Shell Scripting Guide (Bash)

Comprehensive reference for Bash shell scripting: variables, control flow, functions, error handling, debugging, and best practices.

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A guide to Bash scripting by topic with examples: variables and expansion, control flow, functions, error handling, file operations, arrays and maps, debugging and best practices.

Use it when

You are writing or reviewing a shell script.
You need a safe idiom for a common scripting task.

Watch out for

Bash-specific features (arrays, [[ ]]) are not available in POSIX sh; set the shebang accordingly.
Use set -euo pipefail and quote variables, and consider a different language for complex logic.

The data

Sections

Topic

Examples

Variables and Expansion

Name	Code
Simple assignment	name="John" echo "Hello, $name"
Command substitution	current_dir=$(pwd) echo "We are in $current_dir"
Default values	echo "${PORT:-8080}" # prints 8080 if PORT unset echo "${PORT:=8080}" # sets PORT to 8080 if unset echo "${PORT:?Error: PORT required}" # exits if unset
String manipulation	str="hello_world.txt" echo "${str%.}" # hello_world (remove shortest suffix) echo "${str##.}" # txt (remove longest prefix) echo "${str/_/-}" # hello-world.txt (replace first _)

Control Flow

Name	Code
If statement	if [ -f "$file" ]; then echo "File exists" elif [ -d "$file" ]; then echo "Is a directory" else echo "Not found" fi
For loop	for item in "${array[@]}"; do echo "Processing: $item" done # Or with range for i in {1..10}; do echo "Count: $i" done
While loop	while IFS= read -r line; do echo "Line: $line" done < "input.txt"
Case statement	case "$1" in start) echo "Starting...";; stop) echo "Stopping...";; restart) echo "Restarting...";; *) echo "Usage: $0 {start\|stop\|restart}"; exit 1;; esac

Functions

Name	Code
Define and call	greet() { local name="$1" echo "Hello, $name!" } greet "World" # Hello, World!
Return values	is_even() { local num=$1 return $(( num % 2 )) } if is_even 42; then echo "Even" else echo "Odd" fi
Output capture	get_date() { date "+%Y-%m-%d" } today=$(get_date) echo "Today is $today"

Error Handling

Name	Code
Exit on error	set -e # Exit immediately on error set -u # Error on undefined variables set -o pipefail # Fail pipeline on first error # Combined set -euo pipefail
Trap signals	cleanup() { echo "Cleaning up..." rm -f /tmp/tempfile exit } trap cleanup EXIT SIGINT SIGTERM
Check command success	if ! command -v docker &> /dev/null; then echo "Docker is not installed" exit 1 fi

File Operations

Name	Code
Read file line by line	while IFS= read -r line; do echo "$line" done < "input.txt"
Check file conditions	if [ -f "$file" ]; then echo "regular file"; fi if [ -d "$dir" ]; then echo "directory"; fi if [ -x "$bin" ]; then echo "executable"; fi if [ -z "$str" ]; then echo "empty string"; fi if [ -n "$str" ]; then echo "non-empty"; fi
Temp files	tempfile=$(mktemp) echo "Writing to $tempfile" # ... use it ... rm -f "$tempfile"

Arrays and Maps

Name	Code
Indexed array	fruits=("apple" "banana" "cherry") echo "${fruits[0]}" # apple echo "${#fruits[@]}" # length 3 echo "${fruits[@]}" # all elements
Associative array (Bash 4+)	declare -A config config["host"]="localhost" config["port"]=8080 echo "${config[host]}" # localhost

Debugging

Name	Code
Enable debug mode	set -x # Print commands and their arguments as executed # ... debug code ... set +x # Disable debug # Or run script with: # bash -x script.sh
Check syntax without running	bash -n script.sh # Returns silently if syntax is valid
ShellCheck static analysis	# Install: apt install shellcheck / brew install shellcheck # Run: shellcheck script.sh # Catches: unquoted variables, missing shebang, common pitfalls

Best Practices

Name	Code
Script header	#!/usr/bin/env bash set -euo pipefail IFS=$'\n\t'
Usage function	usage() { cat <<EOF Usage: $0 [options] -h Show help -v Verbose mode -f Input file EOF exit 1 } while getopts "hf:v" opt; do case $opt in h) usage;; f) file="$OPTARG";; v) verbose=true;; *) usage;; esac done
Portable scripting	# Prefer POSIX sh -c over bashisms when portability matters # Use printf instead of echo for consistent behavior # Quote all variable expansions # Use $(...) over backticks

Fields

The shape of this dataset, read from the data itself: every column, its type and a real value. Agents can use this to plan a query without fetching the whole document.

Section	Field	Type	Example
sections	`topic`	string	Variables and Expansion
sections	`examples`	array	[4 items]

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