44

How to get parent pid from a given children pid?

I know I can mannully check it under /proc, I am wonder if there is a smart/better way to achieve this in Ubuntu. Note the parent may or may not be killed.

Thanks

50

How to get a parent PID (PPID) from a child's process ID (PID) using the command-line

Use ps -o ppid=

  • e.g. ps -o ppid= 2072 returns 2061, which you can easily use in a script etc. ps -o ppid= -C foo gives the PPID of process with command foo. You can also use the old fashioned ps | grep: ps -eo ppid,comm | grep '[f]oo'.
  • Fuller explanation: ps -f 2072 returns
    UID        PID  PPID  C STIME TTY      STAT   TIME CMD
    izx       2072  2061  0 07:16 ?        S      0:00 /usr/lib/pulseaudio/pulse/gconf-helper
    
  • The pstree relation is: pstree -s -p 2072:
    init(1)───pulseaudio(2061)───gconf-helper(2072)
    
  • Do you know of a way to make pstree show invocations (command + arguments) without splitting lines? My installation automatically splits lines if I add arguments, even if the result could easily fit on one line. The man page appears to say as much, but doesn't give a reason. My workaround involves extracting PID's from pstree and using ps -o args ... on the results, but that has a bit of a smell. – John P Mar 1 '17 at 4:13
  • Just something that surprised me: The space after ppid= but before the pid seems to be necessary. I got different results if I omitted it. – Gregory Arenius Feb 8 at 0:27
5

Using only variable to get parent PID :

echo $PPID

if you need the command from this parent pid:

cat /proc/$PPID/comm

if you need the full command line (with all options):

cat /proc/$PPID/cmdline

Explanation

  • $PPID is defined by the shell, it's the PID of the parent process
  • in /proc/, you have some dirs with the PID of each processes. Then, if you cat /proc/$PPID/comm, you echo the command name of the PID

Check man proc

  • 1
    This is the best answer. procfs is a kernel API. execute a command and parse the output is a nonsense. – Massimo Apr 9 at 5:28
  • This is also potentially the most portable, PPID is defined as part of the POSIX standard so any POSIX compliant shell should be setting this appropriately. See pubs.opengroup.org/onlinepubs/9699919799/utilities/… – RobV 19 hours ago
1

Using pstree by command name

Using pstree you can search by the child process name and get the Process ID (PID) along with the parents, grandparents and any children of the child process:

$ pstree -hp | grep sleep
           |-cron(763)---cron(795)---sh(839)---display-auto-br(841)---sleep(8414)

In this case sleep is the child command and it's PID is 8414. It's parent ID is 841 and is called display-auto-brightness. The grandparent is a shell (sh) with a process ID of 839. The great-grandparent is cron with a process ID of 795. The great-great-grandparent is also cron with a process ID of 763.

If you want to search by Process ID of sleep instead of name you can use:

$ pstree -hp | grep 14653
           |-cron(763)---cron(795)---sh(839)---display-auto-br(841)---sleep(14653)

Notice the sleep process ID changed to 14653. The parent (PID 841) sleeps for 1 minute, wakes up for a split second and then starts a new sleep command which gets a new process ID. This is another reason why searching for sleep is easier than searching by process ID.

This code was taken from: Automatically adjust display brightness based on sunrise and sunset and adapted to this question.


To see a nested chain all the way back to boot process use the PID instead of name:

$ pstree -aps 8541
systemd,1 splash fastboot kaslr
  └─cron,763 -f
      └─cron,795 -f
          └─sh,839 -c    /usr/local/bin/display-auto-brightness
              └─display-auto-br,841 /usr/local/bin/display-auto-brightness
                  └─sleep,8541 60

Note: Another minute has passed and the sleep command gets a new PID (8541).

  • In your example, this method wouldn't have worked for the cron process with PID 807. – Olorin Mar 6 '18 at 4:38
  • @Olorin In both examples you could grep on 807 or cron and the same line would be retruned as well as more children if spawned. But the question was about finding the parent, not all the children. – WinEunuuchs2Unix Mar 6 '18 at 5:28
  • Exactly - how do you find the parent of PID 807 with that output? You can't, becvause it doesn't include the parent of 807. – Olorin Mar 6 '18 at 6:41
  • @Olorin The parent of cron is systemd which is PID 1. I don't think you can kill PID 1. – WinEunuuchs2Unix Mar 6 '18 at 11:09
  • I didn't say anything about killing anything. Just to clarify, now you're claiming that taking the first line of pstree or grepping for the command name will always give the the parent PID? – Olorin Mar 6 '18 at 13:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.