Is it possible to get pid of first command (not background)? Zsh, not bash - process

Question

I have in zsh:
(sleep 100;program1 & another program & another)&
How to get PID of 'sleep' process (I need to kill it)?
$! - returns pid not of sleep process
jobs -p - also useless here
killall -9 sleep -useless, because it will kill all sleep processes, not only this.

Answer 1

One option is to print the pid from sleep from within the set of commands. This can be done by backgrounding the sleep process, getting the pid with $!, and then using wait to block until it exits.
% (sleep 100 &; print sleep_pid:$!; wait $!; print cmd1 && print cmd2) &
[1] 18055
sleep_pid:18056
% kill 18056
cmd1
cmd2
[1] + done ( sleep 100 & print sleep_pid:$!; wait $!; print cmd1 && print cmd2; )
%
If you need to access the pid programmatically, you can write it to a temp file or a named pipe.

Answer 2

Just found out one of the solutions:
sleep_pid=`pstree -p $!|grep -o "[[:digit:]]*"|tail -1`

Answer 3

Even if the shell you're using is a Bourne-style and thus supports the exec builtin with these semantics, you generally shouldn't try to avoid using sh -c (or equivalent) to create a new, separate shell process for this purpose, because:
Once the shell has become myCommand, there's no shell waiting to run subsequent commands. sh -c 'echo $$; exec myCommand; foo would not be able to attempt to run foo after replacing itself with myCommand. Unless you're writing a script that runs this as its last command, you can't just use echo $$; exec myCommand in a shell where you are running other commands.
You cannot use a subshell for this. (echo $$; exec myCommand) may be syntactically nicer than sh -c 'echo $$; exec myCommand', but when you run $$ inside ( ), it gives the PID of the parent shell, not of the subshell itself. But it is the subshell's PID that will be the PID of the new command. Some shells provide their own non-portable mechanisms for finding the subshell's PID, which you could use for this. In particular, in Bash 4, (echo $BASHPID; exec myCommand) does work.
Finally, note that some shells will perform an optimization where they run a command as if by exec (i.e., they forgo forking first) when it is known that the shell will not need to do anything afterward. Some shells try to do this anytime it is the last command to be run, while others will only do it when there are no other commands before or after the command, and others will not do it at all. The effect is that if your forget to write exec and just use sh -c 'echo $$; myCommand' then it will sometimes give you the right PID on some systems with some shells. I recommend against ever relying on such behavior, and instead always including exec when that's what you need.
Before I can run myCommand, I need to set a number of environment variables in my bash script. Will these carry over to the environment in which the exec command is running? –
user5359531
May 11 '18 at 15:41
Looks like my environment does carry over into the exec command. However, this approach does not work when myCommand starts other processes, which are the ones you need to work with; when I issue a kill -INT where pid was obtained this way, the signal does not reach the sub-processes started by myCommand, whereas if I run myCommand in the current session and Ctrl+C, the signals propagate correctly. –
user5359531
May 11 '18 at 16:43
1
I tried this, but the pid of the myCommand process seems to be the pid output by echo $$ +1. Am I doing something wrong? –
crobar
Aug 28 '18 at 10:13
My command looks like this: sh -c 'echo $$; exec /usr/local/bin/mbdyn -f "input.file" -o "/path/to/outputdir" > "command_output.txt" 2>&1 &' –
crobar
Aug 28 '18 at 10:51
This is brilliant, but it's not working for me when I try to get the echoed value into a variable so I can actually use it later to kill the process, e.g. PID=$(sh -c 'echo $$; exec myCommand') just hangs, whereas if I remove the PID=$(...) wrapper it displays the PID and continues immediately!