Linux proc/pid/fd for stdout is 11?

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 开发者_开发问答-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

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.

The output of strace can be helpful here.

The relevant section is

fcntl64(1, F_DUPFD, 10)                 = 11
close(1)                                = 0
fcntl64(11, F_SETFD, FD_CLOEXEC)        = 0
dup2(2, 1)                              = 1
stat64("/home/random/bin/ls", 0xbf94d5e0) = -1 ENOENT (No such file or
+++++++>directory)
stat64("/usr/local/bin/ls", 0xbf94d5e0) = -1 ENOENT (No such file or directory)
stat64("/usr/bin/ls", 0xbf94d5e0)       = -1 ENOENT (No such file or directory)
stat64("/bin/ls", {st_mode=S_IFREG|0755, st_size=96400, ...}) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
+++++++>child_tidptr=0xb75a8938) = 22748
wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 22748
--- SIGCHLD (Child exited) @ 0 (0) ---
dup2(11, 1)                             = 1

So, the shell moves the existing stdout to an available file descriptor above 10 (namely, 11), then moves the existing stderr onto its own stdout (due to the >&2 redirect), then restores 11 to its own stdout when the ls command is finished.