Bash "coalesce" functionality

Question

I'm trying to do the following on a single line:

if [ -z "$foo" ]
then
    source "$foo/subdir/yo.sh"
else
    source "$bar/yo.sh"
fi

(note the subdir only present if foo is used). The simplest solution so far is:

source "${foo:-$bar}${foo:+/subdir}/yo.sh"

In other words, take either foo or bar, then append /subdir if foo exists. Is there a more elegant way to do this?

Answer 1

Try this:

$((printf $foo 2>/dev/null && printf '/subdir') || printf $bar)/yo.sh

Kind of convoluted though.

The key aspect to this solution is finding a way to both output the contents of $foo, if any, and simultaneously branch based on whether something was in $foo. printf with no arguments is an error and $foo with no contents results in no arguments to printf. After that the rest is easy.

For this reason the first printf needs to be printf but the others could be replaced with echo -n if you prefer.