Linux – ps: How to i recursively get all child process for a given pid

linuxprocesspsunix

How can I get the entire process tree spawned by a given process displayed as a tree and only that tree i.e. no other processes?

The output could e.g. look like

 4378 ?        Ss     0:10 SCREEN
 4897 pts/16   Ss     0:00  \_ -/bin/bash
25667 pts/16   S+     0:00  |   \_ git diff
25669 pts/16   S+     0:00  |       \_ less -FRSX
11118 pts/32   Ss+    0:00  \_ -/bin/bash
11123 pts/32   S+     0:00      \_ vi

I couldn't get the desired result purely with parameters to ps.

The following gives the desired result but seems a bit involved:

#!/bin/bash

pidtree() {
  echo -n $1 " "
  for _child in $(ps -o pid --no-headers --ppid $1); do
    echo -n $_child `pidtree $_child` " "
  done
}

ps f `pidtree 4378`

Does anyone have an easier solution?

Best Answer

The pstree is a very good solution, but it is a little bit reticent. I use ps --forest instead. But not for a PID (-p) because it prints only the specific process, but for the session (-g). It can print out any information ps can print in a fancy ASCII art tree defining the -o option.

So my suggestion for this problem:

ps --forest -o pid,tty,stat,time,cmd -g 2795

If the process is not a session leader, then a little bit more trick has to be applied:

ps --forest -o pid,tty,stat,time,cmd -g $(ps -o sid= -p 2795)

This gets the session id (SID) of the current process first and then call ps again with that sid.

If the column headers are not needed add a '=' after each column definition in '-o' options, like:

ps --forest -o pid=,tty=,stat=,time=,cmd= -g $(ps -o sid= -p 2795)

An example run and the result:

$ ps --forest -o pid=,tty=,stat=,time=,cmd= -g $(ps -o sid= -p 30085)
27950 pts/36   Ss   00:00:00 -bash
30085 pts/36   S+   00:00:00  \_ /bin/bash ./loop.sh
31888 pts/36   S+   00:00:00      \_ sleep 5

Unfortunately this does not work for screen as it sets the sid for each child screen and all grandchild bash.

To get all the processes spawned by a process the whole tree needs to be built. I used for that. At first it builds a hash array to contain all PID => ,child,child... . At the end it calls a recursive function to extract all the child processes of a given process. The result is passed to another ps to format the result. The actual PID has to be written as an argument to instead of <PID>:

ps --forest $(ps -e --no-header -o pid,ppid|awk -vp=<PID> 'function r(s){print s;s=a[s];while(s){sub(",","",s);t=s;sub(",.*","",t);sub("[0-9]+","",s);r(t)}}{a[$2]=a[$2]","$1}END{r(p)}')

For a SCREEN process (pid=8041) the example output looks like this:

  PID TTY      STAT   TIME COMMAND
 8041 ?        Ss     0:00 SCREEN
 8042 pts/8    Ss     0:00  \_ /bin/bash
 8092 pts/8    T      0:00      \_ vim test_arg test_server
12473 pts/8    T      0:00      \_ vim
12972 pts/8    T      0:00      \_ vim
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