Linux number of files in directory curiosity

Question

Just out of curiosity, is there any method in Linux to know the number of files in a directory in O(1) (independently of the number of files) without having to list the directory first?

I'm searching for an alternative to ls | wc -l.

Thanks!

Answer 1

As far as I know, there is no better alternative. This information might be off-topic to this question and you may already know this that under Linux (in general under Unix) directories are just special file which contains the list of other files (I understand that the exact details will be dependent on specific file system but this is the general idea). And there is no call to find the total number of entries without traversing the whole list. Please make me correct if I'm wrong.

Answer 2

readdir is not as expensive as you may think. The knack is avoid stat'ing each file, and (optionally) sorting the output of ls.

/bin/ls -1U | wc -l

avoids aliases in your shell, doesn't sort the output, and lists 1 file-per-line (not strictly necessary when piping the output into wc).

The original question can be rephrased as "does the data structure of a directory store a count of the number of entries?", to which the answer is no. There isn't a more efficient way of counting files than readdir(2)/getdents(2).

Answer 3

One can get the number of subdirectories of a given directory without traversing the whole list by stat'ing (stat(1) or stat(2)) the given directory and observing the number of links to that directory. A given directory with N child directories will have a link count of N+2, one link for the ".." entry of each subdirectory, plus two for the "." and ".." entries of the given directory.

However one cannot get the number of all files (whether regular files or subdirectories) without traversing the whole list -- that is correct.

The "/bin/ls -1U" command will not get all entries however. It will get only those directory entries that do not start with the dot (.) character. For example, it would not count the ".profile" file found in many login $HOME directories.

One can use either the "/bin/ls -f" command or the "/bin/ls -Ua" command to avoid the sort and get all entries.

Perhaps unfortunately for your purposes, either the "/bin/ls -f" command or the "/bin/ls -Ua" command will also count the "." and ".." entries that are in each directory. You will have to subtract 2 from the count to avoid counting these two entries, such as in the following:

expr `/bin/ls -f | wc -l` - 2     # Those are back ticks, not single quotes.

The --format=single-column (-1) option is not necessary on the "/bin/ls -Ua" command when piping the "ls" output, as in to "wc" in this case. The "ls" command will automatically write its output in a single column if the output is not a terminal.

Answer 4

use ls -1 | wc -l