parents

#!/usr/bin/env nix-shell
#! nix-shell -i bash -p bash coreutils gnused
# shellcheck shell=bash disable=SC3000-SC3999

# Remove this disable line if usage is ever ran with arguments.
# shellcheck disable=SC2120
usage() {
    [[ "$#" -eq 0 ]] || error "$@" || :
    cat >&2 <<EOF
usage: parents PID

Get the parent processes of a given PID, going in order from nearest
to most distant relative. Requires that the /proc filesystem exists.

Kylie McClain <kylie@somas.is>
EOF
    exit 64 # EX_USAGE
}

error() {
    printf 'error: %s\n' "$@" >&2
    exit 1
}

quote() {
    local quoted="${1//\'/\'\\\'\'}"
    case "$1" in
        *[[:space:]]*) quoted="'${quoted}'" ;;
    esac
    printf "%s" "${quoted}"
}

get_cmdline() {
    local arguments i
    arguments=()
    i=0

    mapfile -t -d $'\0' arguments </proc/"$1"/cmdline || return 1
    while [[ "${i}" -ne "${#arguments[@]}" ]]; do
        arguments[i]=$(quote "${arguments[i]}")
        printf '%s ' "${arguments[i]}"
        i=$((i + 1))
    done
    printf '%s' "${arguments[i]}"
}

get_parent() {
    local parent

    # read/check existence in one command
    parent=$(sed -n '/PPid:/ { s/PPid:.//; p; q }' /proc/"$1"/status 2>/dev/null) || return 1
    if [[ "${parent}" -eq 0 ]] || [[ -z "${parent}" ]]; then
        return 1
    fi

    echo "${parent}"
}

get() {
    pid=$(get_parent "$1")
    cmdline=$(get_cmdline "${pid}")
    printf '%s\t%s\n' "${pid}" "${cmdline}"
}

[[ "$#" -eq 1 ]] || usage

pid="$1"
cmdline=$(get_cmdline "${pid}" 2>/dev/null) || error "error: non-existent PID: %s\n" "${pid}"

get "${pid}"
until get "${pid}" && [[ "${pid}" = 1 ]]; do
    exit
done