bash.md

title	Bash shell
description	Bourne-Again SHell, an `sh`-compatible command-line interpreter.
created	2021-12-19
updated	2021-12-19

Sample program

#!/usr/bin/env bash

echo 'Hello from Bash!'

Comments

Declare the comment:

# my comment

I/O commands

Print the string with a trailing \n:

echo 'Hello from Bash!'
# or
printf '%s\n' 'Hello from Bash!'

Read the string to a variable:

read my_variable

• Echo Command in Linux with Examples
• Bash printf Command

Variables

Defining and erasing

Declare the global/local variable:

declare my_variable='Hello from Bash!'
local my_variable='Hello from Bash!'

Remove the variable:

unset my_variable

Define Variables using export

export my_variable='Hello from Bash!'

Remove variable defined by export

export -n my_variable

Slicing

Slice the variable:

echo ${my_variable:1:10}
echo ${my_variable:2}
echo ${my_variable:0:-2}

Arithmetic

Incrementing and decrementing

Increment/decrement the variable:

((my_variable++))
((my_variable--))

• How to Increment and Decrement Variable in Bash (Counter)

Integer manipulations

Calculate the number sum:

echo $((1 + 2))

Operator	Performs
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulo
**	Exponentiation

Float manipulations

Calculate the number sum:

awk 'BEGIN { print 1.0 + 2.0 }'

Operator	Performs
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulo
**	Exponentiation

• Awk Command in Linux with Examples

String manipulation

Matching

Match the string against a regular expresion:

[[ 'Hello from Bash!' =~ 'Bash' ]]

Extended regular expressions described.

Pattern	Matches
x?	Zero or one x chars
x*	Any count x chars
x+	One or more x chars
x{n}	n times x chars
x{n,m}	n to m times x chars
x{n,}	n or more times x chars
[xy]	x or y char
[^xy]	not x or y char

Modifying

Remove the shortest/longest matching pattern from beginning:

echo ${my_variable#*=}
echo ${my_variable##*=}

Remove the shortest/longest matching pattern from ending:

echo ${my_variable%*=}
echo ${my_variable%%*=}

Replace the first/all matching pattern:

echo ${my_variable/Bash/bash}
echo ${my_variable//Bash/bash}

Conditionals

Compare two variables:

if [[ $my_variable -lt $another_variable ]]; then
  ···
elif [[ $my_variable -eq $another_variable ]]; then
  ···
else
  ···
fi

Integer operator	Meaning
-lt	[L]ess [t]han
-eq	[Eq]ual
-gt	[G]reater [t]han
-le	[L]ess than or [e]qual to
-ge	[G]reater than or [e]qual to
-ne	[N]ot [E]qual

String operator	Meaning
==	[Eq]ual
!=	[N]ot [E]qual

File operator	Meaning
-f	[F]ile exists
-d	[D]irectory exists
-r	File or directory exists and [r]eadable
-w	File or directory exists and [w]ritable
-x	File or directory exists and e[x]ecutable

• Bash if..else Statement

Loops

Iterate over the number range:

for i in {1,10}; do
  ...
done

• Bash For Loop

Process communication

Files

Write the string with a trailing \n to a file:

echo 'Hello from Bash!' > my_file

Write (append) the string with a trailing \n to a file:

echo 'Hello from Bash!' >> my_file

Piping

Pass the first command stdout output as an input to a second command:

my_command | another_command 

Command substitution

Replace the command invocation with its stdout output:

echo $(expr $my_variable + 1)

Process substitution

Replace the command invocation with a temporary file name with a command stdout output:

echo <(expr $my_variable + 1)

Functions

Defining and erasing

Declare the function:

my_function() {
  ···
}

Remove the function:

unset my_function

• Bash Functions

Arrays

Defining arrays:

Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

Iteration:

for i in "${arrayName[@]}"; do
  echo $i
done