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When I run this script, it does not display the watch output on the screen, and does not timeout after 5s.

§ cat script.sh #!/bin/bash timeout 5s watch -n 1 ps

This is what I see on the screen when I run the script:

enter image description here

However, running the content of the script directly in the terminal works as expected: i.e.,

§ timeout 5s watch -n 1 ps

What exactly is happening under the covers that is preventing watch from showing the content on the screen when invoked from a script?

Furthermore, using the --foreground option of timeout in the script makes it work as expected, but I don't understand why.

If it matters, here are versions of the tools:

§ bash --version bash --version GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin19) Copyright (C) 2007 Free Software Foundation, Inc. § watch -v watch from procps-ng 3.3.12-dirty § timeout --version timeout (GNU coreutils) 8.29 Copyright (C) 2017 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Written by Padraig Brady.
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The behavior of timeout is different when it is run from a shell script. The --foreground option forces the default "interactive" behavior, even when run from a script. From the manpage:

--foreground when not running timeout directly from a shell prompt, allow COMMAND to read from the TTY and get TTY signals; in this mode, children of COMMAND will not be timed out

And watch needs to write to the terminal in order to do all of its fancy ANSI tricks.

If you want to see that in action, try the following:

$ script -c 'timeout --foreground 5s watch -n 1 ps' $ less typescript
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    Makes sense. But are we meant to use the --foreground option whenever we use it in a script? Writing to output is one thing, but shouldn't it at least timeout even if we don't use that switch? I gathered as much from the documentation about this flag, but I don't get how the ability to get TTY signals is related to the fact that it does not time out. Commented May 5, 2021 at 22:03

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