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Re: Parameter expansion problem involving ${*#*--}


From: Chet Ramey
Subject: Re: Parameter expansion problem involving ${*#*--}
Date: Tue, 3 Oct 2000 11:31:04 -0400

> Machine Type: i386-redhat-linux-gnu
> 
> Bash Version: 2.04
> Patch Level: 11
> Release Status: release
> 
> Description:
>       Parameter expansion including pattern matching misbehaves in an
>       admittedly obscure case.  More precisely, the expansion of
> 
>         ${*#*--}
> 
>       returns the wrong value for certain values of $* containing more than
>       one occurrence of the substring `--'.  The problem seems to be
>       confined to the special variable $* and does not occur with "normal"
>       (alphanumeric) variables.

No, bash is behaving properly.  You have encountered the difference between
unquoted and quoted expansions of $*.

> Repeat-By:
>       The problem can be reproduced using the following shell script:
> 
>         #!/bin/bash
> 
>         echo 'Value of ${*}:      ' ${*}
>         echo 'Value of ${*#*--}:  ' ${*#*--}
>         echo 'Assigning $* to $bla'
>         bla="$*"
>         echo 'Value of ${bla}:    ' ${bla}
>         echo 'Value of ${bla#*--}:' ${bla#*--}
> 
>         Assuming that the script is stored in an executable file called
>         `bash-bug', upon execution of
> 
>         % bash-bug -- --delete
> 
>       the output is
> 
>         Value of ${*}:       -- --delete

${*} is a list, not a single word.

>         Value of ${*#*--}:   delete

The manual page says, in the description of the ${parameter#word} expansion:

        If parameter is @ or *, the pattern removal operation is applied
        to each positional parameter in turn, and the expansion is the
        resultant list.

The first word is `--', and is removed completely.  The second word is
--delete, and is converted to delete.  This is the output you get.

>         Assigning $* to $bla
>         Value of ${bla}:     -- --delete

By using the double quotes, you just converted a list into a string.
(Actually, it would have behaved the same without the double quotes,
since variable assignment doesn't perform word splitting and you
haven't changed $IFS.)

>         Value of ${bla#*--}: --delete

This is an operation on a single string.

>         Note the difference in the expansion of ${*#*--} and ${bla#*--}.

As the man page documents.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
( ``Discere est Dolere'' -- chet)

Chet Ramey, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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