sleep 10 | read hangs interactive shell after ^Z

[McDutchie]

Found on comp.unix.shell:

From: Christian Weisgerber <[email protected]>
Newsgroups: comp.unix.shell
Subject: ksh93: pipelines vs. job control
Date: Mon, 13 May 2024 14:35:32 -0000 (UTC)
Message-ID: <[email protected]>
User-Agent: slrn/1.0.3 (FreeBSD)

... or "how to accidentally hang your ksh93 session".

Bash has this shell option:

    lastpipe
            If set, and job control is not active, the shell runs
            the last command of a pipeline not executed in the
            background in the current shell environment.

The bit about job control sounds like a weird stipulation, but makes
sense once you think about it.  I wonder how ksh93 handles that.
Famously, the AT&T ksh executes the last command of a pipeline in
the current shell:

  $ x=foo
  $ { /bin/sleep 10; echo bar; } | read x
  $ echo $x
  bar

However, a background pipeline runs in a subshell, as documented
in the man page:

  $ x=foo
  $ { /bin/sleep 10; echo bar; } | read x &
  [1]     78726
  $ 
  [1] +  Done                    { /bin/sleep 10; echo bar; } | read x &
  $ echo $x  
  foo

But we are in job control environment.  I can start a pipeline in
the foreground, suspend it, and put it into the background.  ksh93
can't know about that in advance.

  $ { /bin/sleep 10; echo bar; } | read x  
  ^Z

... and now the pipeline is suspended...

    PID  PGID STAT   COMMAND
  71496 71496 S      - ksh93 ksh93
  62980 62980 T+     `-- ksh93 ksh93
   3778 62980 T+p      `-- /bin/sleep 10

... and you're stuck.  There's no shell prompt, so you can't bg or fg
anything, intr (^C) or quit (^\) don't help, and there's no tty
control character to continue a suspended process (group).

You need to take radical measures like hanging up, or sending a
SIGCONT or SIGKILL from a different terminal.

I can't quite tell if this behavior constitutes a bug, but it seems
to follow from the design decision to not run the last command of
a pipeline in a subshell.  And it can trap the unwary.


A more minimal example:

  $ /bin/sleep 10 | read
  ^Z

  PID  PGID STAT   COMMAND
  30796 30796 S      - ksh93 ksh93
  31981 31981 T+p    `-- /bin/sleep 10

-- 
Christian "naddy" Weisgerber                          [email protected]