shellscripting.md

Shell Scripting

Script Basics

A shell script is a text file containing commands that the shell executes in sequence. The first line should be a shebang that tells the system which interpreter to use. #!/bin/bash #!/usr/bin/env [bash](/man/bash)

Make the script executable and run it. [chmod](/man/chmod) +x script.sh ./script.sh

Use set options at the top of scripts to catch errors early. set -euo pipefail

-e exits on error, -u treats unset variables as errors, -o pipefail catches failures in pipes.

Variables

Variable assignment has no spaces around the = sign. Use double quotes around variable references to prevent word splitting.

Expression	Description
VAR=value	Assign a value
$VAR	Read the value
${VAR}	Read with explicit boundary
"$VAR"	Read safely with quoting
${VAR:-default}	Use default if VAR is unset or empty
${VAR:=default}	Assign default if VAR is unset or empty
${VAR:+alternate}	Use alternate if VAR is set and not empty
${VAR:?error msg}	Exit with error if VAR is unset or empty
${#VAR}	Length of the value
readonly VAR	Make variable read-only
export VAR	Make variable available to child processes

Special Variables

These are set automatically by the shell and are read-only.

Variable	Description
$0	Name of the script
$1..$9	Positional parameters (arguments)
${10}	Positional parameters beyond 9
$#	Number of arguments
$@	All arguments as separate words
$*	All arguments as a single string
$?	Exit status of the last command
$$	PID of the current shell
$!	PID of the last background command
$_	Last argument of the previous command

String Operations

Bash provides built-in string manipulation without needing external commands.

Expression	Description
${VAR#pattern}	Remove shortest match from start
${VAR##pattern}	Remove longest match from start
${VAR%pattern}	Remove shortest match from end
${VAR%%pattern}	Remove longest match from end
${VAR/old/new}	Replace first occurrence
${VAR//old/new}	Replace all occurrences
${VAR:offset}	Substring from offset
${VAR:offset:length}	Substring from offset with length
${VAR^}	Uppercase first character
${VAR^^}	Uppercase all characters
${VAR,}	Lowercase first character
${VAR,,}	Lowercase all characters

Conditionals

Use [[ ]] for conditionals in bash scripts. It supports pattern matching and is safer than the older [ ] form. Always put spaces inside the brackets. if [[ -f "file.txt" ]]; then [echo](/man/echo) "exists"; fi

Operator	Description
-f file	True if file exists and is a regular file
-d file	True if file exists and is a directory
-e file	True if file exists (any type)
-r file	True if file is readable
-w file	True if file is writable
-x file	True if file is executable
-s file	True if file is not empty
-z string	True if string is empty
-n string	True if string is not empty
==	String equality
!=	String inequality
=~	Regex match (inside [[ ]])
-eq	Numeric equality
-ne	Numeric inequality
-lt	Numeric less than
-le	Numeric less than or equal
-gt	Numeric greater than
-ge	Numeric greater than or equal

if [[ $count -gt 0 ]]; then [echo](/man/echo) "positive"; elif [[ $count -eq 0 ]]; then [echo](/man/echo) "zero"; else [echo](/man/echo) "negative"; fi

The case statement matches a value against patterns. case "$1" in start) [echo](/man/echo) "Starting";; stop) [echo](/man/echo) "Stopping";; *) [echo](/man/echo) "Usage: $0 {start|stop}";; esac

Loops

The for loop iterates over a list of items. for file in *.txt; do [echo](/man/echo) "$file"; done for i in {1..10}; do [echo](/man/echo) "$i"; done for ((i=0; i<10; i++)); do [echo](/man/echo) "$i"; done

The while loop runs as long as the condition is true. while [read](/man/read) -r line; do [echo](/man/echo) "$line"; done < file.txt

The until loop runs until the condition becomes true. until [[ -f "ready.flag" ]]; do [sleep](/man/sleep) 1; done

Use break to exit a loop early and continue to skip to the next iteration.

Functions

Functions group reusable commands. Arguments are accessed with $1, $2, etc. inside the function body. Use local to keep variables scoped to the function. greet() { local name="$1"; [echo](/man/echo) "Hello, $name"; } greet "World"

A function returns its last command's exit status, or use return to set an explicit exit code (0-255). To return strings, use command substitution. get_date() { [date](/man/date) +%Y-%m-%d; } today=$(get_date)

Arrays

Bash supports indexed arrays. Declare and manipulate them as follows.

Expression	Description
arr=(a b c)	Declare an array
arr[0]=value	Set element by index
${arr[0]}	Access element by index
${arr[@]}	All elements
${#arr[@]}	Number of elements
${arr[@]:1:2}	Slice: 2 elements starting at index 1
arr+=(d e)	Append elements
${!arr[@]}	All indices
unset arr[1]	Remove an element

Associative arrays require explicit declaration. declare -A map map[key]="value" [echo](/man/echo) "${map[key]}"

Arithmetic

Use $(( )) for arithmetic expressions. All standard operators are available.

Expression	Description
$((a + b))	Addition
$((a - b))	Subtraction
$((a * b))	Multiplication
$((a / b))	Integer division
$((a % b))	Modulo
$((a ** b))	Exponentiation
$((a++))	Post-increment
$((++a))	Pre-increment
let "a += 5"	Arithmetic assignment

Input and Output

Use read to get input from the user or from a file. [read](/man/read) -p "Enter name: " name [read](/man/read) -s -p "Password: " pass [read](/man/read) -r -a items <<< "a b c"

Use printf for formatted output — it is more portable than echo. [printf](/man/printf) "Name: %s, Age: %d\n" "$name" "$age"

A here document passes multi-line text to a command. [cat](/man/cat) <<EOF

Lines of text go here. Variables like $HOME are expanded.

EOF

Use <<'EOF' (quoted) to prevent variable expansion inside the here document.

Exit Codes and Traps

Every command returns an exit code: 0 means success, anything else means failure. Use exit to set the script's return code. exit 0 exit 1

Use trap to run cleanup code when the script exits or receives a signal. trap '[rm](/man/rm) -f /tmp/mylock' EXIT trap '[echo](/man/echo) "Interrupted"' INT TERM

Combine set options for robust scripts.

Option	Description
set -e	Exit immediately on error
set -u	Error on undefined variables
set -o pipefail	Pipe fails if any command fails
set -x	Print each command before executing (debug)