Question

I was looking through the /etc/bash_completion script found in some Debian packages. I was interested in using the code that looks through a specific directory (/etc/bash_completion.d/ by default) and sources every file in that directory.

Unfortunately, trying to run the script causes errors under the Mac OS X version of bash. The lines in question are:

for i in $BASH_COMPLETION_DIR/*; do
    [[ ${i##*/} != @(*~|*.bak|*.swp|\#*\#|*.dpkg*|.rpm*) ]] &&
    [ \( -f $i -o -h $i \) -a -r $i ] && . $i
done

Specifically, my version of bash (3.2.17) chokes on the @() construction. I get that the point of that first test is to make sure we don’t source any editor swap files or backups, etc. I’d like to understand exactly what that @() syntax does, and, if possible how to get something similar (and similarly elegant) running on my ancient copy of bash. Can anyone offer insight?

Answer 1

It’s just an extension to the shell comparison which is equivalent to the grep “or” operator (|).

Depending on your bash version, it may not be available or you may have to set extglob with the shopt built-in. See the following session transcript:

    pax@daemonspawn> $ bash --version
        GNU bash, version 3.2.48(21)-release (i686-pc-cygwin)
        Copyright (C) 2007 Free Software Foundation, Inc.
    pax@daemonspawn> echo @(*~|*.pl)
        bash: syntax error near unexpected token '('
    pax@daemonspawn> shopt extglob
        extglob off
    pax@daemonspawn> shopt -s extglob
    pax@daemonspawn> echo @(*~|*.pl)
        qq.pl qq.sh~ xx.pl
    pax@daemonspawn> 

That allows the following to work:

    ?(pattern-list)
        Matches zero or one occurrence of the given patterns
    *(pattern-list)
        Matches zero or more occurrences of the given patterns
    +(pattern-list)
        Matches one or more occurrences of the given patterns
    @(pattern-list)
        Matches one of the given patterns
    !(pattern-list)
        Matches anything except one of the given patterns

If you can’t get it working with shopt, you can generate a similar effect with older methods such as:

    #!/bin/bash
    for i in $BASH_COMPLETION_DIR/*; do
        # Ignore VIM, backup, swp, files with all #'s and install package files.
        # I think that's the right meaning for the '\#*\#' string.
        # I don't know for sure what it's meant to match otherwise.

        echo $i | egrep '~$|\.bak$|\.swp$|^#*#$|\.dpkg|\.rpm' >/dev/null 2>&1
        if [[ $? == 0 ]] ; then
            . $i
        fi
    done

Alternatively, if there’s multiple complex determinations that will decide whether you want it sourced, you can use a doit variable that’s initially set to true, and set it to false if any of those conditions trigger. For example, the following script qq.sh:

    #!/bin/bash
    for i in * ; do
        doit=1

        # Ignore VIM backups.
        echo $i | egrep '~$' >/dev/null 2>&1
        if [[ $? -eq 0 ]] ; then
            doit=0
        fi

        # Ignore Perl files.
        echo $i | egrep '\.pl$' >/dev/null 2>&1
        if [[ $? -eq 0 ]] ; then
            doit=0
        fi

        if [[ ${doit} -eq 1 ]] ; then
            echo Processing $i
        else
            echo Ignoring $i
        fi
    done

did this in my home directory:

    Processing Makefile
    Processing binmath.c
    : : : : :
    Processing qq.in
    Ignoring qq.pl    --+
    Processing qq.sh    |
    Ignoring qq.sh~   --+--- see?
    Processing qqin     |
    : : : : :           |
    Ignoring xx.pl    --+