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List:       bash-bug
Subject:    Re: Bash reference manual: shell parameter expansion examples
From:       Chet Ramey <chet.ramey () case ! edu>
Date:       2024-02-23 19:45:49
Message-ID: 34c5d3dd-c0fe-4a84-909f-d5d71cdb6f18 () case ! edu
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On 2/23/24 10:44 AM, James Weigle wrote:
> Hi!
> 
> In the Bash reference manual, there are a series of examples of testing if
> a variable is null or unset:
> 
>     - Under *${parameter:-word}*, the usage is *${var-unset}*.
>     - Under *${parameter:=word}*, the usage is *${var:=DEFAULT}*.
>     - Under *${parameter:?word}*, the usage is *${var:?var is unset or null}*
>     .
>     - Under *${parameter:+word}*, the usage is *${var:+var is set and not
>     null}*.
> 
> I got a little confused at the first example, because it's the *only*
> example where the colon is omitted. It still works—but why is that one
> using a different form?

After this paragraph:

   "When not performing substring expansion, using the forms described
below (e.g., ‘:-'), Bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is unset.
Put another way, if the colon is included, the operator tests for both
PARAMETER's existence and that its value is not null; if the colon is
omitted, the operator tests only for existence."

that you noted, the current manual has these:


           $ v=123
           $ echo ${v-unset}
           123
           $ echo ${v:-unset-or-null}
           123
           $ unset v
           $ echo ${v-unset}
           unset
           $ v=
           $ echo ${v:-unset-or-null}
           unset-or-null


-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/


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