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I think this post is not off-topic.We had three traditional file descriptors in Unix contexts:

0 == STDIN 1 == STDOUT 2 == STDERR

But in new articles, blogs, posts, answers, or so on, I read more than above FDs, For example:
find out which file descriptors share the same “open file description”
Why is using a shell loop to process text considered bad practice?
I/O Redirection
Using exec
File descriptors & shell scripting
However Gilles answered in last link, But after some googling I didn't found a reference about the given FDs. Also when I use the following hack :

root@debian:/home/mohsen# ls /dev/fd/ 0 1 2 3

I don't watch more than 4 , But I saw 4 5 or 7 in some examples, I have the following three serious question:

  1. What's /dev/fd/3 FD?
  2. Where're the rest of them?
  3. Do you know a related reference about rest of them?
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Any time you open a file (or anything that you can open like a file, like a socket) you get a file descriptor, which is represented by an int. The first 3 are automatically created for a process as stdin, stdout and stderr. Any other files that are opened get other descriptors. I suspect that it just increments the number each time, but I don't know if that's a reliable phenomenon or not. So 3 and 4 would be the first 2 files that were opened by the process. And 7 would be the 5th file opened.

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    File handles (that is, the fd numbers) are reused. If you open a file and get the descriptor 3, then close it, then open another file, you will again get the descriptor 3 (even though the descriptor now refers to a different file). There are some "deep" reasons for that, and some not-so-deep reasons. The deep reasons involve descriptor inheritance and what happens with daemon processes and the like. The main not-so-deep reason is that file descriptors available to a process are a finite resource, and you definitely want to avoid leaking them. Commented May 20, 2015 at 13:53

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