

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Shell Programming

Some Resources

Understand shell command options

explainshell.com. For example, https://explainshell.com/explain?cmd=rsync+-avz+--progress+--partial+-e

Check shell scripts

How To Validate the Syntax of a Linux Bash Script Before Running It

ShellCheck & download the binary from Launchpad.

If a statement missed a single quote the shell may show an error on a different line (though the error message is still useful). Therefore it is useful to verify the syntax of the script first before running it.

Writing Secure Shell Scripts

Writing Secure Shell Scripts

Bioinformatics

Bioinformatics one-liners

Data science

Data Science at the Command Line Obtain, Scrub, Explore, and Model Data with Unix Power Tools

Special characters

15 Special Characters You Need to Know for Bash

Progress bar

How to Add a Simple Progress Bar in Shell Script

Simple calculation

echo

echo $(( 11/5 ))
# or
echo $((11/5))

Note: only return an integer number.

bc: an arbitrary precision calculator language

bc -l <<< "11/5"
# Without '-l' we only get the integer part
# Or interactive
bc -i
scale=5
11/5
quit

where -l means to use the predefined math routines and <<< is a here string. Note bc returns a real number.

Here documents

<<

#!/bin/bash

cat <<!FUNKY!
hello
this is a here
document
$var on line
!FUNKY!

To disable pathname/parameter/variable expansion, command substitution, arithmetic expansion such as $HOME, ..., add quotes to EOF; 'EOF'.

<<< here string

http://linux.die.net/abs-guide/x15683.html

Redirect

stdin, stdout, and stderr

What Are stdin, stdout, and stderr on Linux?

Redirecting output. File descriptor number 1 (2) means standard output (error). 

./myProgram > stdout.txt        # redirect std out to <stdout.txt>
./myProgram 2> stderr.txt       # redirect std err to <stderr.txt> by using the 2> operator
./myProgram > stdout.txt 2> stderr.txt # combination of above two
./myProgram > stdout.txt 2>&1   # redirect std err to std out <stdout.txt>
./myProgram >& /dev/null        # prevent writing std out and std err to the screen
ps >> outptu.txt                # append

Redirecting input 

./myProgram < input.txt

Using cat or echo to create a new file that needs sudo right

The following command does not work 

sudo cat myFile > /opt/myFile

Solution 1 (sudo sh -c). We can use something like 

sudo sh -c 'cat myFile > /opt/myFile'

Solution 2 (sudo tee). See 'How To Configure Nginx as a Web Server and Reverse Proxy for Apache on One Ubuntu 16.04 Server' 

echo "<?php phpinfo(); ?>" | sudo tee /var/www/html/info.php

If we want to append something to an existing file, use -a option in the tee command.

Create a simple text file with multiple lines; write data to a file in bash script

Each of the methods below can be used in a bash script. 

# Method 1: printf. We can add \t for tab delimiter
$ printf '%s \n' 'Line 1' 'Line 2' 'Line 3' > out.txt

# Method 2: echo. We can add \t for tab delimiter
$ echo -e 'Line 1\t12\t13
$ Line 2\t22\t23
$ Line 3\t32\t33' > out.txt

# Method 3: echo
$ echo $'Line 1\nLine 2\nLine 3' > out.txt

# Method 4: here document, http://tldp.org/LDP/abs/html/here-docs.html
# For the TAB character, use Ctrl-V, TAB.
# Note that first line can be: cat <<EOF > out.txt
# The filename can be a variable if this is used inside a bash file
$ cat > out.txt <<EOF
> line1   Second
> lin2    abcd
> line3ss dkflaf
> EOF
$

See also How to use a here documents to write data to a file in bash script

To escape the quotes, use a back slash. For example 

echo $'#!/bin/bash\nmodule load R/3.6.0\nRscript --vanilla -e "rmarkdown::render(\'gse6532.Rmd\')"'

will obtain 

#!/bin/bash
module load R/3.6.0
Rscript --vanilla -e "rmarkdown::render('gse6532.Rmd')"

>&

&> file is not part of the official POSIX shell spec, but has been added to many Bourne shells as a convenience extension (it originally comes from csh). In a portable shell script (and if you don't need portability, why are you writing a shell script?), use > file 2>&1 only.

Redirect Output and Errors To /dev/null

http://www.cyberciti.biz/faq/how-to-redirect-output-and-errors-to-devnull/ 

command > /dev/null 2>&1
# OR
command &>/dev/null

In addition we can put a process in the background by adding the '&' sign; see the dclock example.

tee -redirect to both a file and the screen same time

To redirect to both a file and the screen the same time, use tee command. See 

command1 |& tee log.txt
## or ##
command1 -arg |& tee log.txt
## or ##
command1 2>&1 | tee log.txt

# use the option '-a' for *append*
echo "new line of text" | sudo tee -a /etc/apt/sources.list

# redirect output of one command to another
ls file* | tee output.txt | wc -l

# streaming file (e.g. running an arduino sketch on Udoo)
# for streaming files, cp command (still need Ctrl + c) will not 
# show anything on screen though copying is executed.
cat /dev/ttymxc3 | tee out.txt      # Ctrl + c
command > >(tee stdout.log) 2> >(tee stderr.log >&2)

Methods To Create A File In Linux

10 Methods To Create A File In Linux

Prepend

BASH Prepend A Text / Lines To a File

Pipe

The operator is |. 

ps > psout.txt
sort psout.txt > pssort.out

can be simplified to 

ps | sort > pssort.out

For example, 

$ head /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync

$cat /etc/passwd | cut -d: -f7 | sort | uniq -c | sort -nr
     18 /bin/sh
     13 /bin/false
      2 /bin/bash
      1 /bin/sync

where cut command will extract the 7th field separated by the : character and write to the output stream. sort command will sort alphabetically sorts the line it reads from its input and returns the new sort to its output. The uniq command will remove and count duplicated lines. The final sort command will sort its input numerically in reverse order.

Dash (-) at the end of a command mean?

Process substitution

https://en.wikipedia.org/wiki/Process_substitution

Powerfulness of pipes

Consider the following commands (samtools gives its output on stdout which is a good opportunity to use pipes) 

samtools mpileup -go temp.bcf -uf genome.fa  dedup.bam
bcftools call -vmO v -o sample1_raw.vcf temp.bcf

The disadvantage of this approach is it will create a temporary file (temp.bcf in this case). If the size of the temporary file is enormous large (several hundred of GB), it will waste/eat up the hard disk space no to say the time used to create the temporary file. If we use pipes, we can save the time and disk space of the temporary file. 

samtools mpileup -uf genome.fa  dedup.bam | bcftools call -vmO v -o sample1_raw.vcf

Send a stdout to a remote computer

See here (bypass SSH password) for a case (utilize cat, ssh and >> commands).

Execute a bash script downloaded (without saving first) from the internet

See the example of install Gitlab 

sudo curl -sS https://packages.gitlab.com/install/repositories/gitlab/raspberry-pi2/script.deb.sh | sudo bash

where -s means silent and -S means showing error messages if it fails. Note that curl will download the file to standard output. So using the pipe operator is a reasonable sequence after running the curl.

Use wget to download and decompress at one line

https://stackoverflow.com/questions/16262980/redirect-pipe-wget-download-directly-into-gunzip 

wget -O - ftp://ftp.direcory/file.gz | gunzip -c > file.out

where "-O -" means to print to standard output (sort of like the default behavior of "curl"). See https://www.gnu.org/software/wget/manual/wget.html

Use pipe and while loop to process multiple files

See an example at while.

Pipe vs redirect

  • Pipe is used to pass output to another program or utility.
  • Redirect is used to pass output to either a file or stream.

In other words, thing1 | thing2 does the same thing as thing1 > temp_file && thing2 < temp_file.

Shebang (#!)

A shebang is the character sequence consisting of the characters number sign and exclamation mark (that is, "#!") at the beginning of a script. See the Wikipedia page.

The syntax looks like 

#! interpreter [optional-arg]

For example,

  • #!/bin/sh — Execute the file using sh, the Bourne shell, or a compatible shell
  • #!/bin/csh -f — Execute the file using csh, the C shell, or a compatible shell, and suppress the execution of the user’s .cshrc file on startup
  • #!/usr/bin/perl -T — Execute using Perl with the option for taint checks

When Is It Better to Use #!/bin/bash Instead of #!/bin/sh in a Shell Script?

http://www.howtogeek.com/276607/when-is-it-better-to-use-bin-bash-instead-of-bin-sh-in-a-shell-script/

Howto Make Script More Portable With #!/usr/bin/env As a Shebang

https://www.cyberciti.biz/tips/finding-bash-perl-python-portably-using-env.html

This is useful if the interpreter location is different on Linux and Mac OSs. 

# Linux
$ which Rscript
/usr/bin/Rscript
# Mac
$ which Rscript
/usr/local/bin/Rscript

We can use the following on the first line of the shell script. 

#!/usr/bin/env Rscript

Comments

For a single line, we can use the '#' sign. Shell Script Put Multiple Line Comments under Bash/KSH.

For a block of code, we use 

#!/bin/bash
echo before comment
: <<'END'
bla bla
blurfl
END
echo after comment

Variables

food=Banana
echo $food
food="Apple"
echo $food

When do I need to use the export command

Consider the following 

MY_DIRECTORY=/path/to/my/directory
export MY_DIRECTORY
./my_script.sh

If you don’t use the export command in the above example, the MY_DIRECTORY variable will not be available to the my_script.sh script. It will only be available within the current shell session as a local shell variable.

When you set a variable in a shell session without using the export command, it is only available within that shell session as a local shell variable. This means that the variable and its value are only accessible within the current shell session and are not passed to child processes (e.g. my_script.sh) or other programs that are started from the command line.

Cf. When I put LS_COLORS in the .bashrc file, I don't need to use the export command.

export -n command: remove from environment

https://linuxconfig.org/learning-linux-commands-export

It will export an environment variable to the subshell/forked process. For example 

$ export MYVAR=10      # export a variable
$ export -n MYVAR      # remove a variable

To see the current process ID, use 

echo $$

To create a new process, use 

bash

When using the export command without any option and arguments it will simply print all names marked for an export to a child process. 

$ export
declare -x EDITOR="nano"
declare -x HISTTIMEFORMAT="%d/%m/%y %T "
declare -x HOME="/home/brb"
declare -x LANG="en_US.UTF-8"
declare -x LESSCLOSE="/usr/bin/lesspipe %s %s"
declare -x LESSOPEN="| /usr/bin/lesspipe %s"
declare -x LOGNAME="brb"
...
declare -x PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
declare -x PWD="/home/brb"
declare -x SHELL="/bin/bash"
...
declare -x USER="brb"
declare -x VISUAL="nano"

echo command

String manipulation

http://www.thegeekstuff.com/2010/07/bash-string-manipulation/

dirname and basename commands

http://www.tldp.org/LDP/LG/issue18/bash.html 

# On directories
$ dirname ~/Downloads
/home/chronos/user
$ basename ~/Downloads
Downloads

# On files
$ dirname ~/Downloads/DNA_Helix.zip
/home/chronos/user/Downloads

$ basename ~/Downloads/DNA_Helix.zip
DNA_Helix.zip
$ basename ~/Downloads/DNA_Helix.zip .zip
DNA_Helix
$ basename ~/Downloads/annovar.latest.tar.gz
annovar.latest.tar.gz
$ basename ~/Downloads/annovar.latest.tar.gz .gz
annovar.latest.tar
$ basename ~/Downloads/annovar.latest.tar.gz .tar.gz
annovar.latest
$ basename ~/Downloads/annovar.latest.tar.gz .latest.tar.gz
annovar

Escape characters and quotes

echo $USER  # brb

echo My name is $USER

echo "My name is $USER"  # My name is brb

echo 'My name is $USER'  # 'My name is $USER'; single quote will not interpret the variable
          # we use the single quotes if we want to present the characters literally or 
          # pass the characters to the shell.
grep '.*/udp' /etc/services  # normally . and * and slash characters have special meaning
   
echo \$USER   # we escape $ so $ lost its special meaning

echo '\'

echo \'text\'  # 'text'

When to use double quotes with a variable

when to use double quotes with a variable in shell script?

Concatenate string variables (not safe)

http://stackoverflow.com/questions/4181703/how-can-i-concatenate-string-variables-in-bash 

a='hello'
b='world'
c=$a$b
echo $c

# Bash also supports a += operator 
$ A="X Y"
$ A+="Z"
$ echo "$A"

Often we need to use "double quotes" around the string variables if the string variables represent some directories. 

mkdir "tmp 1"
touch "tmp 1/tmpfile"

tmpvar="tmp 1"
echo tmpvar
# tmp 1

ls $tmpvar
ls: cannot access tmp: No such file or directory
ls: cannot access 1: No such file or directory
ls "$tmpvar"
# tmpfile

However, for integers 

echo $a
24
((a+=12))
echo $a
36

Note that the double parentheses construct in ((a+=12)) permits arithmetic expansion and evaluation.

${parameter} - Concatenate a string variable and a constant string; variable substitution

Parameter substitution ${}. Cf $() for command execution 

x=foo
y=bar
z=$x$y        # $z is now "foobar"
z="$x$y"      # $z is still "foobar"
z="$xand$y"   # does not work
z="${x}and$y" # does work, "fooandbar"

And 

your_id=${USER}-on-${HOSTNAME}
echo "$your_id"

echo "Old \$PATH = $PATH"
PATH=${PATH}:/opt/bin  # Add /opt/bin to $PATH for duration of script.
echo "New \$PATH = $PATH"

And using "{" in order to create a new string based on an existing variable 

pdir="/tmp/files/today"
fname="report"
mkdir -p $pdir

touch $pdir/$fname  # OK
ls -l $pdir/$fname

touch $pdir/$fname_new  # No error but it does not do anything
                        # because this variable does not exist yet
ls $pdir/$fname_new

touch $pdir/${fname}_new
ls $pdir/${fname}_new   # Works

$(command) - Command Execution and Assign Output of Shell Command To a Variable; Command substitution

Bash Assign Output of Shell Command To Variable 

$(command)
`command`    # ` is a backquote/backtick, not a single quotation sign
             # this is a legacy support; not recommended by https://www.shellcheck.net/

Note all new scripts should use the $(...) form, which was introduced to avoid some rather complex rules.

Example 1. 

sudo apt-get install linux-headers-$(uname -r)

Example 2. 

user=$(echo "$UID")

Example 3. 

#!/bin/sh
echo The current directory is $PWD
echo The current users are $(who)
sudo chown `id -u` SomeDir  # change the ownership to the current user. Dangerous!
                            # Or sudo chown `whoami` SomeDirOrSomeFile
exit 0

Example 4. Create a new file with automatically generated filename 

touch file-$(date -I)

Example 5. Use $(your expression) to run nest expressions. For example, 

# cd into the directory containing the 'touch' command. 
cd $(dirname $(type -P touch))

BACKUPDIR=/nas/backup
LASTDAYPATH=${BACKUPDIR}/$(ls ${BACKUPDIR} | tail -n 1)

The concept of putting the result of a command into a script variable is very powerful, as it makes it easy to use existing commands in scripts and capture their output.

Arithmetic Expansion 

$((...))

is a better alternative to the expr command. More examples: 

for i in $(seq 1 3)
  do echo SRR$(( i + 1027170 ))'_1'.fastq 
done

Note that the single quote above is required. The above will output SRR1027171_1.fastq, SRR102172_1.fastq and SRR1027173_1.fastq.

Parameter Expansion 

${parameter}

Double Parentheses (())

Bash Shell Scripting for beginners (Part 1) fedoramagazine. Double parentheses are simple, they are for mathematical equations.

extract substring

https://www.cyberciti.biz/faq/how-to-extract-substring-in-bash/ 

${parameter:offset:length}

Example: 

## define var named u ##
u="this is a test"

var="${u:10:4}"
echo "${var}"

Or use the cut command. 

u="this is a test"
echo "$u" | cut -d' ' -f 4
echo "$u" | cut --delimiter=' ' --fields=4
##########################################
## WHERE
##   -d' ' : Use a whitespace as delimiter
##   -f 4  : Select only 4th field
##########################################
var="$(cut -d' ' -f 4 <<< $u)"
echo "${var}"

Environment variables

How to Set Environment Variables in Bash on Linux 

$HOME
$PATH
$0 -- name of the shell script
$# -- number of parameters passed (so it does include the program itself)
$$ process ID of the shell script, often used inside a script for generating unique temp filenames
$? -- the exit value of the last run command; 0 means OK and none-zero means something wrong
$_ -- previous command's last argument

Example 1 (check if a command run successfully): 

some_command
if [ $? -eq 0 ]; then
    echo OK
else
    echo FAIL
fi
# OR
if some_command; then
    printf 'some_command succeeded\n'
else
    printf 'some_command failed\n'
fi

$ tabix -f -p vcf ~/SeqTestdata/usefulvcf/hg19/CosmicCodingMuts.vcf.gz
brb@brb-P45T-A:/tmp$ echo $?
0
$ tabix -f -p vcf ~/Downloads/CosmicCodingMuts.vcf.gz
Not a BGZF file: /home/brb/Downloads/CosmicCodingMuts.vcf.gz
tbx_index_build failed: /home/brb/Downloads/CosmicCodingMuts.vcf.gz
$ echo $?
1

Example 2 (check whether a host is reachable) 

ping DOMAIN -c2 &> /dev/null
if [ $? -eq 0 ];
then
  echo Successful
else
  echo Failure
fi

where -c is used to limit the number of packets to be sent and &> /dev/null is used to redirect both stderr and stdout to /dev/null so that it won't be printed on the terminal.

Example 3 (check if users have supply a correct number of parameters): 

#!/bin/bash
if [ $# -ne 2 ]; then
  echo "Usage: $0 ProgramName filename"
  exit 1
fi

match_text=$1
filename=$2

Example 4 (make a new directory and cd to it) 

mkdir -p "newDir/subDir"; cd "$_"

How to List Environment Variables

How to List Environment Variables on Linux 

printenv

Unset/Remove an environment variable

$ export MSG="HELLO WORLD"
$ echo $MSG
HELLO WORLD
$ unset MSG
$ echo $MSG

$

Set an environment variable and run a command on the same line, env command

Parameter variables

$1, $2, .... -- parameters given to the script
$* -- list of all the parameters, in a single variable
$@ -- subtle variation on $*. 
$! -- the process id of the last command run in the background.

Example 1. 

#!/bin/bash
echo "$1 likes to eat $2 and $3 every day."
echo "bye:-)"

Example 2. 

$ touch /tmp/tmpfile_$$

$ set foo bar bam
$ echo $#
3
$ echo $@
foo bar bam
$ set foo bar bam &
[1] 28212
$ echo $!
28212
[1]+  Done                    set foo bar bam

Example 3. $@ parameter for a variable number of parameters 

$ cat stats.sh
for FILE1 in "$@"
do
wc $FILE1
done
$ sh stats.sh songlist1 songlist2 songlist3

We can also use parentheses around the variable name. 

QT_ARCH=x86_64
QT_SDK_BINARY=QtSDK-4.8.0-${QT_ARCH}.tar.gz
QT_SD_URL=https://xxx.com/$QT_SDK_BINARY

How do I rename the extension for a batch of/multiple files? See man bash Shell Parameter Expansion 

# Solution 1:
for file in *.html; do
    mv "$file" "`basename "$file" .html`.txt"
done

# Solution 2:
for file in *.html
do
 mv "$file" "${file%.html}.txt"
done

Get filename without Path

How to Extract Filename & Extension in Shell Script 

fullfilename="/var/log/mail.log"
filename=$(basename "$fullfilename")
echo $filename

Extension without filename

How to Extract Filename & Extension in Shell Script 

fullfilename="/var/log/mail.log"
filename=$(basename "$fullfilename")
ext="${filename##*.}"
echo $ext

Discard the extension name and "%" symbol

$ vara=fillename.ext
$ echo $vara
fillename.ext
$ echo ${vara::-4} # works on Bash 4.3, eg Ubuntu
fillename
$ echo ${vara::${#vara}-4} # works on Bash 4.1, eg Biowulf readhat

http://stackoverflow.com/questions/27658675/how-to-remove-last-n-characters-from-a-bash-variable-string

Another way (not assuming 3 letters for the suffix) https://www.cyberciti.biz/faq/unix-linux-extract-filename-and-extension-in-bash/ 

dest="/nas100/backups/servers/z/zebra/mysql.tgz"
## get file name i.e. basename such as mysql.tgz
tempfile="${dest##*/}"
 
## display filename 
echo "${tempfile%.*}"

Or better with (See Extract filename and extension in Bash and Shell parameter expansion). How to Extract Filename & Extension in Shell Script 

fullfilename="/var/log/mail.log"
filename=$(basename "$fullfilename")
fname="${filename%.*}"
echo $fname   # mail

$ UEFI_ZIP_FILE="UDOOX86_B02-UEFI_Update_rel102.zip"
$ UEFI_ZIP_DIR="${UEFI_ZIP_FILE%.*}"
$ echo $UEFI_ZIP_DIR
UDOOX86_B02-UEFI_Update_rel102

$ FILE="example.tar.gz"
$ echo "${FILE%%.*}"
example
$ echo "${FILE%.*}"
example.tar
$ echo "${FILE#*.}"
tar.gz
$ echo "${FILE##*.}"
gz

Space in variable value

Suppose we have a script file called 'foo' that can remove spaces from a file name. Note: tr command is used to delete characters specified by the '-d' parameter. 

#!/bin/sh
NAME=`ls $1 | tr -d ' '`
echo $NAME
mv $1 $NAME

Now we try the program: 

$ touch 'file 1.txt'
$ ./foo 'file 1.txt'
ls: cannot access file: No such file or directory
ls: cannot access 1.txt: No such file or directory

mv: cannot stat ‘file’: No such file or directory

The way to fix the program is to use double quotes around $1 

#!/bin/sh
NAME=`ls "$1" | tr -d ' '`
echo $NAME
mv "$1" $NAME

and test it 

$ ./foo "file 1.txt"
file1.txt

If we concatenate the variable, put the double quotes around the variables, not the whole string. 

$ rm "$outputDir/tmp/$tmpfd/tmpa"  # fine

$ rm "$outputDir/tmp/$tmpfd/tmp*.txt"
rm: annovar6-12/tmp/tmp_bt20_raw/tmp*.txt: No such file or directory

$ rm "$outputDir"/tmp/$tmpfd/tmp*.txt

See https://unix.stackexchange.com/questions/131766/why-does-my-shell-script-choke-on-whitespace-or-other-special-characters

getopts function - parse options from shell script command line

Check if command line argument is missing (? :) and specifying the default (:-)

Search for ternary (conditional) operator and check out parameter Expansion in Bash Reference Manual. 74 Bash Operators Examples 

#!/usr/bin/env bash

NAME=${1?Error: no name given}
NAME2=${2:-friend}

echo "HELLO! $NAME and $NAME2"

Shell expansion

https://www.gnu.org/software/bash/manual/html_node/Shell-Expansions.html#Shell-Expansions

Curly brace {} expansion and array

cp -v *.{txt,jpg,png} destination/
  • All about {Curly Braces} in Bash
    • Array Builder 
      echo {0..10}

echo {10..0..2}
echo {z..a..2}

mkdir test{10..12}  # test10, test11, test12 directories
rm -rf test{10..12}
    • Parameter expansion 
      # convert jpg to png
for i in *.jpg; do convert $i ${i%jpg}png; done

a="Hello World!"
echo Goodbye${a#Hello}
# Goodbye World!
    • Output Grouping
  • How to Use Arrays in a Bash Script

Square brackets

Using Square Brackets in Bash: Part 1

Globbing: Using wildcards to get all the results that fit a certain pattern is precisely 

ls *.jpg  # the asterisk means "zero or more characters"
ls d*k?   # ?, which means "exactly one character"

touch file0{0..9}{0..9} # This will create files file000, file001, file002, etc., through file097, file098 and file099.
ls file0[78]?           #  list the files in the 70s and 80s
ls file0[259][278]      #  list file022, file027, file028, file052, file057, file058, file092, file097, and file98

Conditions

We can use the test command to check if a file exists. The command is test -f <filename>.

[] is just the same as writing test, and would always leave a space after the test word. 

if test -f fred.c; then ...; fi

if [ -f fred.c ]
then
...
fi

if [ -f fred.c ]; then
...
fi

Boolean variables

How to declare Boolean variables in bash and use them in a shell script 

failed=0 # False
jobdone=1 # True
## more readable syntax ##
failed=false 
jobdone=true

if [ $failed -eq 1 ]
then
    echo "Job failed"
else
    echo "Job done"
fi

We can define them as a string and make our code more readable.

What is the difference between test, [ and [[ ?

http://mywiki.wooledge.org/BashFAQ/031

[ ("test" command) and [[ ("new test" command) are used to evaluate expressions. [[ works only in Bash, Zsh and the Korn shell, and is more powerful; [ and test are available in POSIX shells.

test implements the old, portable syntax of the command. In almost all shells (the oldest Bourne shells are the exception), [ is a synonym for test (but requires a final argument of ]).

[[ is a new improved version of it, and is a keyword, not a program.

String comparison

==  ==> strings are equal (== is a synonym for =)
=   ==> strings are equal 
!=  ==> strings are not equal
-z  ==> string is null
-n  ==> string is not null

For example, the following script check if users have provided an argument to the script. 

$!/bin/sh
if [ -z "$1"]; then
  echo "Provide a \"file name\", using quotes to nullify the space."
  exit 1
fi
mv -i "$1" `ls "$1" | tri -d ' '`

where the -i parameter is to reconfirm the overwrite by the mv command.

To check whether Xcode (either full Xcode or command line developer tools only) has been installed or not on Mac 

if [ -z "$(xcode-select -p 2>&1 | grep error)" ]
then 
   echo "Xcode has been installed";
else
   echo "Xcode has not been installed";
fi

# only print out message if xcode was not found
if [ -n "$(xcode-select -p 2>&1 | grep error)" ]
then 
   echo "Xcode has not been installed";
fi

note the 'error' keyword comes from macOS when the Xcode has not been installed. Also the double quotes around $( ) is needed to avoid the error [: too many arguments” error.

Check if string starts with such as "#". 

if [[ "$var" =~ ^#.*  ]]; then
    echo "yes"
fi

Arithmetic/Integer comparison

expr1 -eq expr2  ==> check equal
expr1 -ne expr2  ==> check not equal
expr1 -gt expr2  ==> expr1 > expr2
expr1 -ge expr2  ==> expr1 >= expr2
expr1 -lt expr2  ==> expr1 < expr2
expr1 -le expr2  ==> expr1 <= expr2
! expr  ==> opposite of expr

File conditionals

-d file  ==> True if the file is a directory
-e file  ==> True if the file exists
-f file  ==> True if the file is a regular file
-r file  ==> True if the file is readable
-s file  ==> True if the file has non-zero size
-w file  ==> True if the file is writable
-x file  ==> True if the file is executable

Example 1: Suppose we want to know if the first argument (if given) match a specific string. We can use (note the space before and after '==') 

#!/bin/bash
if [ $1 == "console" ]; then
  echo 'Console'
else
  echo 'Non-console'
fi

Example 2: Check If File Is Empty Or Not Using Shell Script 

#!/bin/bash
_file="$1"
[ $# -eq 0 ] && { echo "Usage: $0 filename"; exit 1; }
[ ! -f "$_file" ] && { echo "Error: $0 file not found."; exit 2; }
 
if [ -s "$_file" ] 
then
	echo "$_file has some data."
        # do something as file has data
else
	echo "$_file is empty."
        # do something as file is empty 
fi

Check if running as root

if [ $UID -ne 0 ];
then
  echo "Run as root"
  exit 1;
fi

Control Structures

if

if condition
then
  statements
elif [ condition ]; then
  statements
else 
  statements
fi

For example, we can run a cp command if two files are different. 

if ! cmp -s "$filesrc" "$filecur"
then
     cp $filesrc $filecur
fi

String Comparison

http://stackoverflow.com/questions/2237080/how-to-compare-strings-in-bash 

answer=no
if [ -f "genome.fa" ]; then
  echo -n 'Do you want to continue [yes/no]: '
  read answer
fi

if [ "$answer" == "no" ]; then
echo AAA
fi

if [ "$answer"=="no" ]; then
# failed if condition
echo BBB
fi
  1. You want the quotes around $answer, because if $answer is empty.
  2. Space in bash is important.
    • Spaces between if and [ and ] are important
    • A space before and after the double equal signs is important all. So if we reply with 'yes', the code still runs 'echo BBB' statement.

while

while condition do
  statements
done

until 

until condition
do 
  statements
done

case

How to Use Case Statements in Bash Scripts

Semicolon

Command1; command2; command3; command4

Every commands will be executed whether the execution is successful or not.

AND list &&

How To Run A Command After The Previous One Has Finished On Linux 

statement1 && statement2 && statement3 && ...

If command1 finishes successfully then run command2. 

touch /tmp/f1
echo "data" >/tmp/f2
[ -s /tmp/f1 ] 
echo $?    # 1
[ -s /tmp/f2 ]
echo $?    # 0

[ -s /tmp/f1 ] && echo "not empty" || echo "empty"  # empty
[ -s /tmp/f2 ] && echo "not empty" || echo "empty"  # not empty

OR list ||

statement1 || statement2 || statement3 || ...

If command1 fails then run command2.

For example, 

codename=$(lsb_release -s -c)
if [ $codename == "rafaela" ] || [ $codename == "rosa" ]; then
  codename="trusty"
fi

Chaining rule (command1 && command2 || command3)

Coupled commands with control operators in Bash

10 Useful Chaining Operators in Linux with Practical Examples.

  • Ampersand Operator (&),
  • semi-colon Operator (;),
  • AND Operator (&&),
  • OR Operator (||),
  • NOT Operator (!),
  • AND – OR operator (&& – ||),
  • PIPE Operator (|),
  • Command Combination Operator {},
  • Precedence Operator (),
  • Concatenation Operator (\).

A combination of ‘AND‘ and ‘OR‘ Operator is much like an ‘if-else‘ statement. 

$ ping -c3 www.google.com && echo "Verified" || echo "Host Down"

How to program with Bash: Syntax and tools 

# command1 && command2
$ Dir=/root/testdir ; mkdir $Dir/ && cd $Dir

# command1 || command2
$ Dir=/root/testdir ; mkdir $Dir || echo "$Dir was not created."

# preceding commands ; command1 && command2 || command3 ; following commands
# "If command1 exits with a return code of 0, then execute command2, otherwise execute command3." 
$ Dir=/root/testdir ; mkdir $Dir && cd $Dir || echo "$Dir was not created."
$ Dir=~/testdir ; mkdir $Dir && cd $Dir || echo "$Dir was not created."

for + do + done

for variable in values
do 
  statements
done

The values can be an explicit list 

i=1
for day in Mon Tue Wed Thu Fri
do
 echo "Weekday $((i++)) : $day"
done

or a variable 

i=1
weekdays="Mon Tue Wed Thu Fri"
for day in $weekdays
do
 echo "Weekday $((i++)) : $day"
done
# Output
# Weekday 1 : Mon
# Weekday 2 : Tue
# Weekday 3 : Wed
# Weekday 4 : Thu
# Weekday 5 : Fri

Note that we should not put a double quotes around $weekdays variable. If we put a double quotes around $weekdays, it will prevent word splitting. See thegeekstuff article. 

i=1
weekdays="Mon Tue Wed Thu Fri"
for day in "$weekdays"
do
 echo "Weekday $((i++)) : $day"
done
# Output
# Weekday 1 : Mon Tue Wed Thu Fri


To loop over all script files in a directory 

FILES=/path/to/PATTERN*.sh
for f in $FILES;
do
(
   "$f"
)&
done
wait

OR 

FILES="
file1
/path/to/file2
/path/to/file3
"
for f in $FILES;
do
(
   "$f"
)&
done
wait

Here we run the script in the background and wait to exit until all are finished.

See loop over files from cyberciti.biz.

Example 1: convert pdfs to tifs using ImageMagick

"for" looping over files, check cyberciti.biz) 

outdir="../plosone"
indir="../fig"

if [[ ! -d  $outdir ]];
then
   mkdir $outdir
fi

in=(file1.pdf file2.pdf file3.pdf)

for (( i=0; i<${#in[@]} ; i++ ))
do
  convert -strip -units PixelsPerInch -density 300 -resample 300 \
          -alpha off -colorspace RGB -depth 8 -trim -bordercolor white \
          -border 1% -resize '2049x2758>' -resize '980x980<' +repage \
          -compress lzw $indir/${in[$i]} $outdir/Figure$[$i+1].tiff
done

Example 2: download with wget and parsing with 'sed'

A second example is to download all the (Ontario gasoline price) data with wget and parsing and concatenating the data with other *nix tools like 'sed': 

# Download data
for i in $(seq 1990 2014)
        do wget http://www.energy.gov.on.ca/fuelupload/ONTREG$i.csv
done

# Retain the header
head -n 2 ONTREG1990.csv | sed 1d > ONTREG_merged.csv

# Loop over the files and use sed to extract the relevant lines
for i in $(seq 1990 2014)
        do
        tail -n 15 ONTREG$i.csv | sed 13,15d | sed 's/./-01-'$i',/4' >> ONTREG_merged.csv
        done

Example 3: download

Download all 20 sra files (60GB in total) from SRP032789. 

for x in $(seq 1027175 1027180) 
   do wget ftp://ftp-trace.ncbi.nlm.nih.gov/sra/sra-instant/reads/ByStudy/sra/SRP/SRP032/SRP032789/SRR$x/SRR$x.sra
done

https://github.com/MarioniLab/EmptyDrops2017/blob/master/data/download_10x.sh 

for x in \
    http://cf.10xgenomics.com/samples/cell-exp/2.1.0/pbmc4k/pbmc4k_raw_gene_bc_matrices.tar.gz \
    http://cf.10xgenomics.com/samples/cell-exp/2.1.0/neurons_900/neurons_900_raw_gene_bc_matrices.tar.gz \
    http://cf.10xgenomics.com/samples/cell-exp/1.1.0/293t/293t_raw_gene_bc_matrices.tar.gz \
    http://cf.10xgenomics.com/samples/cell-exp/1.1.0/jurkat/jurkat_raw_gene_bc_matrices.tar.gz \
    http://cf.10xgenomics.com/samples/cell-exp/2.1.0/t_4k/t_4k_raw_gene_bc_matrices.tar.gz \
    http://cf.10xgenomics.com/samples/cell-exp/2.1.0/neuron_9k/neuron_9k_raw_gene_bc_matrices.tar.gz
do
    wget $x
    destname=$(basename $x) 
    stub=$(echo $destname | sed "s/_raw_.*//")
    mkdir -p $stub
    tar -xvf $destname -C $stub
    rm $destname
done

Example 4: convert files from DOS to Unix

Convert all files from DOS to Unix format 

for f in *.txt; do   tr -d '\r' < $f > tmp.txt;   mv tmp.txt $f  ; done
# Or
for file in $*; do   tr -d '\r' < $f > tmp.txt;   mv tmp.txt $f  ; done

Example 5: print all files in a directory

for f in /etc/*.conf
do
   echo "$f"
done

Example 6: use ping to find all the live machines on the network

for ip in 192.168.0.{1..255} ;
do
  ping $ip -c 2 &> /dev/null ;
  
  if [ $? -eq 0 ];
  then
    echo $ip is alive
  fi

done

Example 7: sed on multiple files

for i in *.htm*; do sed -i 's/String1/String2/' "$i"; done

Note if the string contains special characters like forward slashes (eg https://www.google.com), we need to escape them by using the backslash sign.

Example 8: run in parallel

for ip in 192.168.0.{1..255} ;
do
   (
      ping $ip -c2 &> /dev/null ;
  
      if [ $? -eq 0 ];
      then
       echo $ip is alive
      fi
   )&
  done
wait

where we enclose the loop body in ()&. () encloses a block of commands to run as a subshell and & sends it to the background. wait waits for all background jobs to complete.

Good technique !!!

wait command

  • An example where we shall wait until files are deleted before continuing the script. 
    cd /home/ubuntu

if [ -d "R-devel" ]; then
    rm -rf "R-devel" &
    wait # Wait for the deletion to complete
    echo "R-devel folder deleted successfully."
else
    echo "R-devel folder does not exist."
fi

wget -O - https://stat.ethz.ch/R/daily/R-devel.tar.gz | tar -xzk

cd R-devel
./configure --prefix=/opt/R/devel --enable-R-shlib
make

Functions

#!/bin/bash

fun () { echo "This is a function"; echo; }

fun () { echo "This is a function"; echo } # Error!
 
function quit {
   exit
}

function hello {
   echo Hello!
}

function e {
   echo $1 
}  
$ ./e World

How to find bash shell function source code on Linux/Unix

$ type -a function_name

# To list all function names
$ declare -F
$ declare -F | grep function_name
$ declare -F | grep foo

How do I find the file where a bash function is defined? 

declare -F function_name

Function arguments

source ~/bin/setpath # add bgzip & tabix directories to $PATH

function raw2exon {
  # put your comments here
  inputvcf=$1
  outputvcf=$2
  inputbed=$3
  if [[ $4 ]]; then
    oldpath=$PWD
    cd $4
  fi
  
  bgzip -c $inputvcf > $inputvcf.gz
  tabix -p vcf $inputvcf.gz
  
  head -$(grep '#' $inputvcf | wc -l) $inputvcf > $outputvcf # header
  tabix -R $inputbed $inputvcf.gz >> $outputvcf
  wc -l $inputvcf
  wc -l $outputvcf
  rm $inputvcf.gz $inputvcf.gz.tbi
  if [[ $4 ]]; then
    cd $oldpath
  fi
}           

inputbed=S04380110_Regions.bed

raw2exon 'mu0001_raw.vcf' 'mu0001_exon.vcf' $inputbed ~/Downloads/

Exit function

exit command and the exit statuses 

$ cat testfun.sh
#!/bin/bash
ping -q -c 1 $1 >/dev/null 2>&1
if [ $? -ne 0 ]
then
   echo "An error occurred while checking the server status".
   exit 3
fi

exit 0
$ chmod +x testfun.sh
$ ./testfun.sh www.cyberciti.biz999
An error occurred while checking the server status.
$ echo $?
3

List of commands

break  ==> escaping from an enclosing for, while or until loop
:      ==> null command
continue ==> make the enclosing for, while or until loo continue at the next iteration
.      ==> executes the command in the current shell
eval   ==> evaluate arguments
exec   ==> replacing the current shell with a different program
export ==> make the variable named as its parameter available in subshells
expr   ==> evaluate its arguments as an expression
printf ==> similar to echo
set    ==> sets the parameter variables for the shell. Useful for using fields in commands that output spaced-separated values
shift  ==> moves all the parameter variables down by one.
trap   ==> specify the actions to take on receipt of signals.
unset  ==> remove variables or functions from the environment.
mktemp ==> create a temporary file

Run the previous command

Understanding the exclamation mark (!) in bash 

$ apt update  # Permission denied
$ sudo !!     # Equivalent sudo apt update

"!" invokes history expansion. To run the most recent command beginning with “foo”: 

!foo
# Run the most recent command beginning with "service" as root
sudo !service

Cache console output on the CLI?

Try the ‘’’script’’’ command line utility to create a typescript of everything printed on your terminal.

To exit (to end script session) type ‘’’exit’’’ or logout or press control-D.

set -e, set -x and trap

Exit immediately if a command exits with a non-zero status. Type help set in command line. Very useful!

See also the trap command that is related to non-zero exit.

See

bash -x

Call your script with something like 

bash –x –v hello_world.sh

OR 

#!/bin/bash –x -v
echo Hello World!

where

  • -x displays commands and their results
  • -v displays everything, even comments and spaces

This is the same as using set -x in your bash script.

set -x example

Bash script 

set -ex
export DEBIAN_FRONTEND=noninteractive

codename=$(lsb_release -s -c)
if [ $codename == "rafaela" ] || [ $codename == "rosa" ]; then
  codename="trusty"
fi

echo $codename
echo step 1
echo step 2

exit 0

Without -x option: 

trusty
step 1
step 2

With -x option: 

+ export DEBIAN_FRONTEND=noninteractive
+ DEBIAN_FRONTEND=noninteractive
++ lsb_release -s -c
+ codename=rafaela
+ '[' rafaela == rafaela ']'
+ codename=trusty
+ echo trusty
trusty
+ echo step 1
step 1
+ echo step 2
step 2
+ exit 0

trap and error handler

The syntax to use trap command is 

trap command signal

For example, 

$ cat traptest.sh
#!/bin/sh

trap 'rm -f /tmp/tmp_file_$$' INT
echo creating file /tmp/tmp_file_$$
date > /tmp/tmp_file_$$

echo 'press interrupt to interrupt ...'
while [ -f /tmp/tmp_file_$$ ]; do
  echo file exists
  sleep 1
done
echo the file no longer exists

trap - INT
echo creaing file /tmp/tmp_file_$$
date > /tmp/tmp_file_$$
echo 'press interrupt to interrupt ...'
while [ -f /tmp/tmp_file_$$ ]; do
  echo file exists
  sleep 1
done
echo we never get here
exit 0

will get an output like 

$ ./traptest.sh
creating file /tmp/tmp_file_21389
press interrupt to interrupt ...
file exists
file exists
^Cthe file no longer exists
creaing file /tmp/tmp_file_21389
press interrupt to interrupt ...
file exists
file exists
^C

The first when we use trap, it will delete the file when we hit Ctrl+C. The second time when we use trap, we do not specify any command to be exected when an INT signal occurs. So the default behavior occurs. That is, the final echo and exit statements are never executed.

Note that the following two are different. 

trap - INT
trap '' INT

The second command will IGNORE signals (Ctrl+C in this case) so if we apply this statement above, we will not be able to use Ctrl+C to kill the execution.

DEBUG trap to step through line by line

You can use the "DEBUG" trap to step through a bash script line by line

Bash shell find out if a command exists or not

http://www.cyberciti.biz/faq/unix-linux-shell-find-out-posixcommand-exists-or-not/

POSIX

POSIX built-in commands

# command -v will return >0 when the command1 is not found
command -v command1 >/dev/null && echo "command1 Found In \$PATH" || echo "command1 Not Found in \$PATH"

$ help command
command: command [-pVv] command [arg ...]
    Execute a simple command or display information about commands.
    
    Runs COMMAND with ARGS suppressing  shell function lookup, or display
    information about the specified COMMANDs.  Can be used to invoke commands
    on disk when a function with the same name exists.
    
    Options:
      -p	use a default value for PATH that is guaranteed to find all of
    	the standard utilities
      -v	print a description of COMMAND similar to the `type' builtin
      -V	print a more verbose description of each COMMAND
    
    Exit Status:
    Returns exit status of COMMAND, or failure if COMMAND is not found.

$ type command     
command is a shell builtin
$ type export
export is a shell builtin
$ type wget
wget is /usr/bin/wget
$ type tophat
-bash: type: tophat: not found
$ type sleep
sleep is /bin/sleep

$ command -v tophat
$ command -v wget
/usr/bin/wget

On macOS, 

$ help command
command: command [-pVv] command [arg ...]
    Runs COMMAND with ARGS ignoring shell functions.  If you have a shell
    function called `ls', and you wish to call the command `ls', you can
    say "command ls".  If the -p option is given, a default value is used
    for PATH that is guaranteed to find all of the standard utilities.  If
    the -V or -v option is given, a string is printed describing COMMAND.
    The -V option produces a more verbose description.

type -P

type -P command1 &>/dev/null && echo "Found" || echo "Not Found"

$ help type
type: type [-afptP] name [name ...]
    Display information about command type.
    
    For each NAME, indicate how it would be interpreted if used as a
    command name.
    
    Options:
      -a	display all locations containing an executable named NAME;
    	includes aliases, builtins, and functions, if and only if
    	the `-p' option is not also used
      -f	suppress shell function lookup
      -P	force a PATH search for each NAME, even if it is an alias,
    	builtin, or function, and returns the name of the disk file
    	that would be executed
      -p	returns either the name of the disk file that would be executed,
    	or nothing if `type -t NAME' would not return `file'.
      -t	output a single word which is one of `alias', `keyword',
    	`function', `builtin', `file' or `', if NAME is an alias, shell
    	reserved word, shell function, shell builtin, disk file, or not
    	found, respectively
    
    Arguments:
      NAME	Command name to be interpreted.
    
    Exit Status:
    Returns success if all of the NAMEs are found; fails if any are not found.
typeset: typeset [-aAfFgilrtux] [-p] name[=value] ...
    Set variable values and attributes.
    
    Obsolete.  See `help declare'.

Find all bash builtin commands

https://www.cyberciti.biz/faq/linux-unix-bash-shell-list-all-builtin-commands/ 

$ help
$ help | less
$ help | grep read

Find if a command is internal or external

$ type -a COMMAND-NAME-HERE
$ type -a cd
$ type -a uname
$ type -a :

$ command -V ls
$ command -V cd
$ command -V food

pause by read -p command

http://www.cyberciti.biz/tips/linux-unix-pause-command.html 

read -p "Press [Enter] key to start backup..."

If we want to ask users about a yes/no question, we can use this method 

while true; do
    read -p "Do you wish to install this program? " yn
    case $yn in
        [Yy]* ) make install; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

OR 

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
    case $yn in
        Yes ) make install; break;;
        No ) exit;;
    esac
done

Keyboard input and Arithmetic

http://linuxcommand.org/wss0110.php

read 

#!/bin/bash

echo -n "Enter some text > "
read text
echo "You entered: $text"

Arithmetic 

#!/bin/bash

# An applications of the simple command
# echo $((2+2))
# That is, when you surround an arithmetic expression with the double parentheses, 
# the shell will perform arithmetic evaluation.
first_num=0
second_num=0

echo -n "Enter the first number --> "
read first_num
echo -n "Enter the second number -> "
read second_num

echo "first number + second number = $((first_num + second_num))"
echo "first number - second number = $((first_num - second_num))"
echo "first number * second number = $((first_num * second_num))"
echo "first number / second number = $((first_num / second_num))"
echo "first number % second number = $((first_num % second_num))"
echo "first number raised to the"
echo "power of the second number   = $((first_num ** second_num))"

and a program that formats an arbitrary number of seconds into hours and minutes: 

#!/bin/bash

seconds=0

echo -n "Enter number of seconds > "
read seconds

# use the division operator to get the quotient
hours=$((seconds / 3600))
# use the modulo operator to get the remainder
seconds=$((seconds % 3600))
minutes=$((seconds / 60))
seconds=$((seconds % 60))

echo "$hours hour(s) $minutes minute(s) $seconds second(s)"

xargs

xargs reads items from the standard input, delimited by blanks (which can be protected with double or single quotes or a backslash) or newlines, and executes the command (the default command is echo, located at /bin/echo) one or more times with any initial-arguments followed by items read from standard input.

Example1 - Find files named core in or below the directory /tmp and delete them

find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f

where, -0 If there are blank spaces or characters (including single quote, newlines, et al) many commands will not work. This option take cares of file names with blank space.

Another case: suppose I have a file with filename -sT. It seems not possible to delete it directly with the rm command. 

$ rm "-sT"
rm: invalid option -- 's'
Try 'rm ./-sT' to remove the file ‘-sT’.
Try 'rm --help' for more information.
$ $ ls *T
ls: option requires an argument -- 'T'
Try 'ls --help' for more information.
$ ls "*T"
ls: cannot access *T: No such file or directory
$ ls "*s*"
ls: cannot access *s*: No such file or directory

$ find . -maxdepth 1 -iname '*-sT'
./-sT
$ find . -maxdepth 1 -iname '*-sT' | xargs -0 /bin/rm -f
$ find . -maxdepth 1 -iname '*-sT' | xargs /bin/rm -f   # WORKS

Similarly, suppose I have a file of zero size. The file name is "-f3". I cannot delete it. 

$ ls -lt
total 448
-rw-r--r-- 1 mingc mingc      0 Jan 16 11:35 -f3
$ rm -f3
rm: invalid option -- '3'
Try `rm ./-f3' to remove the file `-f3'.
Try `rm --help' for more information.
$ find . -size  0 -print0 |xargs -0 rm

Example2 - Find files from the grep coammand and sort them by date

grep -l "Polyphen" tmp/*.* | xargs ls -lt

Example3 - Gzip with multiple jobs

CORES=$(grep -c '^processor' /proc/cpuinfo)
find /source -type f -print0 | xargs -0 -n 1 -P $CORES gzip -9

where

  • find -print0 / xargs -0 protects you from whitespace in filenames
  • xargs -n 1 means one gzip process per file
  • xargs -P specifies the number of jobs
  • gzip -9 means maximum compression

GNU Parallel

A simple trick without using GNU Parallel is run the commands in background.

Example: same command, different command line argument

Input from the command line (Synopsis about the triple colon ":::"): 

parallel echo ::: A B C
parallel gzip --best ::: *.html # '--best' means best compression
parallel gunzip ::: *.CEL.gz

Input from a file: 

parallel -a abc-file echo

Input is a STDIN: 

cat abc-file | parallel echo

find . -iname "*after*" | parallel wc -l

Another similar example is to gzip each individual files 



Example: each command containing an index


Instead of



for i in $(seq 1 100)
do
  someCommand data$i.fastq > output$i.txt &
done


, we can use



parallel --jobs 16 someCommand data{}.fastq '>' output{}.txt ::: {1..100}


Example: each command not containing an index


for i in *gz; do 
  zcat $i > $(basename $i .gz).unpacked
done


can be written as



parallel 'zcat {} > {.}.unpacked' ::: *.gz


Example: run several subscripts from a master script


Suppose I have a bunch of script files: script1.sh, script2.sh, ... And an optional master script (file ext does not end with .sh).
My goal is to run them using GNU Parallel.

I can just run them using



parallel './{}' ::: *.sh


where "./" means the .sh files are located in the current directory and {} denotes each individual .sh file.

More detail:



$ mkdir test-par; cd test-par
$ echo echo A > script1.sh
$ echo echo B > script2.sh
$ echo echo C > script3.sh
$ echo echo D > script4.sh
$ chmod +x *.sh

$ cat > script    # master script (not needed for GNU parallel method)
./script1.sh
./script2.sh
./script3.sh
./script4.sh

$ time bash script
A
B
C
D

real	0m0.025s
user	0m0.004s
sys	0m0.004s

$ time parallel './{}' ::: *.sh    # No need of a master script
                                   # may need to add --gnu option if asked.
A
B
C
D

real	0m0.778s
user	0m0.588s
sys	0m0.144s     # longer time because of the parallel overhead


Note


  • When I run scripts (seqtools_vc) sequentially I can get the standard output on screen. However, I may not get these output when I use GNU parallel.
    • 

  • There is a risk/problem if all scripts are trying to generate required/missing files when they detect the required files are absent.


rush - cross-platform tool for executing jobs in parallel


Debugging Scripts



Run a shell script with -x option. Then each lines of the script will be shown on the stdout. We can see which line takes long time or which lines broke the code (it still runs through the script).



$ bash -x script-name



  • Use of set builtin command
    • 

  • Use of intelligent DEBUG function


To run a bash script line by line:




Geany


  • (Ubuntu 12.04 only): By default, it does not have the terminal tab. Install virtual terminal emulator. Run


sudo apt-get install libvte-dev


  • Step 1: Keyboard shortcut. Select a region of code. Edit -> >Commands->Send selection to Terminal. You can also assign a keybinding for this. To do so: go to Edit->Preferences and pick the Keybindings tab. See a screenshot here. I assign F12 (no any quote) for the shortcut. This is a complete list of the keybindings.


  • Step 2: Newline character. Another issue is that the last line of sent code does not have a newline character. So I need to switch to the Terminal and press Enter. The solution is to modify the <geany.conf> (find its location using locate geany.conf. On my ubuntu 14 (geany 1.26), it is under ~/.config/geany/geany.conf) and set send_selection_unsafe=true. See here.
    • 

  • Step 3: PATH variable.


$ tmpname=$(basename $inputVCF)
Command 'basename' is available in '/usr/bin/basename'
The command could not be located because '/usr/bin' is not included in the PATH environment variable.



The solution is to run PATH=$PATH:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin in the Terminal window before running our script.



  • Step 4 (optional): Change background color.


Another handy change to geany is to change its background to black. To do that, go to Edit -> Preferences -> Editor. Once on the Editor options level, select the Display tab to the far right of the dialog, and you will notice a checkbox marked invert syntax highlighting colors.

See this post about changing the default terminal in the Terminal window. The default is xterm (see the output of echo $TERM).



Examples



How to wrap a long linux command


Use backslash character. However, make sure the backslash character is the last character at a line. For example the first example below does not work since there is an extra space character after \.

Example 1 (not work)



sudo apt-get install libcap-dev libbz2-dev libgcrypt11-dev libpci-dev libnss3-dev libxcursor-dev \
   libxcomposite-dev libxdamage-dev libxrandr-dev libdrm-dev libfontconfig1-dev libxtst-dev \ 
   libcups2-dev libpulse-dev libudev-dev



vs example 2 (work)



sudo apt-get install libcap-dev libbz2-dev libgcrypt11-dev libpci-dev libnss3-dev libxcursor-dev \
   libxcomposite-dev libxdamage-dev libxrandr-dev libdrm-dev libfontconfig1-dev libxtst-dev \
   libcups2-dev libpulse-dev libudev-dev



Command line path navigation


pushd and popd are used to switch between multiple directories without the copying nad posting of directory paths. Thy operate on a stack; a last in first out data structure (LIFO).



pushd /var/www
pushd /usr/src
dirs
pushd +2
popd


When we have only two locations, an alternative and easier way is cd -.



cd /usr/src
# Do something
cd /var/www
cd -     # /usr/src


bd – Quickly Go Back to a Parent Directory



Create log file


  • Create a log file with date


logfile="output_$(date +"%Y%m%d%H%M").log"


  • Redirect the error to a log file


logfile="output_$(date +"%Y%m%d%H%M").log"

module load XXX || exit 1

echo "All output redirected to '$logfile'"
set -ex

exec 2>$logfile

# Task 1
start_time=$(date +%s)
# Do something with possible error output
end_time=$(date +%s)
echo "Task 1 Started: tarted: "$start_date"; Ended: "$end_date"; Elapsed time: "$(($end_time - $start_time))" sec">>$logfile

# Task 2
start_time=$(date +%s)
# Do something with possible error output
end_time=$(date +%s)
echo "Task 1 Started: tarted: "$start_date"; Ended: "$end_date"; Elapsed time: "$(($end_time - $start_time))" sec">>$logfile


Text processing


tr (similar to sed)


It seems tr does not take general regular expression.

The tr utility copies the given input to produced the output with substitution or deletion of selected characters. tr abbreviated as translate or transliterate.




It will read from STDIN and write to STDOUT. The syntax is



tr [OPTION] SET1 [SET2]


If both the SET1 and SET2 are specified and ‘-d’ OPTION is not specified, then tr command will replace each characters in SET1 with each character in same position in SET2. For example,



# translate to uppercase
$ echo 'linux' | tr "[:lower:]" "[:upper:]"

# Translate braces into parenthesis
$ tr '{}' '()' < inputfile > outputfile

# Replace comma with line break
$ tr ',' '\n' < inputfile

# Split a long line using the space 
$ echo $line | tr ' ' '\n' 

# Translate white-space to tabs
$ echo "This is for testing" | tr [:space:] '\t'

# Join/merge all the lines in a file into a single line
$ tr -s '\n' ' ' < file.txt  
# note sed cannot match \n easily as tr command. 
# See 
# http://stackoverflow.com/questions/1251999/how-can-i-replace-a-newline-n-using-sed 
# https://unix.stackexchange.com/questions/26788/using-sed-to-convert-newlines-into-spaces


tr can also be used to remove particular characters using -d option. For example,



$ echo "the geek stuff" | tr -d 't'
he geek suff
$ tr -d "\15" < input > output # octal digit 15


A practical example



#!/bin/bash
echo -n "Enter file name : "
read myfile
echo -n "Are you sure ( yes or no ) ? "
read confirmation
confirmation="$(echo ${confirmation} | tr 'A-Z' 'a-z')"
if [ "$confirmation" == "yes" ]; then
   [ -f $myfile ] &&  /bin/rm $myfile || echo "Error - file $myfile not found"
else
   : # do nothing
fi


Second example



$ ifconfig | cut -c-10 | tr -d ' ' | tr -s '\n'
eth0
eth1
ip6tnl0
lo
sit0

# without tr -s '\n'
eth0


eth1


ip6tnl0


lo


sit0


where tr -d ' ' deletes every space character in each line. The \n newline character is squeezed using tr -s '\n' to produce a list of interface names. We use cut to extract the first 10 characters of each line.



Regular Expression and grep



echo -e "today is Monday\nHow are you" | grep Monday

grep -E "[a-z]+" filename
# or
egrep "[a-z]+" filename

grep -i PATTERN FILENAME # ignore case

grep -v PATTERN FILENAME # inverse match

grep -c PATTERN FILENAME # count the number of lines in which a matching string appears

grep -n PATTERN FILENAME # print the line number

grep -R PATTERN DIR      # recursively search many files and follow symbolic links
grep -r PATTERN DIR      # recursively search many files

grep -e "pattern1" -e "pattern2" FILENAME # multiple patterns OR operation (older Linux)
egrep 'pattern1|pattern2' FILENAME        # multiple patterns (newer Linux)
grep -f PATTERNFILE FILENAME # PATTERNFILE contains patterns line-by-line

grep -F PATTERN FILENAME # Interpret PATTERN as a  list  of  fixed  strings,  separated  by
                         # newlines,  any  of  which is to be matched.

grep -r --include \*.Rmd --include \*.R "file\.csv" ./   # search with only Rmd & R files

grep -r --exclude "README" PATTERN DIR               # excluding files in which to search

grep -o \<dt\>.*<\/dt\> FILENAME # print only the matched string (<dt> .... </dt>)

grep -w                  # checking for full words, not for sub-strings
grep -E -w "SRR2923335.1|SRR2923335.1999" # match in words (either SRR2923335.1 or SRR2923335.1999)


  • Extract the IP address from ifconfig command


$ ifconfig eth1
eth1      Link encap:Ethernet  HWaddr 00:14:d1:b0:df:9f  
          inet addr:192.168.1.172  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::214:d1ff:feb0:df9f/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:29113 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:28561660 (28.5 MB)  TX bytes:3516957 (3.5 MB)

$ ifconfig eth1 | egrep -o "inet addr:[^ ]*" | grep -o "[0-9.]*"
192.168.1.172


where egrep -o "inet addr:[^ ]*" will match the pattern starting with inet addr: and ends with some non-space character sequence (specified by [^ ]*). Now in the next pipe, it prints the character combination of digits and '.'.



--include option



Bash Find Out IF a Variable Contains a Substring



grep returns TRUE or FALSE


Can grep return true/false or are there alternative methods



less -S: print long lines


Causes lines longer than the screen width to be chopped rather than folded. man less.



cut: extract columns or character positions from text files


http://www.thegeekstuff.com/2013/06/cut-command-examples/



cut -f 5-7 somefile  # columns 5-7. 
cut -c 5-7 somefile  # character positions 5-7


The default delimiter is TAB. If the field delimiter is different from TAB you need to specify it using -d:



cut -d' ' -f100-105 myfile > outfile
#
cut -d: -f6 somefile   # colon-delimited file
# 
grep "/bin/bash" /etc/passwd | cut -d':' -f1-4,6,7    # field 1 through 4, 6 and 7

cut -f3 --complement somefile # print all the columns except the third column


To specify the output delimiter, we shall use --output-delimiter. NOTE that to specify the Tab delimiter in cut, we shall use $'\t'. See http://www.computerhope.com/unix/ucut.htm. For example, 



cut -f 1,3 -d ':' --output-delimiter=$'\t' somefile


If I am not sure about the number of the final field, I can leave the number off.



cut -f 1- -d ':' --output-delimiter=$'\t' somefile


A simple shell function to show the first 3 columns and 3 rows of the matrix


function show_matrix() {
    if [ -z "$1" ] || [ -z "$2" ]; then
        echo "Usage: show_matrix <filename> <delimiter>"
        return 1
    fi

    if [ "$2" != "tab" ] && [ "$2" != "comma" ]; then
        echo "Delimiter must be 'tab' or 'comma'"
        return 1
    fi

    if [ "$2" == "tab" ]; then
        cut -f1-3 "$1" | head -n 3
    elif [ "$2" == "comma" ]; then
        cut -d',' -f1-3 "$1" | head -n 3
    fi
}
# show_matrix data.txt tab
# show_matrix data.txt comma


awk: operate on rows and/or columns


awk is a tool designed to work with data streams. It can operate on columns and rows. If supports many built-in functionalities, such as arrays and functions, in the C programming language. Its biggest advantage is its flexibility.




Structure of an awk script



awk pattern { action }
awk ' BEGIN{ print "start" } pattern { AWK commands } END { print "end" } ' file


The three of components (BEGIN, END and a common statements block with the pattern match option) are optional and any of them can be absent in the script. The pattern can be also called a condition.

The default delimiter for fields is a space.

Some examples:



awk 'BEGIN { i=0 } { i++ } END { print i}' filename
echo -e "line1\nline2" | awk 'BEGIN { print "start" } { print } END { print  "End" }'

seq 5 | awk 'BEGIN { sum=0; print "Summation:" } { print $1"+"; sum+=$1 } END { print "=="; print sum }'

awk -F : '{print $6}' somefile   # colon-delimited file, print the 6th field (cut can do it)
#
awk --field-searator="\\t" '{print $6}' filename    # tab-delimited (cut can do it)
 
awk -F":" '{ print $1 " " $3 }' /etc/passwd  # (cut can do it)

awk -F "\t" '{OFS="\t"} {$1="mouse"$1; print $0}' genes.gtf > genescb.gtf 
# or
awk -F "\t" 'BEGIN {OFS="\t"} {$1="mouse"$1; print $0}' genes.gtf > genescb.gtf 
# replace ELEMENT with mouseELEMENT for data on the 1st column; tab separator was used for input (-F) and output (OFS)

awk 'NR % 4 == 1 {print ">" $0 } NR % 4 == 2 {print $0}' input > output
# extract rows 1,2,5,6,9,10,13,14,.... from input

awk 'NR % 4 == 0 {print ">" $0 } NR % 4 == 3 {print $0}' input > output
# extract rows 3,4,7,8,11,12,15,16,.... from input 

awk '(NR==2),(NR==4) {print $0}' input
# print rows 2-4.

awk '{ print ($1-32)*(5/9) }'
# fahrenheit-to-celsius calculator, http://www.hcs.harvard.edu/~dholland/computers/awk.html

# http://stackoverflow.com/questions/3700957/printing-lines-from-a-file-where-a-specific-field-does-not-start-with-something
awk '$7 !~ /^mouse/ { print $0 }' input # column 7 not starting with 'mouse'
awk '$7 ~ /^mouse/ { print $0 }' input  # column 7 starting with 'mouse'
awk '$7 ~ /mouse/ { print $0 }' input   # column 7 containing 'mouse'


It seems AWK is useful for finding/counting a subset of rows or columns. It is not most used for string substitution.



Print the string between two parentheses


https://unix.stackexchange.com/questions/108250/print-the-string-between-two-parentheses



$ awk -F"[()]" '{print $2}' file 

$ echo ">gi|52546690|ref|NM_001005239.1| subfamily H, member 1 (OR11H1), mRNA" | awk -F"[()]" '{print $2}'
OR11H1

$ echo ">gi|284172348|ref|NM_002668.2| proteolipid protein 2 (colonic epithelium-enriched) (PLP2), mRNA" | awk -F"[()]" '{print $2}'
colonic epithelium-enriched  # WRONG


Insert a line


https://stackoverflow.com/a/18276534



awk '/KEYWORDS/ { print; print "new line"; next }1' foo.input



Count number of columns in file


https://stackoverflow.com/a/8629351



awk -F'|' '{print NF; exit}' stores.dat  # Change '|' as needed



sed (stream editor): substitution of text



By default, sed only prints the substituted text. To save the changes along the substitutions to the same file, use the -i option.



sed 's/text/replace/' file > newfile
mv newfile file
# OR better
sed -i 's/text/replace/' file


The sed command will replace the first occurrence of the pattern in each line. If we want to replace every occurrence, we need to add the g parameter at the end, as follows:



sed -i 's/pattern/replace/g' file


To remove blank lines



sed '/^$/d' filename


To remove square brackets



# method 1. replace ] & [ by the empty string
$ echo '00[123]44' | sed 's/[][]//g'
0012344
# method 2 - use tr
$ echo '00[123]00' | tr -d '[]'
0012300


To replace all three-digit numbers with another specified word in a file



sed -i 's/\b[0-9]\{3\}\b/NUMBER/g' filename

echo -e "I love 111 but not 1111." | sed 's/\b[0-9]\{3\}\b/NUMBER/g'


where {3} is used for matching the preceding character thrice. \ in \{3\} is used to give a special meaning for { and }. \b is the word boundary marker.

Variable string and quoting



text=hello
echo hello world | sed "s/$text/HELLO/"


Double quoting expand the expression by evaluating it.



sed takes whatever follows the "s" as the separator



Suppose I like to replace "../jquery-ui.min.js" with "jquery-ui.js", I can use



echo '<script src="../jquery-ui.min.js"></script>' | sed 's|../jquery-ui.min.js|jquery-ui.js|g'
# <script src="jquery-ui.js"></script>



$ cat tmp
@SQ	SN:chrX	LN:155270560
@SQ	SN:chrY	LN:59373566
@RG	ID:NEAT
$ sed 's,^@RG.*,@RG\tID:None\tSM:None\tLB:None\tPL:Illumina,g' tmp
@SQ	SN:chrX	LN:155270560
@SQ	SN:chrY	LN:59373566
@RG	ID:None	SM:None	LB:None	PL:Illumina
$ sed 's/^@RG.*/@RG\tID:None\tSM:None\tLB:None\tPL:Illumina/g' tmp
@SQ	SN:chrX	LN:155270560
@SQ	SN:chrY	LN:59373566
@RG	ID:None	SM:None	LB:None	PL:Illumina


Case insensitive


https://www.cyberciti.biz/faq/unixlinux-sed-case-insensitive-search-replace-matching/



# Newer version - add 'i' or 'I' after 'g'
sed 's/find-word/replace-word/gI' input.txt > output.txt
sed -i 's/find-word/replace-word/gI' input.txt

# Older version/macOS
sed 's/[wW][oO][rR][dD]/replace-word/g' input.txt > output.txt
sed 's/[Ll]inux/Unix/g' input.txt > output.txt



macOS


"undefined label" error on Mac OS X



$ sed -i 's/mkyong/google/g' testing.txt 
sed: 1: "testing.txt": undefined label 'esting.txt'

# Solution
$ sed -i '.bak' 's/mkyong/google/g' testing.txt 



Application: Get the top directory name of a tarball or zip file without extract it


dn=`unzip -vl filename.zip | sed -n '5p' | awk '{print $8}'` # 5 is the line number to print
echo -e "$(basename $dn)"

dn=`tar -tf filename.tar.bz2 | grep -o '^[^/]\+' | sort -u`  # '-u' means unique
echo -e $dn

dn=`tar -tf filename.tar.gz | grep -o '^[^/]\+' | sort -u`
echo -e $dn

# Assume there is a sub-directory called htslibXXXX
dn=$(basename `find -maxdepth 1 -name 'htslib*'`)
echo -e $dn


Application: Grab the line number from the 'grep -n' command output


Follow here



grep -n 'regex' filename | sed 's/^\([0-9]\+\):.*$/\1/'  # return line numbers for each matches
# OR
grep -n 'regex' filename | awk -F: '{print $1}'

echo 123:ABCD | sed 's/^\([0-9]\+\):.*$/\1/'             # 123


where \1 means to keep the substring of the pattern and \( & \) are used to mark the pattern. See http://www.grymoire.com/Unix/Sed.html for more examples, e.g. search repeating words or special patterns.

If we want to find the to directory for a zipped file (see wikipedia for the zip format), we can use



unzip -vl snpEff.zip | head | grep -n 'CRC-32' | awk -F: '{print $1}'


Application: Delete first few characters on each row


http://www.theunixschool.com/2014/08/sed-examples-remove-delete-chars-from-line-file.html



  • To remove 1st n characters of every line:


# delete the first 4 characters from each line
$ sed -r 's/.{4}//' file


Application: delete lines


Sed Command to Delete a Line



  • Delete a single line
    • 

  • Delete a range of lines
    • 

  • Delete multiple lines
    • 

  • Delete all lines except specified range
    • 

  • Delete empty lines
    • 

  • Delete lines based on pattern
    • 

  • Delete lines starting with a specific character
    • 

  • Delete lines ending with specific character
    • 

  • Deleting lines that match the pattern and the next line
    • 

  • Deleting line from the pattern match to the end


Application: comment out certain lines


https://unix.stackexchange.com/a/128595. To comment lines 2 through 4 of bla.conf:



sed -i '2,4 s/^/#/' bla.conf



This is useful when I need to comment out line 240 & 242 on shell scripts (related to pdf file) generated from BRB-SeqTools.



Substitution of text: perl



How to delete the first few rows of a text file


https://unix.stackexchange.com/questions/37790/how-do-i-delete-the-first-n-lines-of-an-ascii-file-using-shell-commands

Suppose we want to remove the first 3 rows of a text file



  • sed


$ sed -e '1,3d' < t.txt    # output to screen

$ sed -i -e 1,3d yourfile  # directly change the file


  • tail


$ tail -n +4 t.txt    # output to screen


  • awk


$ awk 'NR > 3 { print }' < t.txt    # output to screen


Delete the last row of a file


sed -i '$d' FILE


Show the first few characters from a text file


head -c 50 file   # return the first 50 bytes


Remove/Delete The Empty Lines In A File


https://www.2daygeek.com/remove-delete-empty-lines-in-a-file-in-linux/



sed -i '/KEYWORD/d' File



cat: merge by rows


cat file1 file2 > output


paste: merge by columns


paste -d"\t" file1 file2 file3 > output

paste file1 file2 file3 | column -s $'\t' > output


Web


Reference: Linux Shell Scripting Cookbook



Copy a complete webiste


wget --mirror --convert-links URL
# OR
wget -r -N -k -l DEPTH URL


HTTP or FTP authentication


wget --user username --password pass URL


Download a web page as plain text (instead of HTML text)


lynx URL -dump > TextWebPage.txt


cURL


curl http://google.com -o index.html --progress
curl http://google.com --silent -o index.html

# Cookies
curl http://example.com --cookie "user=ABCD;pass=EFGH"
curl URL --cookie-jar cookie_file

# Setting a user agent string
# http://www.useragentstring.com/pages/useragentstring.php
curl URL --user-agent "Mozilla/5.0"

# Authenticating 
curl -u user:pass http://test_auth.com
curl -u user http://test_auth.com

# Printing response headers excluding the data
# For example, to check whether a page is reachable or not
# by checking the 'Content-length' parameter.
curl -I URL


Image crawler and downloader


#!/bin/bash
#Desc: Images downloader
#Filename: img_downloader.sh

if [ $# -ne 3 ];
then
  echo "Usage: $0 URL -d DIRECTORY"
  exit -1
fi

for i in {1..4}
do
  case $1 in
  -d) shift; directory=$1; shift ;;
   *) url=${url:-$1}; shift;;
  esac
done

mkdir -p $directory;
baseurl=$(echo $url | egrep -o "https?://[a-z.]+")

echo Downloading $url
curl -s $url | egrep -o "<img src=[^>]*>" | 
sed 's/<img src=\"\([^"]*\).*/\1/g' > /tmp/$$.list

sed -i "s|^/|$baseurl/|" /tmp/$$.list

cd $directory;

while read filename;
do
  echo Downloading $filename
  curl -s -O "$filename" --silent

done < /tmp/$$.list


Find broken links in a website by lynx -traversal


#!/bin/bash 
#Desc: Find broken links in a website

if [ $# -ne 1 ]; 
then 
  echo -e "$Usage: $0 URL\n" 
  exit 1; 
fi 

echo Broken links: 

mkdir /tmp/$$.lynx 
cd /tmp/$$.lynx 

lynx -traversal $1 > /dev/null 
count=0; 

sort -u reject.dat > links.txt 

while read link; 
do 
  output=`curl -I $link -s | grep "HTTP/.*OK"`; 
  if [[ -z $output ]]; 
  then 
    echo $link; 
    let count++ 
  fi 
done < links.txt 

[ $count -eq 0 ] && echo No broken links found.


Track changes to a website


#!/bin/bash
#Desc: Script to track changes to webpage

if [ $# -ne 1 ];
then 
  echo -e "$Usage: $0 URL\n"
  exit 1;
fi

first_time=0
# Not first time

if [ ! -e "last.html" ];
then
  first_time=1
  # Set it is first time run
fi

curl --silent $1 -o recent.html

if [ $first_time -ne 1 ];
then
  changes=$(diff -u last.html recent.html)
  if [ -n "$changes" ];
  then
    echo -e "Changes:\n"
    echo "$changes"
  else
    echo -e "\nWebsite has no changes"
  fi
else
  echo "[First run] Archiving.."

fi
  
cp recent.html last.html


POST/GET


Look at a web site source and look for the 'name' field in a <input> tag.

http://www.w3schools.com/html/html_forms.asp



# -d is used for posting in curl
curl URL -d "postvar1=var1&postvar2=var2"
# OR the 'get' command with the 'post-data' option
get URL --post-data "postvar1=var1&postvar2=var2" -O out.html


Change detection of a website



Working with Files


iconv command



$ file test.R
test.R: ISO-8859 text, with CRLF line terminators
$ iconv -f ISO-8859 -t UTF-8 test.R  # 'ISO-8859' is not supported
$ iconv -t UTF-8 test.R              # partial conversion??
$ iconv -f ISO-8859-1 -T UTF-8 test.R # Works



nl command


Add line numbers to a text file



$ cat demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
This Line Has All Its First Character Of The Word With Upper Case.

Two lines above this line is empty.
And this is the last line.
$ nl demo_file
     1	THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
     2	this line is the 1st lower case line in this file.
     3	This Line Has All Its First Character Of The Word With Upper Case.
       
     4	Two lines above this line is empty.
     5	And this is the last line.


file command


 
$ file thumbs/g7.jpg 
thumbs/g7.jpg: JPEG image data, JFIF standard 1.01, resolution (DPI), density 72x72, segment length 16, Exif Standard: [TIFF image data, little-endian, direntries=10, orientation=upper-left, xresolution=134, yresolution=142, resolutionunit=2, software=Adobe Photoshop CS Windows, datetime=2004:03:31 22:28:58], baseline, precision 8, 100x75, frames 3

$ file index.html
index.html: HTML document, ASCII text

$ file 2742OS_5_01.sh 
2742OS_5_01.sh: Bourne-Again shell script, ASCII text executable

$ file R-3.2.3.tar.gz 
R-3.2.3.tar.gz: gzip compressed data, last modified: Thu Dec 10 03:12:50 2015, from Unix



date


Displaying dates and times your way in Linux



print by skipping rows


http://stackoverflow.com/questions/604864/print-a-file-skipping-x-lines-in-bash



$ tail -n +<N+1> <filename>  # excluding first N lines
                             # print by starting at line N+1.
$ tail -n +11 /tmp/myfile    # starting at line 11, or skipping the first 10 lines


tail -f (follow)


When we use the '-f' (follow) option, we can monitor a growing file. For example, we can create a new file called tmp.txt and run 'tail -f tmp.txt'. Now we open another terminal and run 'for i in {0..100}; do sleep 2; echo $i >> ~/output.txt ; done'. We will see in the 1st terminal that the content of tmp.txt is changed.

A practical example is



  • Monitor system change


sudo tail -f /var/log/syslog


  • Monitor a process and terminate itself when a give process dies


PID=$(pidof Foo)
tail -f textfile --pid $PID


A process Foo (eg. gedit) is appending data to a file, the tail -f should be executed until the process Foo dies.



Low-level File Access


  • file descriptors: 0 means standard input, 1 means standard output, 2 means standard error.
    • 

  • size_t write(int fildes, const void *buf, size_t nbytes);


#include <unistd.h>
#include <stdlib.h>
int main()
{
  if ((write(1, "Here is some data\n", 18)) != 17)
    write(2, "A write error has occurred on file descriptor\n", 46);
  exit(0);
}



  • size_t read(int fildes, void *buf, size_t nbytes); returns the number of data bytes actually read. If a read call returns 0, it had nothing to read; it reached the end of the file. An error on the call will cause it to return -1.
    • 

  • To create a new file descriptor we use the open system call. int open(const char *path, int oflags, mode_t mode);


  • The next program will do file copy.


#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
int main()
{
  char c;
  int in, out;
  in = open("file.in", O_RDONLY);
  out = open("file.out", O_WRONLY|O_CREAT, S_IRUSER|S_IWUSR);
  while(read(in,&c,1) == 1)
    write(out,&c,1)
  exit(0);
}



The Standard I/O Library


  • fopen, fclose
    • 

  • fread, fwrite
    • 

  • fflush
    • 

  • fseek
    • 

  • fgetc, getc, getchar
    • 

  • fputc, putc, putchar
    • 

  • fgets, gets
    • 

  • printf, fprintf and sprintf
    • 

  • scanf, fscanf and sscanf


Formatted Input and Output


  • prinf, fprintf and sprintf
    • 

  • scanf, fscanf and sscanf


Stream Errors



File and Directory Maintenance


Scanning Directories


  • opendir, closedir
    • 

  • readdir
    • 

  • telldir
    • 

  • seekdir


UNIX environment


Logging


Resources and Limits


Terminals


Fun command line utilities


Turn Your Terminal Into A Playground: 20+ Funny Linux Command Line Tools: cowsay, fortune, figlet, sl, ASCIIquarium, cmatrix, lolcat, ponysay, charasay, party parrot, ternimal, paclear, lavat, pond, cbonsai, dotacat, finger, pinky, no more secrets, hollywood, bucklespring, bb, toilet, sl-alt, fetch utilities, telehack, display star wars episode.



Reading from and Writing to the Terminal


The termios Structure


Terminal Output


Detecting Keystokes


Curses


A technique between command line and full GUI. 

Example: vi.



Data Management


Development Tools


Books


Top Linux developers' recommended programming books



GNU Make and Makefiles



Writing a Manual Page


Distributing Software


The patch Program


Debugging


debug a bash shell


How To Debug a Bash Shell Script Under Linux or UNIX



gdb


Processes and Signals


Search a process ID by its name


Use pgrep https://askubuntu.com/questions/612315/how-do-i-search-for-a-process-by-name-without-using-grep. For example (tested on Linux and macOS),



$ pgrep RStudio  # assume RStudio is running
27043
$ pgrep geany     # geany is not running.     
$


POSIX threads


Inter-process Communication: Pipes


Sockets








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