Does the shell's & operator perform redirects before or after...

pasta_paul

Consider the following script: rm -f /tmp/bar echo foo >/tmp/bar & tail -f < /tmp/bar When processing the echo line, does the shell first perform the redirect opening /tmp/bar for writing and then launch the asynchronous command? Or does the redirect happen inside the asynchronous process? If the former, then this fragment is race free as the tail line happens strictly after /tmp/foo is created. If the latter, there is a chance that the open for...

Originally asked by Igor Bukanov on Stack Exchange (unix) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

The forking is done first and the open(2) for the redirection done in the child process. Doing the open(2) before the fork(2) would make little sense. It would also make fifos less usable. For instance: cmd1 < fifo & cmd2 > fifo Would hang because the open("fifo", O_RDONLY) doesn't return until fifo is also open in write mode. That would also mean the shell would need to close the fd in the parent after the fork() . It's easy to check with strace or equivalent. $ strace -fP /dev/zero sh -c 'echo > /dev/zero & wait' strace: Process 10918 attached [pid 10918] openat(AT_FDCWD, "/dev/zero", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3 [pid 10918] dup2(3, 1) = 1 [pid 10918] close(3) = 0 [pid 10918]...

Full answer by Stéphane Chazelas: https://unix.stackexchange.com/a/805406 (CC BY-SA 4.0).

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