From what timestamp is the output of a command

Question

I have a program that is downloading some images into a folder. I want to count the number of images in the folder currently. I used a command like ls -1 download_folder | wc -l.

But this command takes a lot of time to run(since I have already downloaded a lot of images). While this command is running, my downloader keeps on downloading the images.

And finally, my command returns the count. I wanted to know, from what timestamp this count is. Is it from the instant I started running the command or it gives the current latest count?

Edit: I tried validating for myself, but could not find any way. Thanks.

Answer 1

The ls command is reading the whole directory, and sorting it, and then passing it to wc, which does not care about order anyway. It will run much faster if you tell ls not to sort, as:

ls -U | wc -l

This will reduce the time window in which new files can be added, and be more accurate. However, there is still a race condition between the additions and the listing, which cannot be overcome. Some file systems hash the names, some append to a linear list, so ls may or may not see any additions while it runs.