Fish
Validate Function Argument
test -z "$argv[1]"; and echo "Error: arg1 must be a string"; and return 1
Verify Variable is Defined
test -z "$FOO"; and echo "FOO is empty"; and return
test -z "$argv[1]"; and echo "Error: arg1 must be a string"; and return 1
test -z "$FOO"; and echo "FOO is empty"; and return