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

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

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--))

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

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

Loops

Iterate over the number range:

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

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