Unix as a Second Language

Unix tip: Recursively removing empty directories

As you undoubtedly know, the rmdir command will remove an empty directory but, if a directory contains files, it will balk and report "Directory not empty". To remove directories which are in fact empty is then easy. In any file system containing multiple directories, you can issue the "rmdir *" command knowing that only the empty directories will be removed.

To recursively remove empty directories, you could try a mix of the find and rmdir commands. For example, this command would remove numerous directories yet leave all the populated ones:

find . -type d -print | xargs rmdir

If you don't want to see all the "Directory not empty" errors, you can send standard error to /dev/null:

find . -type d -print | xargs rmdir 2>/dev/null

Looking for directories with find and then removing them has one drawback, however, If a directory becomes empty in the process of removing a directory that's inside of it, the parent directory will not be removed because find will detect it before it finds its child directory and it won't yet be empty.

You could run one of the commands shown above multiple times, maybe until no more "Directory not empty" errors appear. Alternatively, you could run a script like this one that uses find to create a list of directories, reverses the list so that child directories appear before their parents and then runs a rmdir against each directory in the list. In this way, all of the empty directories are removed in one pass.

#!/bin/bash

# get directory list
find . -type d -print > dirs$$

# reverse the listing
tail -r dirs$$ > revdirs$$

while read dirname
do
    rmdir $dirname 2>/dev/null
done < revdirs$$

# clean up
rm dirs$$
rm revdirs$$

One of the core commands in this script is the tail -r command. This command reverses the contents of a file or of any input piped to it. In fact, this command is so useful for this operation that you could reduce the script above to this one-liner:

find . -type d | tail -r | xargs rmdir

Or, again, if you don't want to see all the "Directory not empty" complaints, pipe the error output of this string of commands to /dev/null:

find . -type d | tail -r | xargs rmdir 2>/dev/null

As a one-liner, this command lends itself to use as an alias.

$ alias rmEmptyDirs="find . -type d | tail -r | xargs rmdir 2>/dev/null"

If you want to test this alias, create a directory under /tmp and populate it with a series of new directories.

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