Unix - Functions in Shell Scripts

In shell scripting, functions are reusable blocks of code that help organize scripts, reduce repetition, and improve readability. A function allows you to define a set of commands once and execute them multiple times within the same script, making maintenance and debugging easier.

Defining a Function

A function in a shell script can be defined in two common ways:

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

or

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

Both formats are valid in Bash. Once defined, the function can be called simply by using its name:

greet

Passing Arguments to Functions

Functions can accept arguments just like scripts. Inside a function:

  • $1, $2, etc. represent positional parameters

  • $# gives the number of arguments

  • $@ represents all arguments

Example:

add_numbers() {
    sum=$(( $1 + $2 ))
    echo "Sum: $sum"
}

add_numbers 5 10

Returning Values from Functions

Shell functions do not return values like other programming languages. Instead:

  • Use echo to output a value and capture it

  • Use return to send an exit status (0–255)

Example using echo:

get_date() {
    echo "$(date)"
}

today=$(get_date)

Example using return:

is_root() {
    if [ "$EUID" -eq 0 ]; then
        return 0
    else
        return 1
    fi
}

Local and Global Variables

By default, variables in functions are global. To restrict scope, use the local keyword:

counter() {
    local count=1
    echo $count
}

This prevents variable conflicts in larger scripts.

Advantages of Using Functions

  • Improves code reusability

  • Enhances script readability

  • Simplifies debugging and testing

  • Makes large shell scripts easier to manage

In summary, functions are a powerful feature in shell scripting that enable modular, clean, and efficient automation in Unix and Linux environments.