Unset

Define

x=foo

Remove

Set to empty string.

x=''

Unset - remove from variables.

unset x

When doing echo $x, you won’t see a difference as in both cases it will render as empty string.

Check

From answer

Check a variable is set:

[ -z ${var+foo} ]

Check if a variable is an empty string or not set - it can’t distinguish.

[ -z $foo ]