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

The -U option for ls is not in POSIX, and in OS X's ls it has a different meaning from GNU ls, which is that it makes -t and -l use creation times instead of modification times. -f is in POSIX as an XSI extension. The manual of GNU ls describes -f as do not sort, enable -aU, disable -ls --color and -U as do not sort; list entries in directory order.

POSIX describes -f like this:

Force each argument to be interpreted as a directory and list the name found in each slot. This option shall turn off -l, -t, -s, and -r, and shall turn on -a; the order is the order in which entries appear in the directory.

Commands like ls|wc -l give the wrong result when filenames contain newlines.

In zsh you can do something like this:

a=(*(DN));echo ${#a}

D (glob_dots) includes files whose name starts with a period and N (null_glob) causes the command to not result in an error in an empty directory.

Or the same in bash:

shopt -s dotglob nullglob;a=(*);echo ${#a[@]}

If IFS contains ASCII digits, add double quotes around ${#a[@]}. Add shopt -u failglob to ensure that failglob is unset.

A portable option is to use find:

find . ! -name . -prune|grep -c /

grep -c / can be replaced with wc -l if filenames do not contain newlines. ! -name . -prune is a portable alternative to -mindepth 1 -maxdepth 1.

Or here's another alternative that does not usually include files whose name starts with a period:

set -- *;[ -e "$1" ]&&echo "$#"

The command above does however include files whose name starts with a period when an option like dotglob in bash or glob_dots in zsh is set. When * matches no file, the command results in an error in zsh with the default settings.

Answer 2

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.