How can I send specific messages to stdout from a bash prompt with multiple commands that is run via nohup?

Question

I want to run multiple commands that are concatenated via && with nohup. The output of the commands are redirected to nohup.out which is fine. But I want send some messages in between to stdout. Is that possible?

A simple example would be like:

nohup sh -c " \
echo some message 1 (to stdout) && \
some command 1 (to nohup.out) && \
echo some message 2 (to stdout) && \
some command 2 (to nohup.out) && \
..." &

I tried to to redirect the messages to stderr within the commands and then redirect stderr back to stdout:

nohup sh -c " \
echo some message 1 >&2 && \
some command 1 (to nohup.out) && \
echo some message 2 >$2 && \
some command 2 (to nohup.out) && \
..." 2>&1 &

But this did not work for me.

Any suggestions?

what do you mean by stderr? where do you think it goes for each of your nohup, sh, echo and some commands? — jhnc
– jhnc, Commented Mar 6, 2024 at 6:37
My "goal" is, that the output of the commands goes to nohup.out and that the echo messages go to the console. — HatteSoul
– HatteSoul, Commented Mar 6, 2024 at 7:19
What do you mean by "to the console"? If you just want to send it to your terminal, why not redirecting those to /dev/tty? — user1934428
– user1934428, Commented Mar 6, 2024 at 9:06

tripleee · Accepted Answer · 2024-03-06 08:54:23Z

By definition, nohup redirects any output to nohup.out.

A simple workaround is to write somewhere else, and maybe have your shell display the contents of "somewhere else".

touch "$HOME"/.nohuplog
tail -f "$HOME"/.nohuplog &
nohup sh -c '
    exec 3>>"$HOME"/.nohuplog
    set -e # to avoid repeated "&&"
    echo "some message 1" >&3
    some command 1 (to nohup.out)
    echo "some message 2" >&3
    some command 2 (to nohup.out)
    ...' &

The exec 3>... so that you can use >&3 and don't have to repeat >>"$HOME"/.nohuplog is just to make the code less repetitive.

You could use a FIFO instead of a file, or etc.

Maybe for improved usability add a trap to explain why the script exited in the case of an error. Bash offers the ERR pseudo-signal for trap for this purpose.

Alternatively, if you redirect standard output and/or standard error, no output will end up in nohup.out; this is frequently a better solution.

nohup sh -c '
    exec >>"$HOME"/.nohuplog
    exec 2>>"$HOME"/.nohuperr
    set -e # to avoid repeated "&&"
    echo "some message 1" >&2
    some command 1 (to nohup.out)
    echo "some message 2" >&2
    some command 2 (to nohup.out)
    ...' &
tail -f "$HOME"/.nohuplog "$HOME"/.nohuperr &

(Probably use a less generic file name for the destination files, too.)

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