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Useful Guide for Bash Scripting

Creating bash file:

touch bashfile.sh

To execute bash file run it like this:

./bashfile.sh

Note: Since ".sh" files are sensitive that's why their permission is off by default.

To give it permission write:

chmod +x bashfile.sh

edit your "bashfile.sh" by using gedit and write: echo "Hello World" then execute it...

Note: gedit is not already install, To install it write apt-get install gedit

Linux Terminal Commands run on bash,Try writing "ls" in bashfile.sh and execute it:

If you want to rewrite last command type !!

Comment's in Bash Sometimes we want to write notes in the file but don't want to show or execute it, For that we use comments:

Single-Line Comments: #This is single-line comment

Multiline Comments:

<<COMMENT

  These are
  multi-line 
  comments

COMMENT

To check current directory type pwd

Reading file

First, create a text file Then write something in it

Now to read the file you can use:

cat textfile.txt

To display few lines (Such as first two) you can use: head -n 2 textfile.txt

where -n is the number of line from the top

Same as that if you want to view last 2 lines you can use: tail -n 2 textfile.txt

where -n is number of line (from the last)

To declare variable in bash use myvar=hello

Remember no gaps between it and to call variable type dollar sign before variable and echo it: echo $myvar

Aliases

alias is a shortcut to reference a command. It can be used to avoid typing long commands So instead of writing this: ls -l -a -h -t You can write this: alias lsss="ls -l -a -h -t" and use it like this: lsss To remove alias you can use this: unalias lsss

To check all alias use command "printenv"

Change Prompt

You can change prompt setting by using:

export PS1="\ncommand here> "

You can also use color in prompt:

export PS1="\e[1;31m\nCODE: \e[39m"

Setting won't be same forever it will change to default as you close the terminal to make them permanent change ".bashrc"

Note: Don't edit anything unless you know what you are doing

I have edit my .bashrc and add the path of my art:

Now whenever i open my terminal it will show the art:

Download file using command line:

wget \

http://releases.ubuntu.com/18.10/ubuntu-18.10-desktop-amd64.iso

You call also use curl for this:

curl \

http://releases.ubuntu.com/18.10/ubuntu-18.10-desktop-amd64.iso \

--output ubuntu.iso

Did you notice you can give multi line command by using \ at the end You should try:

Create tar file:

tar -cf ziped-file.tar myfile

Unzip tar file:

tar -xf ziped-file.tar

Redirection:

Consider there is a long file you want to read by scrolling by using pipe "|" you can do this:

cat textfile.txt | more

Note: Press Enter to scroll...

Redirect output on specific file:

cat textfile1.txt > textfile2.txt

Using this command you are overwriting textfile1.txt into textfile2.txt

If you don't want to overwrite the file you can copy text with:

cat textfile1.txt >> textfile2.txt

To print in new line use /n:

printf "1\n2\n3\n"

Sorting in printf:

sort <(printf "5\n3\n2")

Try using this and guess how it work's:

printf "Salam %s, I'm %s" Ali Umer

Redirection with error doesn't work

python hello.py > a.txt

Can't Redirect Log: To solve this problem you can use this:

python hello.py 2> error.log

Array:

car=('BMW' 'Toyota' 'Honda')

car[2]='Hello' #Declare

unset car[2] #Remove car second value

echo "${car[@]}" #Display all car value

echo "${car[0]}" #Display car first value

echo "${car[1]}" #Display car second value

echo "${car[2]}" #Display car third value

echo "${!car[@]}" #Value of Index (Format 1)

echo "${#car[@]}" #Value of Index (Format 2)

Function's:

function funcName()

{

echo "Hello"

}

funcName

You can also try:

function funcCheck()

{

rv="Salam"

echo "$rv"

}

funcCheck

Conditional Statement:

count=10 #Declare Variable

if [ $count -eq 10 ] #Give Condition

then #If its true

echo "Condition is true"

else

echo "Condition is False"

fi

Try using -ne:

count=10 #Declare Variable

if [ $count -ne 10 ] #Give Condition ne stands for "Not Equals"

then #If its true

echo "Condition is true"

else

echo "Condition is False"

fi

You can also use < and > sign:

count=10 #Declare Variable

if (( $count > 10 )) #Give Condition ne stands for "Not Equals"

then #If its true

echo "Condition is true"

else

echo "Condition is False"

fi

Elif command:

count=10 #Declare Variable

if (( $count > 10 )) #Give Condition ne stands for "Not Equals"

then #If its true

echo "Condition is true"

elif (( $count > 5))

then

echo "Hello Bird !!!"

else

echo "Condition is False"

fi

And Operator:

age=5 #Declare Variable

if [ "$age" -gt 18 ] && [ "$age" -lt 40 ]

then

echo "Welcome"

else

echo "Noooo"

fi

OR Operator:

age=5 #Declare Variable

if [ "$age" -gt 18 -o "$age" -lt 40 ] #You can also use || instead of -o

then

echo "Welcome"

else

echo "Noooo"

fi

for loop:

for i in 1 2 3 4 5

do

echo $i

done

Try also using:

one=Hello

two=My

three=Name

four=is

five=1337

for i in $one $two $three $four $five

do

echo $i

done

for loop (Count)

for i in {1..20} #Print from 1 to 20

do

echo $i

done

for i in {a..z} #Print from a to z (Lower Case)

do

echo $i

done

for i in {A..Z} #Print from A to Z (Upper Case)

do

echo $i

done

Another thing you can do:

for i in {0..20..2} # count from 0 to 20 with the gap of 2

do

echo $i

done

standardize way to write for loop in bash:

for (( i=0; i<5; i++ ))

do

echo $i

done

i=0 is basic declaration

i<5 is a condition where it will stop

i++ is a action on per step

A file reading script:

echo "Enter File to Create"

read filename #Ask for a file to take as input

if -f "$filename" # The -f will check that either file exist or not...

then

echo "Text to Append" #Take user file to put in text...

read filetext

echo "$filetext" >> $filename #Redirect text in file

else

echo "$filename don't exist" #If file doesn't exist

fi

You can also make script like this:

echo "Enter File to read"

read filename

if -f "$filename"

then

  rm $filename

  echo "File Deleted !!!"

else

  echo "$filename don't exist"

fi

On conditional statement you can use these condition's:

-e FILE Exists

-r FILE Readable

-h FILE Symlink

-d FILE Directory

-w FILE Writable

-s FILE Size is > 0 bytes

-f FILE File

-x FILE Executable

FILE1 -nt FILE2 1 is more recent than 2

FILE1 -ot FILE2 2 is more recent than 1

FILE1 -ef FILE2 Same files

-z STRING Empty string

-n STRING Not empty string

STRING == STRING Equal

STRING != STRING Not Equal

NUM -eq NUM Equal

NUM -ne NUM Not equal

NUM -lt NUM Less than

NUM -le NUM Less than or equal

NUM -gt NUM Greater than

NUM -ge NUM Greater than or equal

STRING =~ STRING Regexp

(( NUM < NUM )) Numeric conditions

-o noclobber If OPTIONNAME is enabled

! EXPR Not

X && Y And

X || Y Or

Some math:

See the picture:

Another Way:

n1=4

n2=20

echo $(($n1 + $n2 ))

echo $(( $n1 + $n2 ))

echo $(( $n1 * $n2 ))

echo $(( $n1 / $n2 ))

echo $(( $n1 % $n2 ))

Taking user input with script: See the image

To check your current privilege use:

whoami

To check all users:

cat /etc/passwd

To add new user:

sudo useradd syedumerqadri

To remove any user:

sudo userdel syedumerqadri

Note: "cat /etc/passwd | grep syedumerqadri" is used to check specific user

To check all groups use command:

cat /etc/group

To add a new group:

groupadd mygroup

To delete any group:

groupdel mygroup

Permission:

chmod -x myfile.txt #To remove permission

chmod +x myfile.txt #To give permission

Color's cheat sheet:

https://misc.flogisoft.com/bash/tip_colors_and_formatting

For random number use:

$((RANDOM%=200)) # Random number from 0 to 200

Another way to apply math:

a=50 echo $((a + 200))

Replace first occurrence of to in most recent command:

!!:s/<FROM>/<TO>/

To check your command history use:

history

Globe Cheat Sheet: Globe is just simply wildcard !!!

ls hel #List all elements contain words "hel"

ls .*hello #To view hidden file (You can also use "ls *hello")

ls hello.*g?? # ? can be any word

ls ???? #List all with 4 word

ls hello.*g[ab] # last can be either hello.aga or hello.agb

ls report-200[1-9].txt #To list reports from 2001 to 2009

echo [A-Z]*berry.??? #To find

echo [ABCDEFG]*berry.??? #To find

Disable recursive file creation:

set -o noclobber

( It will avoid overlay files (echo "hi" > foo) Give error when creating a file which already exist)

Dictionary Cheat Sheet

declare -A sounds

sounds[dog]="bark"

sounds[cow]="moo"

sounds[bird]="tweet"

sounds[wolf]="howl"

echo ${sounds[dog]} # Dog's sound

echo ${sounds[@]} # All values

echo ${!sounds[@]} # All keys

echo ${#sounds[@]} # Number of elements

unset sounds[dog] # Delete dog

for val in "${sounds[@]}"; do

echo $val

done

for key in "${!sounds[@]}"; do

echo $key

done

Change case of Variable:

STR="HELLO WORLD!"

echo ${STR,} #=> "hELLO WORLD!" (lowercase 1st letter)

echo ${STR,,} #=> "hello world!" (all lowercase)

STR="hello world!"

echo ${STR^} #=> "Hello world!" (uppercase 1st letter)

echo ${STR^^} #=> "HELLO WORLD!" (all uppercase)

Parameter Expansion:

name="Umer"

echo ${name}

echo ${name/U/u}    #=> "umer" (substitution)

echo ${name:0:2}   #=> "Um" (slicing)

echo ${name::2}     #=> "Um" (slicing)

echo ${name::-1}    #=> "Ume" (slicing)

echo ${name:(-1)}   #=> "r" (slicing from right)

echo ${name:(-2):1} #=> "e" (slicing from right)

echo ${food:-Cake}  #=> $food or "Cake"

STR="Hello world"

echo ${STR:6:5}   # "world"

echo ${STR:-5:5}  # "world"

${#FOO} #To find length

Brace Expansion:

echo {A,B}.js

echo {1..5}

echo {a..z}

Some CLI Gadget's:

cal #calender

date #date

bc #calculator

Open specific directory in GUI:

xdg-open

To check who is logged in use:

who

w (w is a more detailed who, showing who’s logged in)

Some text processing:

uniq file.txt   #To remove repeating  and just display one

sort file.txt #Sort

diff a b	# diff will report which lines differ between two files

cmp a b  	#reports which bytes differ between two files