Diff for "BashFAQ/004"

How can I check whether a directory is empty or not? How do I check for any *.mpg files?

In Bash, you can do this safely and easily with the nullglob and dotglob options (which change the behaviour of globbing), and arrays:

    # Bash
    shopt -s nullglob dotglob
    files=(*)
    (( ${#files[*]} )) || echo directory is empty
    shopt -u nullglob dotglob

Or you can pour it into a SubShell to avoid having to reset (in fact, unset! - the code above assumes the shell options were unset before) the shell options again:

    # Bash
    if (shopt -s nullglob dotglob; f=(*); ((! ${#f[@]}))); then
        echo "The current directory is empty."
    fi

Of course, you can use any glob you like instead of *. E.g. *.mpg or /my/music/*.mpg works fine.

Some people dislike nullglob because having unmatched globs vanish altogether confuses programs like ls. Mistyping ls *.zip as ls *.zpi may cause every file to be displayed. Setting nullglob in a SubShell avoids accidentally changing its setting in the rest of the shell, at the price of an extra fork().

If your script needs to run with various non-Bash shell implementations, you can try using an external program like python, perl, or find; or you can try one of these:

    # POSIX
    # Clobbers the positional parameters, so make sure you don't need them.
    set -- *
    if test -e "$1" || test -L "$1"; then
        echo "directory is non-empty"
    fi

(The -L test is required because -e fails if the first file is a dangling symlink.)

    # Bourne
    # (Of course, the system must have printf(1).)
    if test "`printf '%s %s %s' .* *`" = '. .. *' && test ! -f '*'
    then
        echo "directory is empty"
    fi

Yes, they're quite ugly, but they should be more portable than anything depending on ls output. Even ls -A solutions can break (e.g. on HP-UX, if you are root, ls -A does the exact opposite of what it does if you're not root -- and no, I can't make up something that incredibly stupid).

In fact, you may wish to avoid the direct question altogether. Usually people want to know whether a directory is empty because they want to do something involving the files therein, etc. Look to the larger question. For example, one of these find-based examples may be an appropriate solution:

   # Bourne
   find "$somedir" -type f -exec echo Found unexpected file {} \;
   find "$somedir" -maxdepth 0 -empty -exec echo {} is empty. \;  # GNU/BSD
   find "$somedir" -type d -empty -exec cp /my/configfile {} \;   # GNU/BSD

Most commonly, all that's really needed is something like this:

    # Bourne
    for f in ./*.mpg; do
        test -f "$f" || continue
        mympgviewer "$f"
    done

In other words, the person asking the question may have thought an explicit empty-directory test was needed to avoid an error message like mympgviewer: ./*.mpg: No such file or directory when in fact no such test is required.

CategoryShell