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Redirecting output from a known file descriptors to a file is as simple as

./binary 1> ./file 2>&1

But, let's say I have a binary which might be using file descriptors other than 0, 1 and 2.

File descriptors 1 and 2 default to current tty if the commands are run interactively

I'm running the binary interactively, but I think the binary has other file descriptors pointing to current tty because I have already redirected 1 and 2 file descriptors to a file, but still some output is being written to terminal, not in the file.

It might be using n number of other file descriptors, but I don't know which ones it's using.

I just want to redirect all these file descriptors which are pointing to current tty to a file.

I don't care which file descriptor the binary is writing to, all streams should be redirected to this file

Something sort of like

./binary *> ./file
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  • "I just want to redirect all file descriptors to a file." This won't work. Your binary needs to open some files, including, but not limited to, libraries; to see what I mean, run something trivial like strace echo foobar and count the open/openat calls. When you mess with these, things simply won't work. I think you really need to describe what it is that you're planning to do, and which problem you want to solve with that. Commented Jul 19, 2021 at 9:04
  • @MarcusMüller Thanks for the response. Added a bit of info in the question Commented Jul 19, 2021 at 9:13

1 Answer 1

Reset to default
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File descriptors 1 and 2 default to current tty if the commands are run interactively

no, they don't point to current tty - they are stdout and stderr, which might be routed to the current terminal through how your shell and your terminal are set up. Kind of important distinction; your redirection splices the stdout and stderr to some specific file descriptor, but someone interacting with the terminal itself is not affected by it:

If your binary is still writing data to your terminal that doesn't get redirected, it's probably not interacting with stdout; it directly interacts with the pseudoterminal (man 7 pty for more info), e.g. through some library like ncurses.

While that is a character device and you could "snoop" what is written to it, that might not make sense in a "linearized" form. Nothing stops an application from writing words backwards ("move right 2 places, print o, move right 2 places, print l, move right 2 places, print l, move right 2 places, print e, move right 2 places, print h" instead of "hello"). Also, such programs tend to update significant parts of the screen at once!

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