Find the number of files in a directory

Question

Is there any method in Linux to calculate the number of files in a directory (that is, immediate children) in O(1) (independently of the number of files) without having to list the directory first? If not O(1), is there a reasonably efficient way?

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

Answer 1

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).