Get PID from TID

pspthreadsthread

I run iotop to check on programs that are heavy disk users, in case I need to decrease their priority. Usually this is good enough, but iotop only shows thread ID (TID), and sometimes I want to know process ID (PID) so I can find out more about which process is responsible.

Unfortunately, while ps can display TID (a.k.a SPID, LWP), it doesn't have a flag to take a list of TIDs the way it does for a list of PIDs with --pid. The best I can do is list TIDs and then grep the output. For example, if the thread id is 792, I can do

$ ps -eLf | grep ' 792 '

which works reasonably well, but is a little inelegant.

Is there a better way?

Best Answer

You can always do:

ps -eLo pid= -o tid= | awk '$2 == 792 {print $1}'

On Linux:

$ readlink -f /proc/*/task/792/../..
/proc/300

Or with zsh:

$ echo /proc/*/task/792(:h:h:t)
300

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How to find the PID of nginx process

I usually find pgrep more convenient than ps | grep. You might also want to look at killall if the end goal is blasting a bunch of processes by name.

How to get the pid to the last background app

To background itself, that application forks a child process and exits the parent. The shell knows of the parent process as that's the one it has forked itself before executing the command itself, but it has no visibility on the children or grand-children that that process may have spawned.

A simpler example of such a backgrounding command would be:

$ sh -c 'ps -j; sleep 10 &'
  PID  PGID   SID TTY          TIME CMD
 6562  6562 14469 pts/13   00:00:00 sh
 6563  6562 14469 pts/13   00:00:00 ps
14469 14469 14469 pts/13   00:00:00 zsh

My zsh knows about that sh process and its process group.

However, all it sees is that 6562 process exiting. It has no way to know that 6562 process has spawned a 6563 one.

$ ps -j
  PID  PGID   SID TTY          TIME CMD
 6564  6562 14469 pts/13   00:00:00 sleep
 6565  6565 14469 pts/13   00:00:00 ps
14469 14469 14469 pts/13   00:00:00 zsh

However, you can see the process running sleep is also in that 6562 process group (though, there's nothing stopping commands start new process groups, or even sessions (as daemons typically do)).

Those process groups are only created when the shell is interactive though.

Another thing you could do is:

cmd | cat &
wait

If the processes that cmd spawns don't close their stdout, then cat won't die until they have all died.

Best Answer

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