Re: null command and parameter expansion for "Display error if null or unset"

[Paul Jarc]

mwoehlke <mwoehlke@tibco.com> wrote:
> Poor Yorick wrote:
>> The following line does not perform the "echo" command.
>> : ${FAKEVAR?} || echo hello
>
> What you probably want is:
> [ "$FAKEVAR" ] || echo hello

That doesn't distinguish between an unset variable and a variable set
to an empty value.  If that distinction is important, you can use:
test "${FAKEVAR+set}" = set

You can also wrap it up in a function for convenience:
is_set() {
  eval "test \"\${${1?}+set}\" = set"
}


paul