From: leo.dale...@ntymail.com
To: bug-bash@gnu.org
Subject: wait fails to grab exit codes with multiple process substitutions
Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -fdebug-prefix-map=/build/bash-2bxm7h/bash-5.0=.
-fstack-protector-strong -Wformat -Werror=format-security -Wall
-Wno-parentheses -Wno-format-security
uname output: Linux debian 4.19.0-5-amd64 #1 SMP Debian 4.19.37-5+deb10u2
(2019-08-08) x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu
Bash Version: 5.0
Patch Level: 3
Release Status: release
Description:
Hi,
I was trying to write a script that could run an arbitrary amount of
asynchronous commands, and then as soon as one finishes, perform actions
according to its exit code and output.
I attempted to do this by using process substitution and the wait
command. But wait's ability to retrieve the exit code of a finished process
that is a current shell's children seems inconsistent.
The script starts ten asynchronous commands by redirecting a process
substitution's file descriptor to one of the current shell using exec.
The script then uses wait with the process IDs of the asynchronous
tasks to retrieve their exit codes.
wait fails on nine of the ten tasks, giving the error "wait: pid X is
not a child of this shell". But weirdly enough, it succeeds when retrieving
the exit code of the last asynchronous task to be run.
The exact script I use is included in Repeat-By section, as a way to
trigger the bug.
Thank you and have a nice day.
Repeat-By:
The following script reproduces the behavior I described in the
Description section:
#!/bin/bash
# bg.sh
# Executing commands asynchronously, retrieving their exit codes and
outputs upon completion.
asynch_cmds=
echo -e "Asynchronous commands:\nPID FD"
for i in {1..10}; do
exec {fd}< <(sleep $(( i * 2 )) && echo $RANDOM && exit $i) #
Dummy asynchronous task, standard output's stream is redirected to the current
shell
asynch_cmds+="$!:$fd " # Append the task's PID and FD to the
list of running tasks
echo "$! $fd"
done
echo -e "\nExit codes and outputs:\nPID FD EXIT OUTPUT"
while [[ ${#asynch_cmds} -gt 0 ]]; do # While the list of running tasks
isn't empty
for asynch_cmd in $asynch_cmds; do # For each task in the list
pid=${asynch_cmd%:*} # Task's PID
fd=${asynch_cmd#*:} # Task's FD
if ! kill -0 $pid 2>/dev/null; then # If the task ended
wait $pid # Retrieving the task's exit code
echo -n "$pid $fd $? "
cat <&$fd # Retrieving the task's output
asynch_cmds=${asynch_cmds/$asynch_cmd /} #
Removing the task from the list
fi
done
done
By running this script, you will see that wait gives the error I
described. And also that the tenth task (with exit code 10) doesn't trigger the
wait error.
