Shell Script Functions: Complete Guide and Reference

Master shell script functions with this comprehensive guide covering syntax, parameters, return values, variable scope, and best practices for better code organization.

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Shell Script Functions: Complete Guide and Reference

Table of Contents

1. [Introduction to Shell Script Functions](#introduction) 2. [Function Syntax and Declaration](#syntax) 3. [Function Parameters and Arguments](#parameters) 4. [Return Values and Exit Codes](#return-values) 5. [Variable Scope in Functions](#variable-scope) 6. [Advanced Function Concepts](#advanced-concepts) 7. [Best Practices](#best-practices) 8. [Common Use Cases](#use-cases) 9. [Troubleshooting](#troubleshooting) 10. [Examples and Practical Applications](#examples)

Introduction to Shell Script Functions {#introduction}

Shell script functions are reusable blocks of code that perform specific tasks within a shell script. They provide modularity, code reusability, and better organization of complex scripts. Functions help reduce code duplication and make scripts easier to maintain and debug.

Why Use Functions in Shell Scripts

| Benefit | Description | |---------|-------------| | Code Reusability | Write once, use multiple times throughout the script | | Modularity | Break complex tasks into smaller, manageable pieces | | Maintainability | Easier to update and modify specific functionality | | Readability | Makes scripts more organized and easier to understand | | Testing | Individual functions can be tested separately | | Debugging | Isolate issues to specific functions |

Function Syntax and Declaration {#syntax}

Basic Function Syntax

There are several ways to declare functions in shell scripts:

#### Method 1: Using the function keyword

`bash function function_name() { # Function body commands return [exit_code] } `

#### Method 2: POSIX-compliant syntax

`bash function_name() { # Function body commands return [exit_code] } `

#### Method 3: Alternative syntax

`bash function function_name { # Function body commands return [exit_code] } `

Function Declaration Rules

| Rule | Description | Example | |------|-------------|---------| | Naming | Function names must start with letter/underscore | my_function, _helper, calculateSum | | Case Sensitivity | Function names are case-sensitive | MyFunc and myfunc are different | | No Spaces | No spaces around parentheses in POSIX syntax | func() not func () | | Declaration Before Use | Functions must be declared before calling | Define at top of script |

Basic Function Example

`bash #!/bin/bash

Simple function declaration

greet() { echo "Hello, World!" }

Function call

greet `

Output: ` Hello, World! `

Function Parameters and Arguments {#parameters}

Passing Arguments to Functions

Functions can accept arguments similar to how scripts accept command-line arguments.

#### Positional Parameters in Functions

| Parameter | Description | Example Usage | |-----------|-------------|---------------| | $0 | Script name (not function name) | echo "Script: $0" | | $1, $2, $3... | First, second, third argument | echo "First arg: $1" | | $# | Number of arguments passed | if [ $# -eq 2 ] | | $@ | All arguments as separate words | for arg in "$@" | | $ | All arguments as single string | echo "Args: $" |

#### Function with Parameters Example

`bash #!/bin/bash

Function that accepts parameters

greet_user() { local name=$1 local age=$2 if [ $# -lt 2 ]; then echo "Usage: greet_user " return 1 fi echo "Hello, $name! You are $age years old." return 0 }

Function calls

greet_user "Alice" 25 greet_user "Bob" # This will show usage message `

Output: ` Hello, Alice! You are 25 years old. Usage: greet_user `

Advanced Parameter Handling

#### Default Parameter Values

`bash #!/bin/bash

Function with default parameters

create_file() { local filename=${1:-"default.txt"} local content=${2:-"Default content"} echo "$content" > "$filename" echo "Created file: $filename" }

Usage examples

create_file # Uses all defaults create_file "custom.txt" # Custom filename, default content create_file "data.txt" "Custom data" # Custom filename and content `

#### Variable Number of Arguments

`bash #!/bin/bash

Function that processes multiple arguments

calculate_sum() { local sum=0 local count=0 # Process all arguments for number in "$@"; do if [[ $number =~ ^[0-9]+$ ]]; then sum=$((sum + number)) count=$((count + 1)) else echo "Warning: '$number' is not a valid number" fi done echo "Sum of $count numbers: $sum" return 0 }

Usage

calculate_sum 10 20 30 40 calculate_sum 5 abc 15 xyz 25 `

Output: ` Sum of 4 numbers: 100 Warning: 'abc' is not a valid number Warning: 'xyz' is not a valid number Sum of 2 numbers: 45 `

Return Values and Exit Codes {#return-values}

Understanding Return Values

Functions in shell scripts return exit codes (0-255) rather than traditional return values. The return statement sets the exit status of the function.

#### Return Code Conventions

| Return Code | Meaning | Usage | |-------------|---------|--------| | 0 | Success | Function completed successfully | | 1 | General error | Generic failure condition | | 2 | Misuse of shell command | Invalid arguments or usage | | 126 | Command cannot execute | Permission problem | | 127 | Command not found | Function or command doesn't exist | | 128+n | Fatal error signal "n" | Script terminated by signal |

#### Function Return Example

`bash #!/bin/bash

Function that validates input and returns appropriate codes

validate_email() { local email=$1 # Check if email parameter is provided if [ -z "$email" ]; then echo "Error: Email address required" return 1 fi # Basic email validation if [[ $email =~ ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ]]; then echo "Valid email: $email" return 0 else echo "Invalid email format: $email" return 2 fi }

Test the function and check return codes

validate_email "user@example.com" echo "Return code: $?"

validate_email "invalid-email" echo "Return code: $?"

validate_email echo "Return code: $?" `

Output: ` Valid email: user@example.com Return code: 0 Invalid email format: invalid-email Return code: 2 Error: Email address required Return code: 1 `

Capturing Function Output

#### Using Command Substitution

`bash #!/bin/bash

Function that generates output

get_system_info() { local info_type=$1 case $info_type in "hostname") hostname ;; "user") whoami ;; "date") date +"%Y-%m-%d %H:%M:%S" ;; *) echo "Unknown info type: $info_type" return 1 ;; esac }

Capture function output

current_host=$(get_system_info "hostname") current_user=$(get_system_info "user") current_date=$(get_system_info "date")

echo "System Information:" echo "Host: $current_host" echo "User: $current_user" echo "Date: $current_date" `

Variable Scope in Functions {#variable-scope}

Global vs Local Variables

Understanding variable scope is crucial for writing maintainable shell functions.

#### Variable Scope Types

| Scope Type | Declaration | Accessibility | Lifetime | |------------|-------------|---------------|----------| | Global | variable=value | Entire script | Until script ends | | Local | local variable=value | Function only | Until function returns | | Environment | export variable=value | Child processes | Until session ends |

#### Variable Scope Example

`bash #!/bin/bash

Global variable

global_var="I am global"

demonstrate_scope() { local local_var="I am local" local global_var="I am local override" # Shadows global variable echo "Inside function:" echo " Local variable: $local_var" echo " Global variable (shadowed): $global_var" # Modify a global variable another_global="Set inside function" }

echo "Before function call:" echo " Global variable: $global_var" echo " Another global: ${another_global:-'(not set)'}"

demonstrate_scope

echo "After function call:" echo " Global variable: $global_var" # Original value preserved echo " Local variable: ${local_var:-'(not accessible)'}" echo " Another global: $another_global" # Set by function `

Output: ` Before function call: Global variable: I am global Another global: (not set) Inside function: Local variable: I am local Global variable (shadowed): I am local override After function call: Global variable: I am global Local variable: (not accessible) Another global: Set inside function `

Best Practices for Variable Scope

#### Always Use Local Variables

`bash #!/bin/bash

Good practice: Use local variables

process_data() { local input_file=$1 local output_file=$2 local temp_file="/tmp/process_$" local line_count=0 while IFS= read -r line; do # Process line echo "Processed: $line" >> "$temp_file" line_count=$((line_count + 1)) done < "$input_file" mv "$temp_file" "$output_file" echo "Processed $line_count lines" }

Bad practice: Using global variables

process_data_bad() { input_file=$1 # Global variable output_file=$2 # Global variable temp_file="/tmp/process_$" # Global variable # ... rest of function } `

Advanced Function Concepts {#advanced-concepts}

Recursive Functions

Functions can call themselves, creating recursive behavior.

#### Recursive Function Example

`bash #!/bin/bash

Recursive function to calculate factorial

factorial() { local n=$1 # Base case if [ $n -le 1 ]; then echo 1 return 0 fi # Recursive case local prev_result=$(factorial $((n - 1))) echo $((n * prev_result)) }

Calculate factorial of 5

result=$(factorial 5) echo "5! = $result"

Recursive directory listing

list_directory() { local dir=$1 local depth=${2:-0} local indent="" # Create indentation for ((i=0; i`

Function Arrays and Complex Data Structures

#### Passing Arrays to Functions

`bash #!/bin/bash

Function that processes an array

process_array() { local -n arr_ref=$1 # Name reference to array local operation=$2 case $operation in "sum") local sum=0 for element in "${arr_ref[@]}"; do sum=$((sum + element)) done echo "Sum: $sum" ;; "max") local max=${arr_ref[0]} for element in "${arr_ref[@]}"; do if [ $element -gt $max ]; then max=$element fi done echo "Max: $max" ;; "count") echo "Count: ${#arr_ref[@]}" ;; esac }

Test with array

numbers=(10 25 3 47 15 8) process_array numbers "sum" process_array numbers "max" process_array numbers "count" `

Function Libraries and Sourcing

#### Creating Function Libraries

Create a separate file for common functions:

File: lib/common_functions.sh `bash #!/bin/bash

Logging functions

log_info() { echo "[INFO $(date '+%Y-%m-%d %H:%M:%S')] $*" }

log_error() { echo "[ERROR $(date '+%Y-%m-%d %H:%M:%S')] $*" >&2 }

log_warn() { echo "[WARN $(date '+%Y-%m-%d %H:%M:%S')] $*" }

File operations

backup_file() { local file=$1 local backup_dir=${2:-"./backup"} if [ ! -f "$file" ]; then log_error "File not found: $file" return 1 fi mkdir -p "$backup_dir" cp "$file" "$backup_dir/$(basename "$file").$(date +%Y%m%d_%H%M%S)" log_info "Backed up $file" }

Network functions

check_connectivity() { local host=${1:-"google.com"} local timeout=${2:-5} if ping -c 1 -W $timeout "$host" >/dev/null 2>&1; then log_info "Connectivity to $host: OK" return 0 else log_error "Connectivity to $host: FAILED" return 1 fi } `

Main script using the library: `bash #!/bin/bash

Source the function library

source "lib/common_functions.sh"

Use library functions

log_info "Starting application" backup_file "/etc/passwd" "/tmp/backup" check_connectivity "example.com" log_info "Application completed" `

Best Practices {#best-practices}

Function Design Principles

| Principle | Description | Example | |-----------|-------------|---------| | Single Responsibility | Each function should do one thing well | validate_email() vs process_user_data() | | Clear Naming | Function names should describe what they do | calculate_tax() vs calc() | | Parameter Validation | Always validate input parameters | Check for required arguments | | Error Handling | Handle errors gracefully | Return appropriate exit codes | | Documentation | Comment complex functions | Explain parameters and return values |

Error Handling in Functions

`bash #!/bin/bash

Robust function with comprehensive error handling

process_file() { local input_file=$1 local output_file=$2 local operation=${3:-"copy"} # Parameter validation if [ $# -lt 2 ]; then echo "Usage: process_file [operation]" >&2 return 1 fi # Input file validation if [ ! -f "$input_file" ]; then echo "Error: Input file '$input_file' not found" >&2 return 2 fi if [ ! -r "$input_file" ]; then echo "Error: Input file '$input_file' not readable" >&2 return 3 fi # Output directory validation local output_dir=$(dirname "$output_file") if [ ! -d "$output_dir" ]; then echo "Creating output directory: $output_dir" mkdir -p "$output_dir" || { echo "Error: Cannot create output directory" >&2 return 4 } fi # Perform operation case $operation in "copy") cp "$input_file" "$output_file" || { echo "Error: Failed to copy file" >&2 return 5 } ;; "move") mv "$input_file" "$output_file" || { echo "Error: Failed to move file" >&2 return 5 } ;; *) echo "Error: Unknown operation '$operation'" >&2 return 6 ;; esac echo "Successfully processed: $input_file -> $output_file" return 0 } `

Function Testing Framework

`bash #!/bin/bash

Simple testing framework for functions

test_count=0 test_passed=0 test_failed=0

Test assertion functions

assert_equals() { local expected=$1 local actual=$2 local test_name=$3 test_count=$((test_count + 1)) if [ "$expected" = "$actual" ]; then echo "PASS: $test_name" test_passed=$((test_passed + 1)) else echo "FAIL: $test_name" echo " Expected: '$expected'" echo " Actual: '$actual'" test_failed=$((test_failed + 1)) fi }

assert_return_code() { local expected_code=$1 local actual_code=$2 local test_name=$3 test_count=$((test_count + 1)) if [ $expected_code -eq $actual_code ]; then echo "PASS: $test_name" test_passed=$((test_passed + 1)) else echo "FAIL: $test_name" echo " Expected return code: $expected_code" echo " Actual return code: $actual_code" test_failed=$((test_failed + 1)) fi }

Function to test

add_numbers() { local a=$1 local b=$2 echo $((a + b)) }

Test cases

test_add_numbers() { local result result=$(add_numbers 2 3) assert_equals "5" "$result" "add_numbers(2, 3)" result=$(add_numbers 0 0) assert_equals "0" "$result" "add_numbers(0, 0)" result=$(add_numbers -5 10) assert_equals "5" "$result" "add_numbers(-5, 10)" }

Run tests

echo "Running tests..." test_add_numbers

echo echo "Test Results:" echo " Total: $test_count" echo " Passed: $test_passed" echo " Failed: $test_failed" `

Common Use Cases {#use-cases}

System Administration Functions

`bash #!/bin/bash

System monitoring functions

check_disk_space() { local threshold=${1:-90} local mount_point=${2:-"/"} local usage=$(df "$mount_point" | awk 'NR==2 {print $5}' | sed 's/%//') if [ $usage -gt $threshold ]; then echo "WARNING: Disk usage is ${usage}% (threshold: ${threshold}%)" return 1 else echo "OK: Disk usage is ${usage}%" return 0 fi }

check_service_status() { local service_name=$1 if systemctl is-active --quiet "$service_name"; then echo "Service $service_name is running" return 0 else echo "Service $service_name is not running" return 1 fi }

backup_database() { local db_name=$1 local backup_dir=${2:-"/backup"} local timestamp=$(date +%Y%m%d_%H%M%S) local backup_file="$backup_dir/${db_name}_${timestamp}.sql" mkdir -p "$backup_dir" if mysqldump "$db_name" > "$backup_file"; then echo "Database backup created: $backup_file" return 0 else echo "Database backup failed" return 1 fi } `

Text Processing Functions

`bash #!/bin/bash

Text processing utilities

extract_emails() { local input_file=$1 grep -oE '\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b' "$input_file" }

count_words() { local text=$1 echo "$text" | wc -w }

remove_duplicates() { local input_file=$1 local output_file=${2:-"${input_file}.dedup"} sort "$input_file" | uniq > "$output_file" echo "Removed duplicates: $input_file -> $output_file" }

format_csv() { local input_file=$1 local delimiter=${2:-","} column -t -s "$delimiter" "$input_file" } `

Troubleshooting {#troubleshooting}

Common Function Errors and Solutions

| Error Type | Symptoms | Solution | |------------|----------|----------| | Function Not Found | command not found | Ensure function is declared before use | | Wrong Parameter Count | Unexpected behavior | Validate $# parameter count | | Variable Scope Issues | Variables not accessible | Use local keyword properly | | Return Code Problems | Wrong exit status | Use return instead of exit | | Recursive Stack Overflow | Script hangs/crashes | Add proper base case |

Debugging Functions

`bash #!/bin/bash

Enable debug mode

set -x # Print commands as they execute set -e # Exit on error set -u # Exit on undefined variable

Debug function with tracing

debug_function() { local func_name=${FUNCNAME[0]} local line_no=${BASH_LINENO[0]} echo "DEBUG: Entering $func_name at line $line_no" >&2 echo "DEBUG: Arguments: $*" >&2 # Function logic here local result="processed" echo "DEBUG: Exiting $func_name with result: $result" >&2 echo "$result" }

Function call stack tracing

print_call_stack() { echo "Call stack:" >&2 local frame=0 while [ ${FUNCNAME[frame]} ]; do echo " $frame: ${FUNCNAME[frame]} (${BASH_SOURCE[frame]}:${BASH_LINENO[frame-1]})" >&2 frame=$((frame + 1)) done } `

Examples and Practical Applications {#examples}

Complete Example: Log File Analyzer

`bash #!/bin/bash

Log file analyzer with multiple functions

LOG_FILE="/var/log/syslog" TEMP_DIR="/tmp/log_analysis_$"

Initialize analysis environment

init_analysis() { mkdir -p "$TEMP_DIR" echo "Log Analysis Report - $(date)" > "$TEMP_DIR/report.txt" echo "==============================" >> "$TEMP_DIR/report.txt" }

Clean up temporary files

cleanup() { rm -rf "$TEMP_DIR" echo "Cleanup completed" }

Trap to ensure cleanup on exit

trap cleanup EXIT

Analyze log file size and basic stats

analyze_file_stats() { local log_file=$1 local report_file="$TEMP_DIR/report.txt" if [ ! -f "$log_file" ]; then echo "Error: Log file not found: $log_file" >&2 return 1 fi echo "" >> "$report_file" echo "File Statistics:" >> "$report_file" echo " File: $log_file" >> "$report_file" echo " Size: $(du -h "$log_file" | cut -f1)" >> "$report_file" echo " Lines: $(wc -l < "$log_file")" >> "$report_file" echo " Last Modified: $(stat -c %y "$log_file")" >> "$report_file" }

Extract error messages

extract_errors() { local log_file=$1 local error_file="$TEMP_DIR/errors.txt" local report_file="$TEMP_DIR/report.txt" grep -i "error\|fail\|critical" "$log_file" > "$error_file" 2>/dev/null local error_count=$(wc -l < "$error_file") echo "" >> "$report_file" echo "Error Analysis:" >> "$report_file" echo " Total Errors: $error_count" >> "$report_file" if [ $error_count -gt 0 ]; then echo " Recent Errors:" >> "$report_file" tail -5 "$error_file" | sed 's/^/ /' >> "$report_file" fi }

Analyze IP addresses

analyze_ip_addresses() { local log_file=$1 local ip_file="$TEMP_DIR/ips.txt" local report_file="$TEMP_DIR/report.txt" # Extract IP addresses grep -oE '\b([0-9]{1,3}\.){3}[0-9]{1,3}\b' "$log_file" | \ sort | uniq -c | sort -nr > "$ip_file" echo "" >> "$report_file" echo "IP Address Analysis:" >> "$report_file" echo " Unique IPs: $(wc -l < "$ip_file")" >> "$report_file" echo " Top 5 IPs:" >> "$report_file" head -5 "$ip_file" | sed 's/^/ /' >> "$report_file" }

Generate summary report

generate_report() { local report_file="$TEMP_DIR/report.txt" echo "" >> "$report_file" echo "Analysis completed at $(date)" >> "$report_file" # Display report cat "$report_file" # Optionally save report cp "$report_file" "./log_analysis_$(date +%Y%m%d_%H%M%S).txt" }

Main execution function

main() { local log_file=${1:-$LOG_FILE} echo "Starting log analysis for: $log_file" init_analysis analyze_file_stats "$log_file" extract_errors "$log_file" analyze_ip_addresses "$log_file" generate_report echo "Analysis complete. Report saved." }

Execute main function if script is run directly

if [ "${BASH_SOURCE[0]}" = "${0}" ]; then main "$@" fi `

Menu-Driven Application Example

`bash #!/bin/bash

Menu-driven system administration tool

Display main menu

show_menu() { clear echo "================================" echo " System Administration Tool" echo "================================" echo "1. Check System Status" echo "2. Monitor Disk Usage" echo "3. View Running Processes" echo "4. Network Information" echo "5. System Logs" echo "6. Exit" echo "================================" echo -n "Please select an option [1-6]: " }

Check system status

check_system_status() { echo "System Status Information:" echo "=========================" echo "Hostname: $(hostname)" echo "Uptime: $(uptime)" echo "Load Average: $(cat /proc/loadavg)" echo "Memory Usage:" free -h echo read -p "Press Enter to continue..." }

Monitor disk usage

monitor_disk_usage() { echo "Disk Usage Information:" echo "======================" df -h echo echo "Largest directories in /var:" du -h /var/* 2>/dev/null | sort -hr | head -10 echo read -p "Press Enter to continue..." }

View running processes

view_processes() { echo "Running Processes:" echo "==================" ps aux --sort=-%cpu | head -20 echo read -p "Press Enter to continue..." }

Show network information

show_network_info() { echo "Network Information:" echo "===================" echo "Network Interfaces:" ip addr show echo echo "Routing Table:" ip route show echo read -p "Press Enter to continue..." }

View system logs

view_system_logs() { echo "Recent System Logs:" echo "===================" echo "Last 20 lines from syslog:" tail -20 /var/log/syslog 2>/dev/null || echo "Syslog not accessible" echo read -p "Press Enter to continue..." }

Main program loop

main() { while true; do show_menu read -r choice case $choice in 1) check_system_status ;; 2) monitor_disk_usage ;; 3) view_processes ;; 4) show_network_info ;; 5) view_system_logs ;; 6) echo "Goodbye!" exit 0 ;; *) echo "Invalid option. Please try again." sleep 2 ;; esac done }

Start the application

main `

This comprehensive guide covers all aspects of creating and using functions in shell scripts, from basic syntax to advanced concepts and practical applications. Functions are essential for writing maintainable, reusable, and organized shell scripts that can handle complex tasks efficiently.

Tags

  • Functions
  • Linux
  • Unix
  • bash
  • shell-scripting

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Shell Script Functions: Complete Guide and Reference