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Comment by jez

6 hours ago

Another fun consequence of this is that you can initialize otherwise-unset file descriptors this way:

    $ cat foo.sh
    #!/usr/bin/env bash

    >&1 echo "will print on stdout"
    >&2 echo "will print on stderr"
    >&3 echo "will print on fd 3"

    $ ./foo.sh 3>&1 1>/dev/null 2>/dev/null
    will print on fd 3

It's a trick you can use if you've got a super chatty script or set of scripts, you want to silence or slurp up all of their output, but you still want to allow some mechanism for printing directly to the terminal.

The danger is that if you don't open it before running the script, you'll get an error:

    $ ./foo.sh
    will print on stdout
    will print on stderr
    ./foo.sh: line 5: 3: Bad file descriptor

7 comments

jez

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47282847  6 hours ago

Interesting. Is this just literally “fun”, or do you see real world use cases?

  • nothrabannosir  2 hours ago

    The aws cli has a set of porcelain for s3 access (aws s3) and plumbing commands for lower level access to advanced controls (aws s3api). The plumbing command aws s3api get-object doesn't support stdout natively, so if you need it and want to use it in a pipeline (e.g. pv), you would naively do something like

      $ aws s3api get-object --bucket foo --key bar /dev/stdout | pv ...

    Unfortunately, aws s3api already prints the API response to stdout, and error messages to stderr, so if you do the above you'll clobber your pipeline with noise, and using /dev/stderr has the same effect on error.

    You can, though, do the following:

      $ aws s3api get-object --bucket foo --key bar /dev/fd/3 3>&1 >/dev/null | pv ...

    This will pipe only the object contents to stdout, and the API response to /dev/null.

  • jez  5 hours ago

    I have used this in the past when building shell scripts and Makefiles to orchestrate an existing build system:

    https://github.com/jez/symbol/blob/master/scaffold/symbol#L1...

    The existing build system I did not have control over, and would produce output on stdout/stderr. I wanted my build scripts to be able to only show the output from the build system if building failed (and there might have been multiple build system invocations leading to that failure). I also wanted the second level to be able to log progress messages that were shown to the user immediately on stdout.

        Level 1: create fd=3, capture fd 1/2 (done in one place at the top-level)
    Level 2: log progress messages to fd=3 so the user knows what's happening
    Level 3: original build system, will log to fd 1/2, but will be captured

    It was janky and it's not a project I have a need for anymore, but it was technically a real world use case.

  • jas-  4 hours ago

    Red hat and other RPM based distributions recommended kickstart scripts use tty3 using a similar method