How to get all parent processes and all subprocesses with `pstree`

Question

Command pstree PID can show all subprocess information of the process specified by PID. However, I also want to know all parent process information of the process PID, how can I get it?

An example:

init
|- parent_process
|    `- current_process
|       |- subprocess_1
|       `- subprocess_2
`- other_process

What I want is when I run pstree current_process_pid, I want to get below output:

init
`- parent_process
    `- current_process
       |- subprocess_1
       `- subprocess_2

When I run pstree subprocess_1_pid, it will output:

init
`- parent_process
    `- current_process
       `- subprocess_1

Answer 1

# With my psmisc 22.20:
pstree -p -s PID

Maybe if with ps -ef:

awk -vPID=$1 '
function getParent ( pid ) {
    if (pid == "" || pid == "0") return;
    while ("ps -ef | grep "pid | getline) {
        if ($2 == pid) {
            print $8"("$2") Called By "$3;
            getParent($3);
            break;
        }
    }
    close ("ps -ef")
}

BEGIN { getParent(PID) }
'

This is ugly assuming ps output column and order. Actually one single run of ps -ef contains all info needed. This don't worth the time, I still recommend updating psmisc, it won't hurt.

EDIT: A mimic using single run ps -ef:

ps -ef | awk -vPID=$1 '
function getpp ( pid, pcmd, proc ) {
    for ( p in pcmd ) {
        if (p == pid) {
            getpp(proc[p], pcmd, proc);
            if (pid != PID) printf("%s(%s)───", pcmd[pid], pid);
        }
    }
}

NR > 1 {
    # pid=>cmd
    pcmd[$2] = $8;
    # pid=>Parent
    pproc[$2] = $3;
}

END {
    getpp(PID, pcmd, pproc);
    printf "\n";
    system("pstree -p "PID);
}'

Answer 2

I found laps options mentioned by @haridsv (pstree -laps <pid>) being a solution. It was a bit verbose for me though, so I'd stick to a shorter aps output.

To get the process tree of the current process (its ID is $$ in Bash):

pstree -aps $$

That prints the process tree like this:

systemd,1
  └─kitty,86739
      └─bash,86742
          └─pstree,86904 -aps 86742

Answer 3

One-liner Linux-only POSIX shell. Works in busybox and every other Linux system:

pl() { for f in /proc/$1/task/*/children;do l=""; [ -r $f ] &&read l<$f;for p in $l;do echo $p; pl $p; done; done; }

Usage: run sleep 10000 & then echo $(pl $$) will show you all subprocesses below this ($$) shell, which will only be the one sleep proc currently. But, if that sleep proc spawned children, then pl would recursively descend until it found all children