Loop-run systemd’s ExecStart command

exec-service.sh

#!/bin/bash
#---
## @file exec-service.sh
## @brief Loop-run systemd’s ExecStart command
## @see https://gist.github.com/kawanet/2eadbbf03f3619a28bb7cd6f36985f8d
## @author Yusuke Kawasaki
## @license MIT
#---

die () {
    echo "$1" >&2
    exit 1
}

warn () {
    echo "$1" >&2
}

prompt () {
    local cmd=("$@")

    if [ -z "$PROMPT" ]; then
        local user=${USER:-$(whoami)}
        if [ "$user" = "root" ]; then
            PROMPT="[$user]#"
        else
            PROMPT="[$user]$"
        fi
    fi

    echo "$PROMPT ${cmd[@]}" >&2
}

if command -v perl > /dev/null 2>&1; then
    strip_nl() { perl -pe 's/\\\n//g' < "$1"; }
else
    strip_nl() { sed ':a;N;$!ba;s/\\\n//g' < "$1"; }
fi

getconf () {
    local unit="$1"
    local key="$2"
    local row=$(strip_nl "$unit" | grep "^$key")
    local value="${row#*=}"
    echo $value
}

start_unit () {
    unit="$1"

    WorkingDirectory=$(getconf "$unit" WorkingDirectory)
    Environment=$(getconf "$unit" Environment)
    ExecStartPre=$(getconf "$unit" ExecStartPre)
    ExecStart=$(getconf "$unit" ExecStart)

    [ -n "$ExecStart" ] || die "Error: ExecStart is required in the service file: $unit"

    if [ -n "$WorkingDirectory" ]; then
        cd "$WorkingDirectory" || die "Error: Failed to cd to '$WorkingDirectory'."
    fi

    if [ -n "$ExecStartPre" ]; then
        prompt $ExecStartPre
        env $Environment $ExecStartPre || die "Error: ExecStartPre failed with code $?"
    fi

    prompt $ExecStart
    env $Environment $ExecStart &
}

forward() {
    sig="$1"
    warn "SIGNAL: $sig"
    for pid in $(pgrep -P $$); do
        kill -s "$sig" "$pid" 2>/dev/null
    done
}

[ "$#" -gt 0 ] || die "Usage: $0 /etc/systemd/system/xxxx.service"

trap 'forward HUP' HUP
trap 'forward USR1' USR1
trap 'forward USR2' USR2
trap 'forward TERM; sleep 1; exit 0' TERM
trap 'forward INT; sleep 1; exit 0' INT

args=("$@")
pids=()
delay=1

start_all() {
    local len=${#args[@]}

    for (( idx=0; idx<len; idx++ )); do
        local pid=${pids[idx]}
        if [ -z "$pid" ] || ! kill -0 "$pid" 2>/dev/null; then
            start_unit "${args[idx]}"
            pids[$idx]=$!
        fi
    done

    [ -n "$(jobs -p)" ] && wait -n
}

start_once() {
    local startTime=$(date +%s)
    start_all
    local status=$?
    local endTime=$(date +%s)
    local runTime=$(( endTime - startTime ))

    if (( runTime < delay )); then
        delay=$(( delay * 2 ))
    else
        delay=1
    fi

    [ "$status" -eq 0 ] && prefix="INFO:" || prefix="ERR:"
    warn "${prefix} Exit code ${status}; ran for ${runTime}s; restarting in ${delay}s..."
    sleep "$delay"
}

while true; do
    start_once
done