How do I get the parent process ID of a given child process?

How to get parent PID from a given child's PID?

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

Thanks

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)
  

Using only shell 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

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).

ps -efj can also be used for the same.

For example,

> ps -efj | head
UID        PID  PPID  PGID   SID  C STIME TTY          TIME CMD
root         1     0     1     1  0 Jul01 ?        00:00:13 /sbin/init splash
root         2     0     0     0  0 Jul01 ?        00:00:00 [kthreadd]
root         3     2     0     0  0 Jul01 ?        00:00:02 [ksoftirqd/0]
root         5     2     0     0  0 Jul01 ?        00:00:00 [kworker/0:0H]
root         7     2     0     0  0 Jul01 ?        00:06:44 [rcu_sched]
root         8     2     0     0  0 Jul01 ?        00:00:00 [rcu_bh]
root         9     2     0     0  0 Jul01 ?        00:00:00 [migration/0]
root        10     2     0     0  0 Jul01 ?        00:00:08 [watchdog/0]
root        11     2     0     0  0 Jul01 ?        00:00:08 [watchdog/1]