Bash function `join` that joins a number of elements to a colon-separated variable, gracefully appending to an existing one when present.

join.bash

#!/usr/bin/env bash

##
## Copyright (C) 2018 Alex Thiessen <alex.thiessen.de+github@gmail.com>
## Copyright (C) 2018 https://unix.stackexchange.com/users/116858/kusalananda
## 
## This program is free software; you can redistribute it and/or modify it under
## the terms of the GNU General Public License as published by the Free Software
## Foundation; either version 2 of the License, or (at your option) any later
## version.
## 
## This program is distributed in the hope that it will be useful, but WITHOUT
## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
## FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
## details.
## 
## You should have received a copy of the GNU General Public License along with
## this program; if not, write to the Free Software Foundation, Inc., 51
## Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
##
## SPDX-License-Identifier: GPL-2.0-or-later
## <https://spdx.org/licenses/GPL-2.0-or-later.html>
##

set -o nounset
set -o errexit

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 "${*}")"
}

# ensure that this fails
if join
then
	return 1
fi

export foo=X
join foo Y
[ "$foo" = "X:Y" ]

export foo=
join foo Y
[ "$foo" = "Y" ]

unset foo
join foo Y
[ "$foo" = "Y" ]

export foo="a b"
join foo Y
[ "$foo" = "a b:Y" ]

export foo="a b"
join foo "c d"
[ "$foo" = "a b:c d" ]

export foo="a b"
join foo "c d" "e f"
[ "$foo" = "a b:c d:e f" ]