How to get child process from parent process

Is it possible to get the child process id from parent process id in shell script?

I have a file to execute using shell script, which leads to a new process process1 (parent process). This process1 has forked another process process2(child process). Using script, I'm able to get the pid of process1 using the command:

cat /path/of/file/to/be/executed

but i'm unable to fetch the pid of the child process.

Just use :

pgrep -P $your_process1_pid

I am not sure if I understand you correctly, does this help?

ps --ppid <pid of the parent>

I've written a script to get all child process pids of a parent process. Here is the code. Hope it will help.

function getcpid() {
    cpids=`pgrep -P $1|xargs`
#    echo "cpids=$cpids"
    for cpid in $cpids;
    do
        echo "$cpid"
        getcpid $cpid
    done
}

getcpid $1

The shell process is $$ since it is a special parameter

On Linux, the proc(5) filesystem gives a lot of information about processes. Perhaps pgrep(1) (which accesses /proc) might help too.

So try cat /proc/$$/status to get the status of the shell process.

Hence, its parent process id could be retrieved with e.g.

  parpid=$(awk '/PPid:/{print $2}' /proc/$$/status)

Then use $parpid in your script to refer to the parent process pid (the parent of the shell).

But I don't think you need it!

Read some Bash Guide (or with caution advanced bash scripting guide, which has mistakes) and advanced linux programming.

Notice that some server daemon processes (wich usually need to be unique) are explicitly writing their pid into /var/run, e.g. the  sshd server daemon is writing its pid into the textual file /var/run/sshd.pid). You may want to add such a feature into your own server-like programs (coded in C, C++, Ocaml, Go, Rust or some other compiled language).

To get the child process and thread, pstree -p PID. It also show the hierarchical tree

ps -axf | grep parent_pid 

Above command prints respective processes generated from parent_pid, hope it helps. +++++++++++++++++++++++++++++++++++++++++++

root@root:~/chk_prgrm/lp#

 parent...18685

 child... 18686


root@root:~/chk_prgrm/lp# ps axf | grep frk

 18685 pts/45   R      0:11  |       \_ ./frk
                                
 18686 pts/45   R      0:11  |       |   \_ ./frk

 18688 pts/45   S+     0:00  |       \_ grep frk

You can get the pids of all child processes of a given parent process <pid> by reading the /proc/<pid>/task/<tid>/children entry.

This file contain the pids of first level child processes. Recursively do this for all children pids.

For more information head over to https://lwn.net/Articles/475688/

You can print the PID of all child processes invoked by a parent process:

pstree -p <PARENT_PID> | grep -oP '\(\K[^\)]+'

This prints a list of pids for the main process and its children recursively

I used the following to gather the parent process and child processes of a specific process PID:

ps -p $PID --ppid $PID --forest | tail -n +2 | awk '{print$1}'

Note, this will not work to find child processes of child processes.

For the case when the process tree of interest has more than 2 levels (e.g. Chromium spawns 4-level deep process tree), pgrep isn't of much use. As others have mentioned above, procfs files contain all the information about processes and one just needs to read them. I built a CLI tool called Procpath which does exactly this. It reads all /proc/N/stat files, represents the contents as a JSON tree and expose it to JSONPath queries.

To get all descendant process' comma-separated PIDs of a non-root process (for the root it's ..stat.pid) it's:

$ procpath query -d, "..children[?(@.stat.pid == 24243)]..pid"
24243,24259,24284,24289,24260,24262,24333,24337,24439,24570,24592,24606,...

#include<stdio.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
    // Create a child process     
    int pid = fork();

    if (pid > 0)
    {

            int j=getpid();

            printf("in parent process %d\n",j);
    }
    // Note that pid is 0 in child process
    // and negative if fork() fails
    else if (pid == 0)
    {





            int i=getppid();
            printf("Before sleep %d\n",i);

            sleep(5);
            int k=getppid();

            printf("in child process %d\n",k);
    }

    return 0;

}