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 c...@case.edu http://tiswww.cwru.edu/~chet/
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