Super-simple monitoring program.

mon spawned from the needlessly complex frustration that tools like monit provide, with their awkward DSLs and setup. mon is written in C, uses less than 400kb of memory, and is incredibly simple to set up.

$ make install

Too lazy to clone?:

$ (mkdir /tmp/mon && cd /tmp/mon && curl -L# https://github.com/tj/mon/archive/master.tar.gz | tar zx --strip 1 && make install && rm -rf /tmp/mon)

Usage: mon [options] <command>
Options:
  -V, --version                 output program version
  -h, --help                    output help information
  -l, --log <path>              specify logfile [mon.log]
  -s, --sleep <sec>             sleep seconds before re-executing [1]
  -S, --status                  check status of --pidfile
  -p, --pidfile <path>          write pid to <path>
  -m, --mon-pidfile <path>      write mon(1) pid to <path>
  -P, --prefix <str>            add a log prefix
  -d, --daemonize               daemonize the program
  -a, --attempts <n>            retry attempts within 60 seconds [10]
  -R, --on-restart <cmd>        execute <cmd> on restarts
  -E, --on-error <cmd>          execute <cmd> on error

The most simple use of mon(1) is to simply keep a command running:

$ mon ./myprogram
mon : pid 50395
mon : child 50396
mon : sh -c "./example/program.sh"
one
two
three

You may daemonize mon and disassociate from the term with -d:

$ mon ./myprogram -d
mon : pid 50413

mon(1) will continue to attempt restarting your program unless the maximum number of --attempts has been exceeded within 60 seconds. Each time a restart is performed the --on-restart command is executed, and when mon(1) finally bails the --on-error command is then executed before mon itself exits and gives up.

For example the following will echo "hey" three times before mon realizes that the program is unstable, since it's exiting immediately, thus finally invoking ./email.sh, or any other script you like.

mon "echo hey" --attempts 3 --on-error ./email.sh
mon : child 48386
mon : sh -c "echo hey"
hey
mon : last restart less than one second ago
mon : 3 attempts remaining
mon : child 48387
mon : sh -c "echo hey"
hey
mon : last restart less than one second ago
mon : 2 attempts remaining
mon : child 48388
mon : sh -c "echo hey"
hey
mon : last restart less than one second ago
mon : 1 attempts remaining
mon : 3 restarts within less than one second, bailing
mon : on error `sh test.sh`
emailed failure notice to tobi@ferret-land.com
mon : bye :)

NOTE: The process id is passed as an argument to both --on-error and --on-restart scripts.

mon(1) is designed to monitor a single program only, this means a few things, firstly that a single mon(1) may crash and it will not influence other programs, secondly that the "configuration" for mon(1) is simply a shell script, no need for funky weird inflexible DSLs.

#!/usr/bin/env bash
pids="/var/run"
app="/www/example.com"
mon -d redis-server -p $pids/redis.pid
mon -d "node $app/app" -p $pids/app-0.pid
mon -d "node $app/jobs" -p $pids/jobs-0.pid
mon -d "node $app/jobs" -p $pids/jobs-1.pid
mon -d "node $app/jobs" -p $pids/jobs-2.pid
mon -d "node $app/image" -p $pids/image-0.pid
mon -d "node $app/image" -p $pids/image-1.pid
mon -d "node $app/image-broker" -p $pids/image-broker.pid

I highly recommend checking out jgallen23's mongroup(1), which provides a great interface for managing any number of mon(1) instances.

By default mon(1) logs to stdio, however when daemonized it will default to writing a log file named ./mon.log. If you have several instances you may wish to --prefix the log lines, or specify separate files.

• SIGQUIT graceful shutdown
• SIGTERM graceful shutdown

Tools built with mon(1):

• mongroup(1) - monitor a group of processes (shell script)
• node-mongroup - node implementation of mongroup(1)

MIT