I've seen... (ok, actually I recommended)... someone do the following:

    if [ -x /usr/bin/blah ]
    then
       BLAH=/usr/bin/blah
    else
       BLAH=/bin/true
    fi

    $BLAH foo bar baz

They wanted BLAH to be a noop if blah didn't exist. I could only think of /bin/true as a noop at the time, but I needed it to ignore parameters.

The man page defines that it does.

(in retrospect, we could have defined an sh function noop {} )

I was going to mention that it came up just days ago on Unix and Linux StackExchange. (-:

* https://unix.stackexchange.com/a/497561/5132

The conditionally defined shell function in one of the other answers is a more appealing approach, especially given the native behaviours of some shells with respect to variable expansion. But also given that /bin/true is not necessarily the pathname.

    % export BLAH='/bin/true too'
    % dash -c '$BLAH 1 2 3'
    % dash: 1: /bin/true: not found
    % sh -c '$BLAH 1 2 3'
    /bin/true: not found
    % ksh -c '$BLAH 1 2 3'
    ksh: /bin/true: not found
    % zsh -c '$BLAH 1 2 3'
    zsh:1: no such file or directory: /bin/true too
    % bash -c '$BLAH 1 2 3'
    bash: /bin/true: No such file or directory
    % 
    % export BLAH='/usr/bin/true too'
    % zsh -c '$BLAH 1 2 3'
    zsh:1: no such file or directory: /usr/bin/true too
    % bash -c '$BLAH 1 2 3'
    % ksh -c '$BLAH 1 2 3'
    % dash -c '$BLAH 1 2 3'
    %

Yeah, that was my answer -- and actually I wrote /usr/bin/true on SO but got sloppy here.

I wrote that answer because I recalled it as a common idiom in autoconf or something -- some automake system I'd seen where the system seeks out the tools it needs and defines them as variables.

Yeah, options include a noop function (which is nice because you can change it to noop () { echo "Skipping $@" >&2 ; }) or the shell builtin :, which definitely ignores its arguments.