Parameter expansion without colon (default value to empty string) in a condition

Question

I know the difference between ${var:-word} and ${var-word}, but I don't see the purpose of using a default value in this condition:

[ -z "${NVM_PROFILE-}" ]

• Why is an empty default value needed there?
• Is it something related to compatibility with other shells (zsh)?

Answer 1

I like set -u.

$ sh -c '[ -z "NVM_PROFILE" ] && echo empty'
empty
$ sh -c 'set -u; [ -z "$NVM_PROFILE" ] && echo empty'
sh: NVM_PROFILE: unbound variable
$ sh -c 'set -u; [ -z "${NVM_PROFILE-}" ] && echo empty'
empty