Get list of processes that were forked off my currently running process?

Question

I am writing a wrapper application to bash scripts and want the application to keep a track of which tools/processes have been launched from user scripts. I would like to know what is the best way to determine the list of child processes that were spawned of this parent process.

I tried

1. Periodically invoking ps command and building a process tree (like ps -ejH) but this misses out on processes that ran to completion very quickly.
2. Using a tool like forkstat that uses the proc connector interface, but that would only run with elevated privileges. While this gives the correct data, running as sudo would not work in my case?

Any suggestions how this can be achieved?

Answer 1

If you're using Linux, you can use strace to trace system calls used by a process. For example:

~ strace -e fork,vfork,clone,execve -fb execve -o log ./foo.sh
foo bar
~ cat log
4817  execve("./foo.sh", ["./foo.sh"], [/* 42 vars */]) = 0
4817  clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f1bb563b9d0) = 4818
4818  execve("/bin/true", ["/bin/true"], [/* 42 vars */] <detached ...>
4817  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=4818, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
4817  clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f1bb563b9d0) = 4819
4817  clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f1bb563b9d0) = 4820
4820  execve("/bin/echo", ["/bin/echo", "foo", "bar"], [/* 42 vars */] <detached ...>
4817  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=4820, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
4817  +++ exited with 0 +++
4819  execve("/bin/sleep", ["sleep", "1"], [/* 42 vars */] <detached ...>

You can see that the script forked off three processes (PIDs 4818, 4819, 4820) using the clone(2) system call, and the execve(2) system calls in those forked off processes show the commands executed.

• -e fork,vfork,clone,execve limits strace output to these system calls
• -f follows child processes
• -b execve detaches from a process when the execve is reached, so we don't see further tracing of child processes.

Answer 2

pstree -p `pgrep NetworkManager`
NetworkManager(1638)─┬─dhclient(3594)
                     ├─{NetworkManager}(1645)
                     ├─{NetworkManager}(1647)
                     └─{NetworkManager}(7363)

I think this is what you were looking for.

use direct pid or pgrep with process name.

-p used to print pids of children.

Answer 3

pgrep command will be helpful. Use these commands :

Print PIDs of each forked process from process($pid) :

pgrep -P $pid

For more detailed information about each forked process use this command :

ps -fp `pgrep -P $pid`

where $pid is your process ID.

Answer 4

I would like to know what is the best way to determine the list of child processes that were spawned of this parent process.

Assuming that you are writing in bash (you do not say). Then you can use job control. This is off by default, when running scripts.

The bash manual says:

set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
...
-m Monitor mode.
Job control is enabled. This option is on by default for interactive shells on systems that support it (see JOB CONTROL above). Background processes run in a separate process group and a line containing their exit status is printed upon their completion.

So add to your script set -m, the you can start commands, with command &, then latter do jobs to list jobs.