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lastpipe and "*may* be run by the shell process"

From: Andreas Kusalananda Kähäri
Subject: lastpipe and "*may* be run by the shell process"
Date: Mon, 11 May 2020 21:15:51 +0200


I was recently answering a bash-related query in another context, and
found myself quoting the bash (5.0.17(1)-release) manual:

        Each command  in a  pipeline is  executed as  a separate
        process (i.e.,  in a  subshell).  See  COMMAND EXECUTION
        ENVIRONMENT for a description of a subshell environment.
        If  the  lastpipe  option  is enabled  using  the  shopt
        builtin (see  the description of shopt  below), the last
        element of a pipeline may be run by the shell process.

What caught my eye was the word "may" in the last sentence, and it
struck me that I couldn't immediately think of a case where this word
couldn't be "will" instead.  That is to say, where setting the lastpipe
shell option did *not* cause the last part of a pipeline to run in the
invoking shell environment.

Not usually using lastpipe myself, it's possible that I just haven't
come across instances where it doesn't work as naively expected.  Does
anyone have examples of where the last part of the pipeline does not run
in the invoking shell environment, even when lastpipe is set?

Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden


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