Create a Bash Script

Create a file named example.sh 

touch example.sh

By convention, bash script has a .sh file extension, but it works without it.

Add shebang

Shell script starts with shebang #! following by the bash shell path.

For example, use which bash to find the path of bash shell, add #! /usr/bin/bash to example.sh and then add commands etc.

#! /usr/bin/bash 
echo "Hello World!"

Make the script executable

chmod +x example.sh

Run the script

$./hello.sh       
Hello World!

Basic Syntax of Bash Scripting

Variables

Define a variable with foo=bar and reference a variable with $foo.

Numerical expressions

$((expression)), for example var=$((1+2)).

Operator Usage
+ addition
- subtraction
* multiplication
/ division
** exponentiation
% modulus

Numerical comparison

Operation Syntax
a == b a -eq b
a >= b a -ge b
a > b a -gt b
a <= b a -le b
a < b a -lt b
a != b a -ne b

Conditional statements

  • if ... then ... fi
  • if ... then ... else ... fi
  • if ... then ... elif ... then ... else ... fi
if [[condition]]
then
    statement
elif [[condition]]
then
    statement
else
    statement
fi

Looping

for loop

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

for color in red yellow green
do
    echo $color
done

while loop

i=0
while [[ $i -le 10]]
do
    echo "$i"
    (( i += 1 ))
done

Read user input

read foo, with a message read -p "Enter your name" foo.

Read file

LINE=1

while read -r CURRENT_LINE
do
    echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "my_file.txt"

Execute commands

|| or operator / && and operator, can be used to conditionally execute commands based on exit codes.

;, commands can be separated in the same line with ;.

Get the output of a command

$(CMD), <(CMD), var=`CMD`

echo "Now is $(date)"

diff <(foo) <(bar)

var=`ls -a | grep foo`
echo $var

Run with arguments

Run with ./example.sh arg1 arg2

for i in $@
do
    echo "Entered arg is $i"
done
Syntax Meaning
$0 Name of the script
$1 to $9 Arguments of the script, $1 is the first argument etc.
$@ All the arguments
$# Number of arguments
$? Return code of the previous command
$$ Process identification number (PID) for the current script
!! Entire last command
$_ Last argument from the last command

Automate scripts via cron jobs

* * * * *, represents minute/hour/day/month/weekday

# cron job
* * * * * sh /path/to/example.sh