Append to PATH-like variable without creating leading colon if unset

Question

I need to append a directory to PKG_CONFIG_PATH. Normally, I would use the standard

export PKG_CONFIG_PATH=${PKG_CONFIG_PATH}:$(pyenv prefix)/lib/pkgconfig

but PKG_CONFIG_PATH has not been previously set on my system. Therefore, the variable begins with a : character, which tells it to look in the current directory first. I do not want that. I settled on the following,

export PKG_CONFIG_PATH=${PKG_CONFIG_PATH}${PKG_CONFIG_PATH:+:}$(pyenv prefix)/lib/pkgconfig

but that just seems so ugly. Is there a better way? What is the appropriate way to conditionally append the colon if and only if the variable has already been set?

Answer 1

You are on the right track with the ${:+} expansion operator, you just need to modify it slightly:

V=${V:+${V}:}new_V

The first braces expand to $V and the colon iff V is set already otherwise to nothing - which is exactly what you need (and probably also one of the reasons for the existence of the operator).

Thus in your case:

export "PKG_CONFIG_PATH=${PKG_CONFIG_PATH:+${PKG_CONFIG_PATH}:}$(pyenv prefix)/lib/pkgconfig"

Answer 2

Lately, I set up GNU stow on my machines to store user-wide stuff like libraries under ~/.local and ran into troubles when defining LD_LIBRARY_PATH, CPATH and LIBRARY_PATH, inadvertently putting a colon there and so breaking stuff.

Then I found your question and the answer wasn't exactly elegant ;-), and I wrote a small function to handle this, please find it here: https://gist.github.com/rico-chet/0229e4c080d9f51a02535dd25a656a8a

## Copyright (C) 2018 Alex Thiessen <alex.thiessen.de+github@gmail.com>
## Copyright (C) 2018 https://unix.stackexchange.com/users/116858/kusalananda
## SPDX-License-Identifier: GPL-2.0-or-later
## <https://spdx.org/licenses/GPL-2.0-or-later.html>

function join() {
    if [ ${#} -eq 0 ]
    then
        echo "\`join\` appends elements separated by colons to a \`bash\` variable " >&2
        echo "usage: join <variable> <element> [element ...]" >&2
        return 1
    fi
    variable="${1}"

    shift
    export ${variable}="${!variable:+${!variable}:}$(IFS=:; echo "${*}")"
}

// edited as suggested by @Kusalananda