Bash unsets *_PID variable before I can wait on coproc

bashprocess

I'm trying to use coprocesses to pipe the output of asynchronous processes into other shell commands and then check the exit status of the asynchronous process:

coproc LS { ls dir; }
while IFS= read i; do echo "$i"; done <&"$LS" # Do something with the output.
wait "$LS_PID"; echo $? # Check exit status.

The problem is that the above code does not work, at least not with Bash 4.3.46(1)-release. After the while statement has executed, Bash will unset $LS_PID (probably because it notices that the process has terminated). Calling wait on the processes still works, e.g.:

coproc LS { ls dir; }
P=$LS_PID
while IFS= read i; do echo "$i"; done <&"$LS" # Do something with the output.
wait "$P"; echo $? # Check exit status.

This will print 0 or 2 depending on whether the directory exists. Thus it is just an inconvenience that I have to save the PID to another variable. But I'm interested in why this happens and whether I can influence this.

Why is $LS_PID unset before I wait on the process? Is this behavior documented somewhere? Is there an easy workaround for this, because I think it's a common use case to both use the output of a command and also check its exit code.

Best Answer

This is the intended behaviour of the COPROC_PID variable it seems, though it's undocumented in the Bash manual.

The correct workaround is to do exactly what you've done; save the value of $COPROC_PID in a variable that will not be unset when the co-process exits.

Note that this will not work if the co-process exits before you have time to get it's PID, so it may be prudent to make sure that your variable has a value before you use it.

Reference: http://gnu-bash.2382.n7.nabble.com/Several-issues-with-coprocesses-regarding-exit-status-td11125.html

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Get the exit code of processes forked from the master process

Processes report their exit status to their parent and if their parent is dead to the process of id 1 (init), though with recent versions of Linux (3.4 or above), you can designate another ancestor as the child subreaper for that role (using prctl(PR_SET_CHILD_SUBREAPER)).

Actually, after they die, processes become zombies until their parent (or init) retrieves their exit status (with waitpid() or other).

In your case, you're saying the children are dying after (as a result of?) run-jobs dying. That means they'll report their exit status to init or to the process designated as child sub-reaper.

If init doesn't log that (and it generally doesn't) and if you don't use auditing or process accounting, that exit status will be lost.

If on a recent version of Linux, you can create your own sub-reaper to get the pid and exit status of those orphan processes. Like with perl:

$ perl -MPOSIX -le '
  require "syscall.ph";
  syscall(&SYS_prctl,36,1) >= 0 or die "cannot set subreaper: $!";

  # example running 1 child and 2 grand children:
  if (!fork) {
    # There, you would run:
    # exec("php", "run-jobs");
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  }'
22425: 88
22426: 12
22427: 123

If you wanted to retrieve information on the dying processes (like command line, user, ppid...), you'd need to do that while they're still in the zombie state, that is before you've done a wait() on them.

To do that you'd need to use the waitid() API with the WNOWAIT option (and then get the information from /proc or the ps command). I don't think perl has an interface to that though, so you'd need to write it in another language like C.

Bash – Wait for Foreground Process to Emit String, Then Send to Background

The idea is to run the server in the background and then grep its output as long as the expected string has appeared 'Something 3' ('Server Initialized' in your case)

#!/bin/bash

main()  
{
    output=$(mktemp "${TMPDIR:-/tmp/}$(basename 0).XXX")
    server &> $output &
    server_pid=$!
    echo "Server pid: $server_pid"
    echo "Output: $output"
    echo "Wait:"
    until grep -q -i 'Something 3' $output
    do       
      if ! ps $server_pid > /dev/null 
      then
        echo "The server died" >&2
        exit 1
      fi
      echo -n "."
      sleep 1
    done
    echo 
    echo "Server is running!" 
}

server() 
{       
    i=0
    while true; do
      let i++
      echo "Something $i"  
      sleep 1
    done
}

main "$@"

Best Answer

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Get the exit code of processes forked from the master process

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