why does shell save, dup, and restore redirected descriptor in the parent, rather than redirect in the child? - linux

Question

I wanted to understand how redirection is implemented in a (POSIX) shell, so I looked at dash (the simplest shell) code, and here is how it works.
A dash script like:
date > foobar.txt
date
is (as an SSCCE) handled like this:
int fd;
int saved;
fd = open64("foobar.txt", O_WRONLY|O_CREAT);
saved = fcntl(1, F_DUPFD, 10);
dup2(fd, 1);
if (!fork()) {
execl("/bin/date", "date", (char *)NULL);
}
dup2(saved, 1);
if (!fork()) {
execl("/bin/date", "date", (char *)NULL);
}
This is strange. Why save, dup and dup again to restore, descriptors in the parent, when it would be much simpler to just dup in the child, and not have to save and restore. This is simpler and I checked it works the same:
int fd;
if (!fork()) {
fd = open64("foobar.txt", O_WRONLY|O_CREAT);
dup2(fd, 1);
execl("/bin/date", "date", (char *)NULL);
}
if (!fork()) {
execl("/bin/date", "date", (char *)NULL);
}
I am sure there must be a good reason and I am not understanding something deeper. What is it?

Answer 1

No good reason as far as I can tell. Bash does things in the opposite order, and the externally observable behavior is the same.
I didn't bother reading the source code, it's easy enough to see what happens using strace. (The : is to prevent the shell from optimizing away the fork.)
$ strace -fetrace=dup2,file,process dash -c 'date > foobar.txt; :'
execve("/usr/bin/dash", ["dash", "-c", "date > foobar.txt; :"], [/* 71 vars */]) = 0
open("foobar.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
dup2(3, 1) = 1
stat("/usr/bin/date", {st_mode=S_IFREG|0755, st_size=105280, ...}) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f6650bf2750) = 1948
strace: Process 1948 attached
[pid 1947] wait4(-1, <unfinished ...>
[pid 1948] execve("/usr/bin/date", ["date"], [/* 71 vars */]) = 0
[pid 1948] exit_group(0) = ?
[pid 1948] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 1948
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=1948, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
dup2(10, 1) = 1
exit_group(0) = ?
+++ exited with 0 +++
$ strace -fetrace=dup2,file,process bash -c 'date > foobar.txt; :'
execve("/usr/bin/bash", ["bash", "-c", "date > foobar.txt; :"], [/* 71 vars */]) = 0
stat("/usr/bin/date", {st_mode=S_IFREG|0755, st_size=105280, ...}) = 0
access("/usr/bin/date", R_OK) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fd5b78349d0) = 2026
strace: Process 2026 attached
[pid 2025] wait4(-1, <unfinished ...>
[pid 2026] open("foobar.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
[pid 2026] dup2(3, 1) = 1
[pid 2026] execve("/usr/bin/date", ["date"], [/* 71 vars */]) = 0
[pid 2026] exit_group(0) = ?
[pid 2026] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 2026
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=2026, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
wait4(-1, 0x7ffeffe03c50, WNOHANG, NULL) = -1 ECHILD (No child processes)
exit_group(0) = ?
+++ exited with 0 +++