Get n filenames in a directory?

Question

I have a single flat directory with over a million files. I just wanted to take a sample of the first few files but FileNames doesn't include a "only the first n" option, and so it took over a minute:

Is there a faster way?

Answer 1

Using OS shells commands seems to be much faster, although their output will need some massaging to obtain only the file names.

For instance, the following works quite well on my system (Win7-64):

Import["!dir /b /a-d C:\\Windows\\*", "Text"] // StringSplit[#, "\n"] &

This command takes 0.11s to execute; for comparison the corresponding FileNames expression (FileNames["*", "C:\\Windows\\"]) takes 0.33s. The difference is even more pronounced with larger/deeper directories.

In the dir command above, /a-d selects files that are not directories; /b produces minimal output which is easier to parse. If you want to traverse subdirectories, you can use the /s option.

Answer 2

New function in Mathematica 11 FileSystemMap with option MaxItems (documentation) can be useful here.

dir = "C:\\Users\\Alexey\\Documents";
n = 10;
f = FileSystemMap[#&, dir, MaxItems -> n] // Keys;

Answer 3

a linux version using find to list files and head to take the first n.

dir = "/path"
StringSplit[
    RunProcess[
         {"/bin/sh",
          "-c",
          "find "<>dir<>" -maxdepth 1 -type f | head -10"
          },"StandardOutput"]]

note find does not return files in any canonical order, you can sort for example, add | sort in front of | head

Note FileNames[][;;10]] is a good bit faster by my testing, but I don't have a folder with a million files to test it on.