Humbling Experience

I’m relatively comfortable with Bash but when the other day I saw this Stack Overflow answer on how to decode URL-encoded string in shell my brain took a moment to process it.

Let me copy it here just to appreciate it.

function urldecode() { : "${*//+/ }"; echo -e "${_//%/\\x}"; }

Let’s explore what is going on here, lookig at it in easier to digest(?) form:

function urldecode() {
    : "${*//+/ }"
    echo -e "${_//%/\\x}"
}

First part is using built-in : and as the (single) argument expanding all the arguments to the function into a single string "${*}" while replacing all occurrences of + with space (double forward slash pattern substitution).

Confusing part is that : build-in kinda does nothing (except for exiting with 0, many of you probably invoked spell of shape while :; do domething; done or as a part of comment/documentation combined with here document). While this looks like nop/pass/… it is not.

The magic is revealed when we inspect the following line where special variable ${_} is used. From Bash documentation in this case it

expands to the last argument to the previous simple command executed in the foreground, after expansion.

Therefore meaning of the previous line was “just” to set ${_}. There all percent signs get replaced with \x (double backslash for escaping in ") and pass that to echo -e for evaluation.

Obviously, people who read me know there is a bug: urldecode -e will return nothing. It is still very elegant way to solve task at hand on command line.