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Answer 1

+5 A:

That's not Perl, its shell. Init scripts are usually written in shell. Specifically it means "if defined, use the HTTPD environment variable, otherwise use /usr/sbin/httpd".

Look here for more info.

Schwern 2010-01-18 06:49:56

do you think OP has a typo error? should it be $HTTPD:- instead of just -

ghostdog74 2010-01-18 07:29:47

`${VAR-default}` works, though its not the documented bash way to do it. I can't say more, I'm not much of a shell programmer.

Schwern 2010-01-18 08:30:17

It certainly is documented: look for the section "Paramater Expansion" in the man page.

glenn jackman 2010-01-18 14:41:24

@glenn I did. I even linked to it. Look closely, the documented version is `${VAR:-default}` where the OP is using `${VAR-default}`. Note the lack of a colon. Maybe somewhere its documented that the colon is optional, but its not immediately obvious from the docs.

Schwern 2010-01-19 01:58:06

@Schwern: It's documented in the man page in the sentence immediately preceding the section about `${VAR:-default}` as you can see in my answer. "Omitting the colon..." It's worded a little differently in the reference you linked to, but it's in the same place. It **is** easy to miss (I did for a long time).

Dennis Williamson 2010-01-19 02:14:39

@Dennis Yep. I missed it because I was searching for `:` and skimming the list of parameter styles.

Schwern 2010-01-19 19:34:07

Answer 2

+2 A:

The colon affects whether the variable is checked for being unset or null versus only checking for whether it's unset.

$ var="goodbye"; echo ${var-hello}
goodbye
$ var="goodbye"; echo ${var:-hello}
goodbye
$ var= ; echo ${var:-hello}
hello
$ var= ; echo ${var-hello}    # var is null, only test for unset so no sub. made

$ unset var; echo ${var:-hello}
hello
$ unset var; echo ${var-hello}
hello

From the Bash man page:

       When not performing substring expansion,  using  the  forms  documented
       below,  bash tests for a parameter that is unset or null.  Omitting the
       colon results in a test only for a parameter that is unset.

       ${parameter:-word}
              Use Default Values.  If parameter is unset or null,  the  expan‐
              sion  of word is substituted.  Otherwise, the value of parameter
              is substituted.

Dennis Williamson 2010-01-18 09:49:55

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${HTTPD-...} Perl syntax

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