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From The Linux Programming Interface

In an interactive shell, these three file descriptors 0, 1 and 2 normally refer to the terminal under which the shell is running.

  1. Does "the terminal under which the shell is running" mean the controlling terminal of the session to which the interactive shell belongs?

    If yes, what if the shell's session doesn't have a controlling terminal?

  2. When the shell is created from its parent process, will the shell automatically create connection betwee file descriptors 0, 1 and 2 and the terminal, in each of the following cases (inheritance from the parent process of the shell):

    • if "the terminal under which the shell is running" or the controlling terminal has already been opened at a file descriptor which is not 0, 1 and 2?

    • if the file descriptors 0, 1 and 2 have already been connected to a file which is not "the terminal under which the shell is running" or the controlling terminal?

  3. What if the shell in the quote is noninteractive?

Thanks.

Btw, just assume "shell" is POSIX or bash.

Related How can we disconnect a file descriptor from any file?

  • 1
    The terminal opens FDs 0, 1, and 2 to whichever executable it runs. If the executable happens to be a shell then so be it. – Ignacio Vazquez-Abrams May 29 '18 at 2:16
  • And if the terminal is connected over a port, like all real terminals used to be, (traditionally) init spawns getty which opens the port and waits for a terminal to connect then execs login which verifies the userid/password and usually (except for a restricted userid) execs a shell -- all with 0,1,2 on the terminal. – dave_thompson_085 May 29 '18 at 3:28
3

No.

This is done by the various implementations of getty, by open-controlling-tty, by the forked pty-slave-side part of an SSH server, by the forked pty-slave-side part of a GUI terminal emulator, or some such.

And notice that the text quoted has no implication that the shell does this. You have created a whole load of questions based upon a false premise pulled out of thin air.

And you knew that getty does this, because you wrote that it did in "When is the process session led by login started?", written at the same time as this question.

Further reading

  • Thanks. I think your nosh is great! I happened to have a problem which nosh's ulimit addresses, and still wonder about how to achieve the similar using regular commands: unix.stackexchange.com/questions/446539/… – Tim May 29 '18 at 12:29

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