How to return a value from a function?

Returning Values from Functions in Shell

In the world of shell scripting, returning values from functions is a crucial aspect of creating reusable and modular code. Unlike programming languages like Python or Java, where functions can directly return values, shell functions have a slightly different approach. In this guide, we'll explore the various techniques you can use to return values from shell functions.

Using the return Command

The most straightforward way to return a value from a shell function is to use the return command. The return command allows you to specify an exit status, which can be interpreted as a return value. Here's an example:

my_function() {
    local result=42
    return $result
}

my_function
echo "The result is: $?"

In this example, the my_function function sets a local variable result to the value 42, and then uses the return command to exit the function with the value of result. The exit status of the function is then stored in the special variable $?, which can be accessed after the function call.

The main limitation of this approach is that the return command can only return an integer value between 0 and 255. If you need to return a more complex value, such as a string or an array, you'll need to use a different technique.

Storing Values in Variables

Another way to return values from a shell function is to store the desired value in a variable that can be accessed outside the function. Here's an example:

my_function() {
    local result=42
    echo "$result"
}

result=$(my_function)
echo "The result is: $result"

In this example, the my_function function simply echoes the value of the result variable. The function call is then captured using command substitution ($(my_function)), and the resulting output is stored in the result variable, which can be accessed outside the function.

This approach is more flexible than using the return command, as it allows you to return any type of value, including strings and arrays. However, it requires a bit more coordination between the function and the calling code, as the caller needs to know the name of the variable to capture the return value.

Using Standard Output

A third approach to returning values from a shell function is to use the standard output of the function. This involves having the function write the desired return value to the standard output, which can then be captured by the caller. Here's an example:

my_function() {
    local result=42
    echo "$result"
}

result=$(my_function)
echo "The result is: $result"

This example is very similar to the previous one, but instead of storing the return value in a variable inside the function, the function writes the value to the standard output, which is then captured by the caller using command substitution.

This approach is particularly useful when you want to return multiple values from a function, as you can simply write each value to a new line in the standard output, and the caller can then split the output into an array or process the values individually.

Mermaid Diagram

Here's a Mermaid diagram that summarizes the different ways to return values from a shell function:

In conclusion, there are several techniques you can use to return values from shell functions, each with its own advantages and trade-offs. The choice of which method to use will depend on the specific requirements of your script and the type of data you need to return.