Re: [Help-bash] The best way to test whether a directory has *.txt files?

[Steven W. Orr]

On 6/28/2012 8:00 AM, Greg Wooledge wrote:
> On Thu, Jun 28, 2012 at 04:44:49PM +0800, Clark WANG wrote:
> > > There can be more than one way to test whether a directory contains
> > > *.txt files (to be used with 'find -exec'). But I'm wondering what is
> > > the best way (in terms of convenience to type) to do it.
>
> There is no "best way".  There are ways that don't work, and there are
> ways that STILL don't work, and then, every once in a great while, you
> may find a way that actually does work.
>
> For example, any trick that expands a glob and then compares the result
> to the literal glob is wrong, because the glob itself is also a valid
> filename.
>
> # Wrong.
> touch '*'
> if [[ `echo *` = '*' ]]; then echo "Derp, I think there are no files"; fi
> # Wrong.
>
> > > Does anybody have any tricks on this?
>
> http://mywiki.wooledge.org/BashFAQ/004
>
> > I've ever seen this:
> >
> > ( shopt -s nullglob; set -- /the/dir/*.txt; (( $# )) )
>
> I'm not fond of subshells for this kind of task, due to the performance
> penalties.  Personally I'd just use:
>
> shopt -s nullglob dotglob
> files=(/the/dir/*.txt); address@hidden
> shopt -u nullglob dotglob
>
> This also has the advantage of leaving an array populated with the
> filenames for later iteration or whatever.  With the subshell approach,
> that array is discarded.


Greg is right on the money. Parens mean sub-shells. They do not mean 
prioritization like they do in other languages.

Here's a snippet of how I reliably do what you want. This is not a minimal 
example. It's just something I have lying around. The trick is to set nullglob 
and to then restore it. The glob is assigned to an array so I can see how many 
matches I got. Zero is not enough and more than 1 is too many.

    DIR=${DIST}${ARCH_STR}
    # set nullglob so the we get no expansion if the glob fails.
    # Assign to an array so the expansion can be done inline.
    shopt -s nullglob
    # It can be either a 2, a 4 or a 10.
    PKG=( 
${DIR}/${RPM_BASE_NAME}${KERN_VAR}-{2,4,10}.*_${RPM_DISTRO}-[1-9].${RPM_ARCH_STR}.rpm 
)
    shopt -u nullglob
    # Check for whether the expansion failed.
    if [[ address@hidden -eq 0 ]]
    then
        echo "ERROR:rpm file not found."
        exit 1
    fi
    # Check if we got too many matches.
    if [[ address@hidden -gt 1 ]]
    then
        echo "INTERNAL ERROR: Too many rpm files found."
        echo "Matching packages: address@hidden"
        exit 1
    fi

I have other code that does similar things that are more general, but this 
works as the the problem is described.


--
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steveo at syslang.net