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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.
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The easiest way to count files in a directory on Linux is to use the “ls” command and pipe it with the “wc -l” command. The “wc” command is used on Linux in order to print the bytes, characters or newlines count.
To get count files in the folder using PowerShell, Use the Get-ChildItem command to get total files in directory and use measure-object to get count files in a folder.
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).
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