Bash scripting

Bash script is used to make linux utilities and in makefile to automate your task.

Without wasting anymore time let’s begin the bash quickstart.

💡Note: First line of every script file contains #!/bin/bash which tells os to use bash | zsh | fish shell. (It is not require though but it is generally good practice to include it)

Creating a bash script file

• touch <filename>.sh → Used to create bash file.
• chmod +x <filename>.sh → Make it executable (by default it is only in read and write mode
• ./<filename>.sh → used to execute the file and it shows output in terminal

comments

# single line comment

# multi line comment starts with << followed by any word that is used to close multi line comment
<<comment-start
your comment
comment-start

variables

# declaring variable
VAR_NAME="some_value" # white space are not allowed after and before '='

# storing command output
LIST_DIR=$(ls -la) # we can execute any command between '$()'

# accessing variable

echo $LIST_DIR
echo $VAR_NAME

Constant

# declaring constants
readonly PI=3.14

readonly PIE=3.14 # readonly variable

echo "The value of PIE is $PIE"

PIE=20 # Throws an error

Taking Input from User

We use read keyword for reading input from user

# directly asking for input
read NAME
echo "Entered name is $NAME"

# asking with prompt string
read -p "Enter your age " AGE
echo "Entered age is $AGE"

Conditional statement

Conditional statement in bash scripting is little bit weird compared to other languages

1. If-else statement

   # if
   if [ condition ]; then
       # code
   fi
   
   # if else
   if [ condition ]; then
       # code
   else
       # else code
   fi
   

2. if-else ladder

   if [ condition1 ]; then
     # commands to execute if condition1 is true
   elif [ condition2 ]; then
     # commands to execute if condition2 is true
   else
     # commands to execute if all conditions are false
   fi
   

Example

# if else
NAME="arshil"
if [ "$NAME" = "arshil" ]; then
    echo "Name is arshil"
else
    echo "Name is not arshil"
fi

# if elif else
read -p "Enter your age: " AGE

if [ $AGE -lt 18 ]; then
    echo "There is a long way to go :)"
elif [ $AGE -ge 18 ] && [ $AGE -le 60 ]; then
    echo "Enjoy your life :)"
else
    echo "Life is about to end :("
fi

Different types of comparison operator

Numeric comparison operator:

Operator	Description	Example
-eq	Equal to	[ $a -eq $b ]
-ne	Not equal to	[ $a -ne $b ]
-lt	Less than	[ $a -lt $b ]
-le	Less than or equal to	[ $a -le $b ]
-gt	Greater than	[ $a -gt $b ]
-ge	Greater than or equal to	[ $a -ge $b ]

String comparison operator:

Operator	Description	Example
=	Equal to	[ "$a" = "$b" ]
!=	Not equal to	[ "$a" != "$b" ]
<	Less than, in ASCII alphabetical order	[ "$a" < "$b" ]
>	Greater than, in ASCII alphabetical order	[ "$a" > "$b" ]
-z	True if the string is empty	[ -z "$a" ]
-n	True if the string is not empty	[ -n "$a" ]

File comparison operator:

Operator	Description	Example
-e	True if the file exists	[ -e file.txt ]
-f	True if the file is a regular file (not a directory)	[ -f file.txt ]
-d	True if the file is a directory	[ -d /path/to/dir ]
-r	True if the file is readable	[ -r file.txt ]
-w	True if the file is writable	[ -w file.txt ]
-x	True if the file is executable	[ -x script.sh ]
-s	True if the file has a size greater than zero	[ -s file.txt ]
-nt	True if file1 is newer than file2	[ file1 -nt file2 ]
-ot	True if file1 is older than file2	[ file1 -ot file2 ]

File operations

if [ -f "script.sh" ] && [ -r "script.sh" ] && [ -w "script.sh" ] && [ -x "script.sh" ]; then
    echo "script.sh exists and it is readable, writable and executable"
else
    echo "script.sh does not exists"
fi

Switch case

Though we don’t use it often, it can sometimes be very handy

echo "Enter your choice"
echo "n = neofetch"
echo "g = gdu"

read choice

case $choice in
    n) neofetch;;
    g) gdu;;
    *) echo "Invalid choice"
esac

Loops

It’s syntax looks similar to rust like loops

for i in {start..end}
do
    # code to perform
done

Example

for i in {1..5}
do
    echo "Number is $i"
done

for i in Arshil Hapani
do
    echo "Word is $i"
done

# we can also use for i in 1 3 3 5 6...

Reading file using for loop

for ch in $(cat hello.txt)
do
    echo $ch
done

While loop

while [ condition ]
do
    # code
done

Example

I=1
while [ $I -le 5 ]
do
    echo "Number is $I"
    let I++
done

Until loop

I don’t think there is need of such loop. It is similar to while until it executes while it’s condition is false. Sounds confusing, right? It checks for false condition while while loop checks for true condition to execute current iteration.

I=1
until [ $I -gt 5 ]
do
    echo "Number is $I"
    let I++
done

Break and continue

for i in {1..10}
do
    if [ $i -eq 4 ]; then
        continue
    elif [ $i -eq 7 ]; then
        break
    fi
    echo "Number is $i"
done

Expressions

In bash we cannot use expressions directly like we do in other languages. We need to use let keyword followed by expression

let <expression>

Example

I=10
J=20

let J-=I+90
echo $J

let I+=J*10
echo $I

Can you guess the output?

Accessing script name

echo "Script name is $0"

---

Functions

Creating simple function

function name {
    echo "Called name"
}
# or
name1() {
    echo "called name1"
}

# calling function
name
name1

Passing arguments

function myFn {
    NAME=$1 # arg1
    SURNAME=$2 # arg2

    echo "Hello Mr. $NAME $SURNAME"
}

# passing argument
myFn Arshil Hapani # direct args passing

Passing argument while running (program parameters)

Symbol	Description
$#	Displays number of arguments
$@	Displays all arguments
$1 $2…	To use the argument

echo "Number of arguments $#"
echo "Args = $@"

echo "First args: $1"
echo "Second args: $2"

Bash variable

Some of the global bash variable which we can use to make our life easier.

H

Here is the comprehensive table describing essential bash variable:

Variable	Description	Example Usage
$0	The name of the script currently being executed.	echo $0
$1, $2, ...	The first, second, and subsequent positional parameters (arguments to the script).	echo $1
$#	The number of positional parameters passed to the script or function.	echo $#
$@	All positional parameters as separate quoted strings.	echo $@
$*	All positional parameters as a single string.	echo $*
$?	The exit status of the last command executed (0 for success, non-zero for failure).	echo $?
$$	The process ID (PID) of the current shell or script.	echo $$
$!	The PID of the last background process.	some_command & echo $!
$USER	The current logged-in user’s username.	echo $USER
$HOME	The home directory of the current user.	echo $HOME
$PATH	A colon-separated list of directories where executable files are searched.	echo $PATH
$PWD	The current working directory.	echo $PWD
$OLDPWD	The previous working directory.	echo $OLDPWD
$IFS	Internal Field Separator used by the shell to determine word boundaries.	echo $IFS
$RANDOM	Generates a random integer between 0 and 32767.	echo $RANDOM
$SECONDS	The number of seconds the script has been running.	echo $SECONDS
$LINENO	The current line number in the script.	echo $LINENO

Some useful keywords

• sleep → Used to make script sleep
• exit → To terminate the script
• logger → used to write logs in /var/messages file