Bash

From Network Security Wiki


Basics

Source: ss64.com,learnxinyminutes.com

Variables

  • Declaring
$1, $2, $3    are the positional parameters.
"$@"          is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
"$*"          is the IFS expansion of all positional parameters, $1 $2 $3 ....
$#            is the number of positional parameters.
$-            current options set for the shell.
$$            pid of the current shell (not subshell).
$_            most recent parameter (or the abs path of the command to start the current shell immediately after startup).
$IFS          is the (input) field separator.
$?            is the most recent foreground pipeline exit status.
$!            is the PID of the most recent background command.
$0            is the name of the shell or shell 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

  • Declare an array with 6 elements
array0=(one two three four five six)
  • Print first element
echo $array0 # => "one"
  • Print first element
echo ${array0[0]} # => "one"
  • Print all elements
echo ${array0[@]} # => "one two three four five six"
  • Print number of elements
echo ${#array0[@]} # => "6"
  • Print number of characters in third element
echo ${#array0[2]} # => "5"
  • Print 2 elements starting from forth
echo ${array0[@]:3:2} # => "four five"
  • Print all elements. Each of them on new line.
 for i in "${array0[@]}"; do
    echo "$i"
 done

Brace Expansion

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

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!

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
  • 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

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
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





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