Bash Functions

Functions in Bash allow you to group commands for later execution. They help make your scripts more modular, reusable, and easier to maintain.

Defining Functions

There are two syntaxes for defining functions in Bash:


# Syntax 1
function_name() {
    commands
}

# Syntax 2
function function_name {
    commands
}

Simple Function Example


#!/bin/bash

# Define a function
greet() {
    echo "Hello, $1!"
}

# Call the function
greet "World"
greet "Bash Programmer"

Returning Values

Bash functions don't return values like in other languages. Instead, they return an exit status (0 for success, non-zero for failure). You can use return to set this status.


#!/bin/bash

is_even() {
    local num=$1
    if (( num % 2 == 0 )); then
        return 0  # success (true)
    else
        return 1  # failure (false)
    fi
}

# Test the function
is_even 4 && echo "Even number" || echo "Odd number"
is_even 7 && echo "Even number" || echo "Odd number"

Returning Data

To return data from a function, you can use command substitution to capture its output:


#!/bin/bash

add() {
    local sum=$(( $1 + $2 ))
    echo $sum  # Output the result
}

result=$(add 5 3)
echo "The sum is: $result"

Local Variables

By default, variables in Bash are global. Use the local keyword to create variables with function scope:


#!/bin/bash

demo_scope() {
    local local_var="I'm local"
    global_var="I'm global"
    echo "Inside function: $local_var, $global_var"
}

demo_scope
echo "Outside function: $local_var, $global_var"  # local_var is undefined here

Passing Arguments

Functions receive arguments similarly to scripts, using positional parameters:


#!/bin/bash

show_args() {
    echo "First arg: $1"
    echo "Second arg: $2"
    echo "All args: $@"
    echo "Number of args: $#"
}

show_args "apple" "banana" "cherry"

Recursive Functions

Bash supports recursive function calls, though there's usually a limit to the recursion depth:


#!/bin/bash

factorial() {
    if (( $1 <= 1 )); then
        echo 1
    else
        local prev=$(( $1 - 1 ))
        local result=$(factorial $prev)
        echo $(( $1 * result ))
    fi
}

echo "Factorial of 5 is: $(factorial 5)"

Best Practices

Use descriptive function names
Keep functions small and focused
Use local for variables that shouldn't be global
Document your functions with comments
Return proper exit statuses (0 for success, non-zero for errors)

Advanced Example

Here's a more complex example demonstrating several function features:


#!/bin/bash

# Function to check if a file exists and is readable
file_exists() {
    if [[ ! -f "$1" ]]; then
        echo "Error: File '$1' does not exist" >&2
        return 1
    fi
    if [[ ! -r "$1" ]]; then
        echo "Error: File '$1' is not readable" >&2
        return 1
    fi
    return 0
}

# Function to count lines in a file
count_lines() {
    file_exists "$1" || return 1
    local lines=$(wc -l < "$1")
    echo "$lines"
}

# Main function
main() {
    local filename="$1"
    local lines

    lines=$(count_lines "$filename") || exit 1
    
    echo "File '$filename' has $lines lines"
    if (( lines > 100 )); then
        echo "That's a large file!"
    fi
}

# Call main with the first argument
main "$1"

Next: Input/Output Redirection