Bash

Basics

Source: ss64.com,learnxinyminutes.com

POSIX

• POSIX stands for Portable Operating System Interface, and is an IEEE standard designed to facilitate application portability.
• POSIX is an attempt by a consortium of vendors to create a single standard version of UNIX.
• If they are successful, it will make it easier to port applications between hardware platforms.

SH

• sh (or the Shell Command Language) is a programming language described by the POSIX standard.
• It has many implementations (ksh88, dash, ...). bash can also be considered an implementation of sh (see below).
• Because sh is a specification, not an implementation, /bin/sh is a symlink (or a hard link) to an actual implementation on most POSIX systems.

BASH

• bash started as an sh-compatible implementation (although it predates the POSIX standard by a few years), but as time passed it has acquired many extensions.
• Many of these extensions may change the behavior of valid POSIX shell scripts, so by itself bash is not a valid POSIX shell.
• Rather, it is a dialect of the POSIX shell language.
• bash supports a --posix switch, which makes it more POSIX-compliant.
• It also tries to mimic POSIX if invoked as sh.

Variables

• Declaring

$0             # Name of the Bash script.
$1 - $9        # First 9 arguments to the Bash script.
$@             # All the arguments supplied to the Bash script, {$1, $2, $3 ...}.
$*             # IFS expansion of all positional parameters, $1 $2 $3 ....
$#             # How many arguments were passed to the Bash script.
$-             # Current options set for the shell.
$$             # The process ID of the current script.
$_             # Most recent parameter (or the abs path of the command to start the current shell immediately after startup).
$IFS           # Input field separator.
$?             # The exit status of the most recently run process.
$!             # PID of the most recent background command.
$USER          # The username of the user running the script.
$HOSTNAME      # The hostname of the machine the script is running on.
$SECONDS       # The number of seconds since the script was started.
$RANDOM        # Returns a different random number each time is it referred to.
$LINENO        # Returns the current line number in the Bash script.

• Using Variables in Bash:

dir="/data/.folder/"
path="/home/system/Desktop/pending_files.txt"
tree $dir | wc -l >> $path

• Correct way to declare variable

Variable="Some string"

• Wrong way to declare Varibales:

Variable = "Some string"        # Bash will decide that Variable is a command it must execute
Variable= 'Some string'         # Bash will decide that 'Some string' is a command it must execute

• Print first 10 characters:

echo ${text:0:10}

Builtin variables

echo "Last program's return value: $?"
echo "Script's PID: $$"
echo "Number of arguments passed to script: $#" 
echo "All arguments passed to script: $@"
echo "Script's arguments separated into different variables: $1 $2..."

Array

array0=(one two three four five six)   # Declare an array with 6 elements
echo $array0                           # Print first element => "one"
echo ${array0[0]}                      # Print first element => "one"  
echo ${array0[@]}                      # Print all elements => "one two three four five six"
echo ${#array0[@]}                     # Print number of elements => "6"
echo ${#array0[2]}                     # Print number of characters in third element => "5"    
echo ${array0[@]:3:2}                  # Print 2 elements starting from fourth => "four five"

• Print all elements. Each of them on new line.

 for i in "${array0[@]}"; do
    echo "$i"
 done

Brace Expansion

echo {1..10}
echo {a..z}

• Display Even Numbers

for number in {2..99..2}         # 2 is the low end, 99 is the high end, 2 is the step after each iteration.
do
    echo $number
done

Builtin variables

Both commands are quivalent

echo "I'm in $(pwd)"                  # execs `pwd` and interpolates output
echo "I'm in $PWD"                    # interpolates the built in `$PWD` variable

Reading Input

echo "What's your name?"
read Name                             # Note we didn't declare a variable
echo Hello, $Name!

Expressions

Let

let a=5+4
echo $a          # 9

let "a = 5 + 4"
echo $a          # 9

let a++
echo $a          # 10

let "a = 4 * 5"
echo $a          # 20

let "a = $1 + 30"
echo $a          # 30 + first command line argument for script

Expr

expr 5 + 4          # 9

expr "5 + 4"        # 5 + 4

expr 5+4            # 5+4

expr 5 \* $1        # 60 ( 5*first command line argument for script[12])

expr 11 % 2         # 1

a=$( expr 10 - 3 )
echo $a             # 7

Double Parentheses

echo $(( 10 + 5 ))   # 15

a=$(( 4 + 5 ))
echo $a              # 9

a=$((3+5))
echo $a              # 8

b=$(( a + 3 ))
echo $b              # 11

b=$(( $a + 4 ))
echo $b              # 12

(( b++ ))
echo $b              # 13

(( b += 3 ))
echo $b              # 16

a=$(( 4 * 5 ))
echo $a              # 20

Length of a Variable

a='Hello World'
echo ${#a}       # 11

b=4953
echo ${#b}       # 4

Read Files

Contents=$(cat file.txt)
echo "START OF FILE\n$Contents\nEND OF FILE"

Use subshells to work across directories

 (echo "First, I'm here: $PWD") && (cd someDir; echo "Then, I'm here: $PWD")
 # First, I'm here: /home/test
 # Then, I'm here: /home/test/scripts
 pwd                  # still in first directory

Case Statements

case "$Variable" in
    #List patterns for the conditions you want to meet
    0) echo "There is a zero.";;
    1) echo "There is a one.";;
    *) echo "It is not null.";;
esac

Loops

• General Syntax:

for Variable in {1..3}
do
    echo "$Variable"
done

• Traditional for loop way:

for ((a=1; a <= 3; a++))
do
    echo $a
done

• Act on files:

for i in file1 file2
do
    cat "$i"
done

• Output from a command

for i in $(ls)
do
    cat "$i"
done

• While loop:

while [ true ]
do
    echo "This is loop..."
    break
done

Functions

• Declaration:

function foo ()
{
    echo "Arguments work just like script arguments: $@"
    echo "And: $1 $2..."
    echo "This is a function"
    return 0
}

foo arg1 arg2

• Another way:

bar ()
{
    echo "Another way to declare functions!"
    return 0
}

bar

Misc

Trap executes a command whenever script receives a signal (here any of the three listed signals):

trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM

Sudo is used to run commands as the superuser:

NAME1=$(whoami)
NAME2=$(sudo whoami)
echo "Was $NAME1, then became more powerful $NAME2"

Bash Scripts

One Liners

• Script to find file extensions

 find $dir -type f | perl -ne 'print $1 if m/\.([^.\/]+)$/' | sort -u

• Convert files with name "webp" file to Png:

find . -name "*.webp" -exec dwebp {} -o {}.png \;
find . -name "*.webp" -delete

</syntaxhighlight>

• Create Multiple Directories with single command:

 mkdir ep{1..16}
 mkdir -p sa{0..10}/{1,2,3,4}
 mkdir -p sa{0..10}/{1..4}

• Download multiple files at once:

 wget https://imagedump.com/i/1/234{1..100}.jpg
 wget -i list.txt

• Saving text use echo to continue on same line:

 echo -n "Count: " >> files.txt
 find . | wc -l >> files.txt

• Word count for directories

 IFS=$'\n'; for i in `find . -type d`; do echo $i; ls $i|wc -l; done; unset IFS

• Find Lengths of multiple video files:

 IFS=$'\n'; for i in `find . -name "*.mp4"`; do mediainfo $i | grep "Duration" | awk 'NR == 1 {print i $3$4,$5$6 $7$8}'; done; unset IFS

Small Scripts

Extract multiple tar files to sub-directories

 for i in `find /home/test/ -type f -name '*.tar.gz'`; do echo $i; j=$(echo $i | cut -d '.' -f1); mkdir $j; tar xvzf $i -C $j; done

Tar Multiple files at once

 for i in `find $(pwd) -type f`; do echo $i; j=$(echo $i | cut -d '.' -f1); echo $j; tar cvzf $"$j.tar.gz" $i; done

Checking Internet connectivity

#!/bin/bash

if ping -c 1 -W 2 google.com > /dev/null; then
 echo "Up"
else
 echo "Down"
fi

Mplayer Play Live Gurbani from 8 to 10AM

 #!/bin/bash
 trickle -d 20 -u 15 mplayer mms://sgpc.net/live &
 sleep 2h
 kill $!

Cron Entry:

0 8 * * * ./mplayer.sh

Download Newspaper front page

URL:

http://newspaper.ajitjalandhar.com/newspages/20130226/20130226_1_1.jpg

 #!/bin/bash
 cd ~/Desktop
 D=$(date +"%Y%m%d")
 FILE="$D"_1_1.jpg
 if [ -f $FILE ];
 then
    exit
 else
    wget "http://newspaper.ajitjalandhar.com/newspages/"$D"/"$D"_1_1.jpg" && notify-send -i /data/Softwares/Wallpapers/icons/ajit.png -t 50 "Ajit Jalandhar" "is saved at your desktop"
 fi

Cron Entry to download daily:

0 7-11 * * * ./ajit.sh

Eye strain preventer

#!/bin/bash
PATH=/usr/games:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

while true; do sleep 1200; sm --foreground=white --background=black Look at an Object 20 feet away for 20 Seconds & sleep 20s && killall sm; done

Add following line to startup programs:

/home/aman/Scripts/eyesaver.sh >/dev/null 2>&1

Extension Doctor

This script will overwrite any existing file.

for f in *.{jpg,JPG,png,PNG,jpeg,JPEG,webp}; do 
    type=$( file "$f" | grep -oP '\w+(?= image)' )
    case $type in  
        PNG)  newext=png ;; 
        JPEG) newext=jpg ;;
        P) newext=webp ;;
        *)    echo "??? what is this: $f"; continue ;; 
    esac
    ext=${f##*.}   # remove everything up to and including the last dot
    if [[ $ext != $newext ]]; then
        # remove "echo" if it's working correctly
        echo mv "$f" "${f%.*}.$newext"
    fi
done

Big Scripts

Directory Stats

#!/bin/bash
#Script to export file/directory stats to a file.

# Path Variables:
dir="/data/"
file="/home/aman/Desktop/pending_files.txt"

# Script starts here:
echo "###########################">> $file

# Date of execution:
date >> $file && echo " ">> $file

# File Stats:
echo -n "No of Files: ">> $file
find $dir -type f | wc -l >> $file && echo " ">> $file

echo -n "No of Directories: ">> $file
find $dir -type d | wc -l >> $file && echo " ">> $file

# Directory size:
echo "Sub directory size: ">> $file
du -h --max-depth=1 $dir | sort -h >> $file && echo " ">> $file

# Huge Files:
echo "100 MB+ Files: ">> $file
find $dir -size +100M -exec ls -sh {} \; | sort -r >> $file && echo " ">> $file

# Image file extension counts:
echo "No of Image Files: ">> $file
echo -n "JPG : " >> $file
find $dir -iname "*.jpg" | wc -l >> $file

echo -n "PNG : " >> $file
find $dir -iname "*.png" | wc -l >> $file

echo -n "GIF : " >> $file
find $dir -iname "*.gif" | wc -l >> $file

echo -n "JPEG: ">> $file
find $dir -iname "*.jpeg" | wc -l >> $file

echo -n "WEBP: ">> $file
find $dir -iname "*.webp" | wc -l >> $file && echo " ">> $file

# Video file extension counts:
echo "No of Video Files: ">> $file
echo -n "WEBM: ">> $file
find $dir -iname "*.webm" | wc -l >> $file

echo -n "MPG : ">> $file
find $dir -iname "*.mpg" | wc -l >> $file

echo -n "3GP : ">> $file
find $dir -iname "*.3gp" | wc -l >> $file

echo -n "DAT : ">> $file
find $dir -iname "*.dat" | wc -l >> $file

echo -n "MP4 : ">> $file
find $dir -iname "*.mp4" | wc -l >> $file

echo -n "AVI : ">> $file
find $dir -iname "*.avi" | wc -l >> $file

echo -n "FLV : ">> $file
find $dir -iname "*.flv" | wc -l >> $file && echo " ">> $file


echo "###########################">> $file
exit

References

• Reference Guide: gnu.org
• Check Scripts for errors: shellcheck.net

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