2

I have read books/tutorials which mention standard input/output/error. My understanding is that:

  1. standard input/output/error are concepts of a process (i.e. if someone mentions standard input/output/error, they are actually talking about standard input/output/error of a particular process. It doesn't make sense to talk about standard input/output/error of a file.)
  2. Standard input in Linux for most processes is keyboard and standard output/error is screen.

My question: if there's the standard, what is the non-standard input/output/error of a process in Linux?

7

The file descriptors 0, 1, and 2 are provided by default when a process is execed, and associated with stdin, stdout, and stderr. If a process needs additional I/O channels, it opens non-standard file descriptors starting from 3.

For 1.: Yes

For 2.: That's true for interactive processes whose I/O is not redirected. Other processes (e.g. cron jobs) can well have different associations.

  • 2
    For 2, I'd say that is a result of "most processes" being started from a shell whose own standard file descriptors are a terminal. All processes inherit the standard file descriptors from their parent. (Even redirections are instructions to the shell to substitute other descriptors in place of the shell's own. A process is unaware of how its own standard descriptors are set.) – chepner Sep 28 '18 at 13:12
  • Indeed, for 2 it's only the case that they are normally whatever the process inherits from its parent, which, for a command typed in an interactive session, will be the interactive device. Plenty of processes are started in contexts nothing like that. – R.. Sep 30 '18 at 19:13

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