How can I check whether a directory is empty or not? How do I check for any *.mpg files, or count how many there are?
See ArithmeticExpression for explanations of arithmetic commands.
Of course, you can use any glob you like instead of *. E.g. *.mpg or /my/music/*.mpg works fine.
Bear in mind that you need read permission on the directory, or it will always appear empty.
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 (for such cases consider setting failglob). Setting nullglob in a SubShell avoids accidentally changing its setting in the rest of the shell, at the price of an extra fork(). If you'd like to avoid having to set and unset shell options, you can pour it all into a SubShell:
The other disadvantage of this approach (besides the extra fork()) is that the array is lost when the subshell exits. If you planned to use those filenames later, then they have to be retrieved all over again.
Both of these examples expand a glob and store the resulting filenames into an array, and then check whether the number of elements in the array is 0. If you actually want to see how many files there are, just print the array's size instead of checking whether it's 0:
You can also avoid the nullglob if you're OK with putting a non-existing filename in the array should no files match (instead of an empty array):
Without nullglob, if there are no files in the directory, the glob will be added as the only element in the array. Since * is a valid filename, we can't simply check whether the array contains a literal *. So instead, we check whether the thing in the array exists as a file. The -L test is required because -e fails if the first file is a dangling symlink.
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. Note the "magic 3 globs"1 as POSIX does not have the dotglob option.
At this stage, the positional parameters have been loaded with the contents of the directory, and can be used for processing.
If you just want to count files:
In the Bourne shell, it's even worse, because there is no test -e or test -L:
Of course, that fails if * exists as something other than a plain file (such as a directory or FIFO). The absence of a -e test really hurts.
Here is another solution using find:
If you want it not to recurse, then you need to tell find not to recurse into directories. This gets really tricky and ugly. GNU find has a -maxdepth option to do it. With standard POSIX find, you're stuck with -prune. This is left as an exercise for the reader.
Never try to parse 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, one 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:
Most commonly, all that's really needed is something like this:
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.
Support for a nullglob-like feature is inconsistent. In ksh93 it can be done on a per-pattern basis by prefixing with ~(N)2:
From: http://permalink.gmane.org/gmane.comp.standards.posix.austin.general/2058, which contains some good discussion. (2)