One liner to set environment variable if doesn't exist, else append

A little improvement on Michael Burr's answer. This works with set -u (set -o nounset) as well:

PATH=${PATH:+$PATH:}/path/to/bin

PATH=${PATH}${PATH:+:}/path/to/bin

• ${PATH} evaluates to nothing if PATH is not set/empty, otherwise it evaluates to the current path
• ${PATH:+:} evaluates to nothing if PATH is not set, otherwise it evaluates to ":"