Linux proc/pid/fd for stdout is 11?

Question

Executing a script with stdout redirected to a file. So /proc/$$/fd/1 should point to that file (since stdout fileno is 1). However, actual fd of the file is 11. Please, explain, why.

Here is session:

$ cat hello.sh
#!/bin/sh -e
ls -l /proc/$$/fd >&2

$ ./hello.sh > /tmp/1
total 0
lrwx------ 1 nga users 64 May 28 22:05 0 -> /dev/pts/0
lrwx------ 1 nga users 64 May 28 22:05 1 -> /dev/pts/0
lr-x------ 1 nga users 64 May 28 22:05 10 -> /home/me/hello.sh
l-wx------ 1 nga users 64 May 28 22:05 11 -> /tmp/1
lrwx------ 1 nga users 64 May 28 22:05 2 -> /dev/pts/0

Answer 1

I have a suspicion, but this is highly dependent on how your shell behaves. The file descriptors you see are:

• 0: standard input
• 1: standard output
• 2: standard error
• 10: the running script
• 11: a backup copy of the script's normal standard out

Descriptors 10 and 11 are close on exec, so won't be present in the ls process. 0-2 are, however, prepared for ls before forking. I see this behaviour in dash (Debian Almquist shell), but not in bash (Bourne again shell). Bash instead does the file descriptor manipulations after forking, and incidentally uses 255 rather than 10 for the script. Doing the change after forking means it won't have to restore the descriptors in the parent, so it doesn't have the spare copy to dup2 from.