Bash Scripters: Stop using subshells to call functions.

When writing in bash, zsh, sh, etc... stop using subshells to call functions. There is a significant speed overhead to using a subshell and there is a much better alternative. Instead, you should just have a convention where a particular variable is always the return value of the function (I use retval). This has the added benefit of also allowing you to return arrays from your functions.

If you don't know what a subshell is, a subshell is another bash shell which is spawned whenever you use $() or `` and is used to execute the code you put inside.

I did some simple testing to allow you to observe the overhead. For two functionally equivalent scripts:

This one uses a subshell:

#!/bin/bash
function a() {
    echo hello
}
for (( i = 0; i < 10000; i++ )); do
    echo "$(a)"
done

This one uses a variable:

#!/bin/bash
function a() {
    retval="hello"
}
for (( i = 0; i < 10000; i++ )); do
    a
    echo "$retval"
done

The speed difference between these two is noticeable and significant.

$ for i in variable subshell; do
> echo -e "\n$i"; time ./$i > /dev/null
> done

variable

real 0m0.367s
user 0m0.346s
sys 0m0.015s

subshell

real 0m11.937s
user 0m3.121s
sys 0m0.359s