Shell scripting tutorial

Getting started

Introduction

This is a quick reference khổng lồ getting started with Bash scripting.

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Example

#!/usr/bin/env bashNAME="John"echo "Hello $NAME!"

Variables

NAME="John"echo $NAMEecho "$NAME"echo "$NAME!"

String quotes

NAME="John"echo "Hi $NAME" #=> Hi Johnecho "Hi $NAME" #=> Hi $NAME

Shell execution

echo "I"m in $(pwd)"echo "I"m in `pwd`"# SameSee Command substitution

Conditional execution

git commit && git pushgit commit || echo "Commit failed"

Functions

get_name() echo "John"echo "You are $(get_name)"See: Functions

Conditionals

if << -z "$string" >>; then echo "String is empty"elif << -n "$string" >>; then echo "String is not empty"fiSee: Conditionals

Strict mode

set -euo pipefailIFS=$" "See: Unofficial bash strict mode

Brace expansion

echo A,B.js

ExpressionDescription

A,B	Same as A B
A,B.js	Same as A.js B.js
1..5	Same as 1 2 3 4 5

See: Brace expansion

Parameter expansions

Basics

name="John"echo $nameecho $name/J/j #=> "john" (substitution)echo $name:0:2 #=> "Jo" (slicing)echo $name::2 #=> "Jo" (slicing)echo $name::-1 #=> "Joh" (slicing)echo $name:(-1) #=> "n" (slicing from right)echo $name:(-2):1 #=> "h" (slicing from right)echo $food:-Cake #=> $food or "Cake"length=2echo $name:0:length #=> "Jo"See: Parameter expansion

STR="/path/to/foo.cpp"echo $STR%.cpp # /path/to/fooecho $STR%.cpp.o # /path/to/foo.oecho $STR%/* # /path/toecho $STR##*. # cpp (extension)echo $STR##*/ # foo.cpp (basepath)echo $STR#*/ # path/to/foo.cppecho $STR##*/ # foo.cppecho $STR/foo/bar # /path/to/bar.cppSTR="Hello world"echo $STR:6:5 # "world"echo $STR: -5:5 # "world"SRC="/path/to/foo.cpp"BASE=$SRC##*/ #=> "foo.cpp" (basepath)DIR=$SRC%$BASE #=> "/path/to/" (dirpath)

Substitution

CodeDescription

$FOO%suffix	Remove suffix
$FOO#prefix	Remove prefix
$FOO%%suffix	Remove long suffix
$FOO##prefix	Remove long prefix
$FOO/from/to	Replace first match
$FOO//from/to	Replace all
$FOO/%from/to	Replace suffix
$FOO/#from/to	Replace prefix

Comments

# Single line comment: "This is amulti linecomment"

Substrings

ExpressionDescription

$FOO:0:3	Substring (position, length)
$FOO:(-3):3	Substring from the right

Length

ExpressionDescription

$#FOO	Length of $FOO

Manipulation

STR="HELLO WORLD!"echo $STR, #=> "hELLO WORLD!" (lowercase 1st letter)echo $STR,, #=> "hello world!" (all lowercase)STR="hello world!"echo $STR^ #=> "Hello world!" (uppercase 1st letter)echo $STR^^ #=> "HELLO WORLD!" (all uppercase)

Default values

ExpressionDescription

$FOO:-val	$FOO, or val if unset (or null)
$FOO:=val	Set $FOO to val if unset (or null)
$FOO:+val	val if $FOO is set (and not null)
$FOO:?message	Show error message & exit if $FOO is unset (or null)

Omitting the : removes the (non)nullity checks, e.g. $FOO-val expands to val if unset otherwise $FOO.

Loops

Basic for loop

for i in /etc/rc.*; bởi echo $idone

C-like for loop

for ((i = 0 ; i

Ranges

for i in 1..5; do echo "Welcome $i"doneWith step sizefor i in 5..50..5; bởi vì echo "Welcome $i"done

Reading lines

cat file.txt | while read line; bởi vì echo $linedone

Forever

while true; vì chưng ···done

Functions

Defining functions

myfunc() echo "hello $1"# Same as above (alternate syntax)function myfunc() echo "hello $1"myfunc "John"

Returning values

myfunc() local myresult="some value" echo $myresultresult="$(myfunc)"

Raising errors

myfunc() return 1if myfunc; then echo "success"else echo "failure"fi

Arguments

ExpressionDescription

$#	Number of arguments
$*	All positional arguments (as a single word)
$	All positional arguments (as separate strings)
$1	First argument
$_	Last argument of the previous command

Note: $
và $* must be quoted in order to perform as described.Otherwise, they vì chưng exactly the same thing (arguments as separate strings).

See Special parameters.

Conditionals

Conditions

Note that << is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.

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ConditionDescription

<< -z STRING >>	Empty string
<< -n STRING >>	Not empty string
<< STRING == STRING >>	Equal
<< STRING != STRING >>	Not Equal
<< NUM -eq NUM >>	Equal
<< NUM -ne NUM >>	Not equal
<< NUM -lt NUM >>	Less than
<< NUM -le NUM >>	Less than or equal
<< NUM -gt NUM >>	Greater than
<< NUM -ge NUM >>	Greater than or equal
<< STRING =~ STRING >>	Regexp
(( NUM	Numeric conditions

More conditionsConditionDescription

<< -o noclobber >>	If OPTIONNAME is enabled
<< ! EXPR >>	Not
<< X && Y >>	And
<< X || Y >>	Or

File conditions

ConditionDescription

<< -e tệp tin >>	Exists
<< -r tệp tin >>	Readable
<< -h tệp tin >>	Symlink
<< -d tệp tin >>	Directory
<< -w tệp tin >>	Writable
<< -s file >>	Size is > 0 bytes
<< -f file >>	File
<< -x file >>	Executable
<< FILE1 -nt FILE2 >>	1 is more recent than 2
<< FILE1 -ot FILE2 >>	2 is more recent than 1
<< FILE1 -ef FILE2 >>	Same files

Example

# Stringif << -z "$string" >>; then echo "String is empty"elif << -n "$string" >>; then echo "String is not empty"else echo "This never happens"fi# Combinationsif << X && Y >>; then ...fi# Equalif << "$A" == "$B" >># Regexif << "A" =~ . >>if (( $a if << -e "file.txt" >>; then echo "file exists"fi

Arrays

Defining arrays

Fruits=("Apple" "Banana" "Orange")Fruits<0>="Apple"Fruits<1>="Banana"Fruits<2>="Orange"

Working with arrays

echo $Fruits<0> # Element #0echo $Fruits<-1> # Last elementecho $Fruits<
> # Number of elementsecho $#Fruits # String length of the 1st elementecho $#Fruits<3> # String length of the Nth elementecho $Fruits<
>"; vì chưng echo $idone

Dictionaries

Defining

declare -A soundssounds="bark"sounds="moo"sounds="tweet"sounds="howl"Declares sound as a Dictionary object (aka associative array).

Working with dictionaries

echo $sounds # Dog"s soundecho $sounds<
> # Number of elementsunset sounds # Delete dog

Iteration

Iterate over valuesfor val in "$sounds<
>"; do echo $keydone

Options

Options

set -o noclobber # Avoid overlay files (echo "hi" > foo)set -o errexit # Used khổng lồ exit upon error, avoiding cascading errorsset -o pipefail # Unveils hidden failuresset -o nounset # Exposes unset variables

Glob options

shopt -s nullglob # Non-matching globs are removed ("*.foo" => "")shopt -s failglob # Non-matching globs throw errorsshopt -s nocaseglob # Case insensitive globsshopt -s dotglob # Wildcards match dotfiles ("*.sh" => ".foo.sh")shopt -s globstar # Allow ** for recursive matches ("lib/**/*.rb" => "lib/a/b/c.rb")Set GLOBIGNORE as a colon-separated danh mục of patterns to lớn be removed from globmatches.

History

Commands

CommandDescription

history	Show history
shopt -s histverify	Don’t execute expanded result immediately

Expansions

ExpressionDescription

!$	Expand last parameter of most recent command
!*	Expand all parameters of most recent command
!-n	Expand nth most recent command
!n	Expand nth command in history
!	Expand most recent invocation of command

Operations

CodeDescription

!!	Execute last command again
!!:s///	Replace first occurrence of lớn in most recent command
!!:gs///	Replace all occurrences of to in most recent command
!$:t	Expand only basename from last parameter of most recent command
!$:h	Expand only directory from last parameter of most recent command

!! & !$ can be replaced with any valid expansion.

Slices

CodeDescription

!!:n	Expand only nth token from most recent command (command is 0; first argument is 1)
!^	Expand first argument from most recent command
!$	Expand last token from most recent command
!!:n-m	Expand range of tokens from most recent command
!!:n-$	Expand nth token to lớn last from most recent command

!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc.

Miscellaneous

Numeric calculations

$((a + 200)) # showroom 200 to $a$(($RANDOM%200)) # Random number 0..199

Subshells

(cd somedir; echo "I"m now in $PWD")pwd # still in first directory

Redirection

python hello.py > output.txt # stdout to lớn (file)python hello.py >> output.txt # stdout to lớn (file), appendpython hello.py 2> error.log # stderr lớn (file)python hello.py 2>&1 # stderr to lớn stdoutpython hello.py 2>/dev/null # stderr to lớn (null)python hello.py &>/dev/null # stdout & stderr khổng lồ (null)python hello.py

Inspecting commands

command -V cd#=> "cd is a function/alias/whatever"

Trap errors

trap "echo Error at about $LINENO" ERRor

traperr() echo "ERROR: $BASH_SOURCE<1> at about $BASH_LINENO<0>"set -o errtracetrap traperr ERR

Case/switch

case "$1" in start | up) vagrant up ;; *) echo "Usage: $0 ssh" ;;esac

Source relative

source "$0%/*/../share/foo.sh"

printf

printf "Hello %s, I"m %s" Sven Olga#=> "Hello Sven, I"m Olgaprintf "1 + 1 = %d" 2#=> "1 + 1 = 2"printf "This is how you print a float: %f" 2#=> "This is how you print a float: 2.000000"

Directory of script

DIR="$0%/*"

Getting options

while << "$1" =~ ^- && ! "$1" == "--" >>; vị case $1 in -V | --version ) echo $version exit ;; -s | --string ) shift; string=$1 ;; -f | --flag ) flag=1 ;;esac; shift; doneif << "$1" == "--" >>; then shift; fi

Heredoc

cat

Reading input

echo -n "Proceed? : "read ansecho $ansread -n 1 ans # Just one character

Special variables

ExpressionDescription

$?	Exit status of last task
$!	PID of last background task
$$	PID of shell
$0	Filename of the shell script
$_	Last argument of the previous command

See Special parameters.

Go to previous directory

pwd # /home/user/foocd bar/pwd # /home/user/foo/barcd -pwd # /home/user/foo

Check for command’s result

if ping -c 1 google.com; then echo "It appears you have a working mạng internet connection"fi

Grep check

if grep -q "foo" ~/.bash_history; then echo "You appear lớn have typed "foo" in the past"fi

Also see