pgrep -P, but for grandchildren not just children

Question

I am using:

pgrep -P $$

to get the child pids of $$. But I actually want a list of grandchildren and great grandchild too.

How do I

Answer 1

Using nothing but bash builtins (not even ps or pgrep!):

#!/usr/bin/env bash

collect_children() {
  # format of /proc/[pid]/stat file; group 1 is PID, group 2 is its parent
  stat_re='^([[:digit:]]+) [(].*[)] [[:alpha:]] ([[:digit:]]+) '

  # read process tree into a bash array
  declare -g children=( )              # map each PID to a string listing its children
  for f in /proc/[[:digit:]]*/stat; do # forcing initial digit skips /proc/net/stat
    read -r line <"$f" && [[ $line =~ $stat_re ]] || continue
    children[${BASH_REMATCH[2]}]+="${BASH_REMATCH[1]} "
  done
}

# run a fresh collection, then walk the tree
all_children_of() { collect_children; _all_children_of "$@"; }

_all_children_of() {
  local -a immediate_children
  local child
  read -r -a immediate_children <<<"${children[$1]}"
  for child in "${immediate_children[@]}"; do
    echo "$child"
    _all_children_of "$child"
  done
}

all_children_of "$@"

On my local system, time all_children_of 1 >/dev/null (invoking the function in an already-running shell) clocks in the neighborhood of 0.018s -- typically, 0.013s for the collect_children stage (the one-time action of reading the process tree), and 0.05s for the recursive walk of that tree triggered by the initial call of _all_children_of.

_{Prior timings were testing only the time needed for the walk, discarding the time needed for the scan.}