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Quick reference to Bash Scripting

This article will help you understand and structure Bash Scripting

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Azher Khan
·Aug 13, 2022·

4 min read

Quick reference to Bash Scripting

Photo by Markus Spiske on Unsplash

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Table of contents

Introduction

Bash Scripting Basics

The Shebang (below) goes on the first line of every Bash script

#!/bin/bash

# is used to make comments, text that comes after it will not be executed

# This line is a comment

Give test-script.sh executable permissions and execute it

chmod +x test-script.sh && ./test-script.sh

Stores the number of arguments passed to the Bash script

$#

Variables that store the values passed as arguments to the Bash script

$1, $2, $3

Exit from the Bash script, optionally add an error code

exit

Keyboard combination to stop Bash script in the middle of execution

Ctrl + C

Execute a command inside of a subshell

$( )

Pause for a specified number of seconds, minutes, hours, or days

sleep

Special Variables

$0        The program or script name
$1-$9     The first nine parameters
$$        The PID for the current process.
$#        The number of passed parameters
$* or $@  List all the passed parameters
$?        most recent foreground pipeline exit status
$!        PID of the most recent background command
$-        current options set for the shell.

Set Options

set -eux        (-e fail if any subcommands fail | -u fail if unknown variables referenced | -x debug)
set -o pipefail (fail if any part of pipe command fails)

Conditional statements

We use if or case to allow us to check if a certain condition is true or false. The logical flow of script will depend on the answer.

Test a condition and execute the then clause if it is true

if then fi

Execute the then clause if the condition is true, otherwise execute the else clause

if then else fi

Test multiple conditions and execute whichever clause is true

if then elif else fi

Script for case statement:

#!/bin/bash

day=$(date +"%A")

case $day in

  Monday | Tuesday | Wednesday | Thursday | Friday)
    echo "Today is a Weekday :)"
    ;;

  Saturday | Sunday)
    echo "Today is the Weekend :D"
    ;;

  *)
    echo "date not Recognised"
    ;;
esac

Script for if then else:

#!/bin/bash
if [ $1 -gt 0 ]; then
  FAILURE=true
  echo "STATUS:  FAIL"
else
  echo "STATUS:  PASS"
fi

Bash Loops

Loops in bash, allow the script to continue executing a set of instructions as long as a condition is said to be true.

Continue to loop for a defined number of range, lines, files, etc

for do done

Continue to loop until a certain condition is met

until do done

Continue to loop as long as certain condition is true

while do done

Exit the loop and continue to the next part of the Bash script

break

Exit the current iteration of the loop but continue to run the loop

continue

Read User Input

Prompt the user for information to enter by using read command:

#!/bin/bash
read -p "Please Enter your name: " person
echo "Hello $person, hope you're having a great day!"

Arithmetic Operators

You can perform addition, subtraction, multiplication, division, and other basic arithmetic inside of a Bash script.

+           Addition
-           Subtraction
*           Multiplication
/           Division
%           Modulus
**          Raise to a power
((i++))     Increment a variable
((i--))     Decrement a variable

Arithmetic Conditional Operators

You can evaluate if a certain condition is true or false by using two numbers.

[ $1 -lt $2 ]       [[ $1 < $2 ]]
[ $1 -gt $2 ]       [[ $1 > $2 ]]
[ $1 -le $2 ]       [[ $1 <= $2 ]]
[ $1 -ge $2 ]       [[ $1 >= $2 ]]
[ $1 -eq $2 ]       [[ $1 == $2 ]]
[ $1 -ne $2 ]       [[ $1 != $2 ]]

String Comparison Operators

This operator is used to evaluate a string. empty or not, and to check if a string is equal, less, or greater in length to another string.

Strings are equal

[ $str1 =    $str2 ]

String are not equal

[ $str1 != $str2 ]

String1 less then String2

[ $str1 <    $str2 ] less then

String1 greater then String2

[ $str1 >    $str2 ]

String1 is not empty

[ -n $str1 ]

String1 is empty

[ -z $str1 ]

Boolean Operators

Boolean operators include and operator &&, or operator || and not equal to !. These operators allow us to test if two or more conditions are true or not.

&&    Logical AND operator
||        Logical OR operator
!        NOT equal to operator

Bash File Evaluation

We can evaluate different characteristics about a file or directory.

-f filename    Check for regular file existence not a directory
-e filename    Check for file existence
-r filename    Check if file is a readable
-w filename    Check if file is writable
-x filename    Check if file is executable
-s filename    Check if file is nonzero size
-b filename    Block special file
-c filename    Special character file
-d dirname    Check for directory existence
-g filename    true if file exists and is set-group-id.
-k filename    Sticky bit
-L filename    Symbolic link
-u filename    Check if file set-ser-id bit is set
 
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