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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