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


From: Paul Jarc
Subject: Re: null command and parameter expansion for "Display error if null or unset"
Date: Mon, 31 Jul 2006 14:17:44 -0400
User-agent: Gnus/5.110003 (No Gnus v0.3) Emacs/21.4 (gnu/linux)

mwoehlke <address@hidden> 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




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