Cron alternative with runit and snooze

salotz — Fri, 23 Oct 2020 22:25:36 +0000

I have tried many times to use cron effectively, but ultimately this
ends in failure each time. Lets look at why I find cron onerous and a
solution I came up with[1].

Skip to this section if you just want to see the solution.

Non-Issues

Conceptually specifying when to run things with a special syntax is not
a bad idea and not why I find cron hard to use.

Just as a reminder this is what a cron job I found in my system for the
dma mail agent:

*/5 *    * * *   root    [ -x /usr/sbin/dma ] && /usr/sbin/dma -q1

The meaning and examples for the timing specs are well known and not
really an issue here. There are lots of alternatives that try to improve
on this, this is not one of them (although it is different in this
regard).

Sys-Admin Orientation

My first issue with cron is that it is inseparable (or at least
practically so) from the base *nix system. That means you can't run it
as a user as it is meant to be the scheduled thing runner for a
multi-user system. So while you can have the system run things on behalf
of you as a user you can't really control it.

In this day and age, multi-user systems are becoming pretty rare outside
HPCC systems in academia and gov't labs. For most people they want to
run simple recurring tasks on their desktop, laptop, VPS server, or
maybe even a homelab server. In all these cases user's are usually
simply mechanisms for implementing access control and not for distinct
human entities.

This kind of sysadmin oriented baggage is pretty pervasive in *nix
systems and cron is not alone here.

Because cron is so deeply embedded into the lizard brain of *nix it is
necessary to have a properly running and configured cron for the
stability of your system.

As a reasonably competent user of linux desktops, the thought of
inadvertently messing up /etc/crontab and its ilk is enough to keep me
away from it.

For instance where exactly am I supposed to put a cron job? On Ubuntu
20.04 (KDE Neon actually) I see this in my /etc dir:

--> ls /etc/ | grep cron
anacrontab
cron.d
cron.daily
cron.hourly
cron.monthly
crontab
cron.weekly

Additionally this is highly variable distro to distro.

For instance here is part of the man page for cron on my system:

DEBIAN SPECIFIC
       Debian introduces some changes to cron that were not originally available  upstream.   The  most
       significant changes introduced are:

       —      Support for /etc/cron.{hourly,daily,weekly,monthly} via /etc/crontab,

       —      Support for /etc/cron.d (drop-in dir for package crontabs),

       —      PAM support,

       —      SELinux support,

       —      auditlog support,

       —      DST and other time-related changes/fixes,

       —      SGID crontab(1) instead of SUID root,

       —      Debian-specific file locations and commands,

       —      Debian-specific configuration (/etc/default/cron),

       —      numerous other smaller features and fixes.

       Support  for  /etc/cron.hourly,  /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly is pro‐
       vided in Debian through the default setting of the /etc/crontab file (see the system-wide  exam‐
       ple  in crontab(5)).  The default system-wide crontab contains four tasks: run every hour, every
       day, every week and every month.  Each of these tasks will execute run-parts providing each  one
       of the directories as an argument.  These tasks are disabled if anacron is installed (except for
       the hourly task) to prevent conflicts between both daemons.

This goes on for several more paragraphs of neckbeard-speak that I don't
pretend to understand. Am I running anacron? Why? What is run-parts?
Its obvious a lot of this is for security purposes that are important in
enterprise environments that a professional sysadmin is paid a hefty
salary to comprehend. So there is probably a simple answer to this mess,
except I don't have direct access to the people who made this mess to
ask them.

This is a scenario where reading the manual leaves me more confused than
I started. I'll probably get chastised for overcomplicating things. In
pre-emptive response I will leave this quote from James Clear's mailing
list I got today:

To simplify before you understand the details is ignorance.

To simplify after you understand the details is genius.

I'm in the former category.

I would like to schedule backups to happen everynight, but I would
rather not upset the fragile balance distro maintainers have made to
keep your desktop running "smoothly".

Cron is simply not meant for a simple user like me.

Control & Supervision

The second major problem I had with cron was the real lack of control
you have over it. You write text into the file and cron goes off and
does its thing, just like Ritchie and Thompson intended.

Everyone with a modicum of understanding of why databases exist sees the
obvious issues with inconsistency that this has. So no longer can you
just edit /etc/crontab but rather you are supposed to go through
helper tools (like crontab) which does all manner of locking and
gatekeeping to desparately pretend its a database.

Using tools that automatically open up editors is another pet-peeve of
mine since you now are bringing in a lot of other assumptions about the
configuration of your system. I hope you know how to safely close files
in nano and vi! Further, I like to keep all my configurations in a
central configuration directory that I can use normal version control
etc. How am I supposed to "load" these into the cron jobs without
manually copy-pasting, at least if your going by the "manual". Aren't
these systems supposed to be scriptable? [2]

As we'll get to in the actual solution I'm presenting here, the loading,
unloading, restarting, pausing etc. of jobs is eerily similar to the
feature sets used in PID 0 programs like systemd.

Indeed systemd has a similar timer system, which has much better
control, but again suffers from being sysadmin oriented. Also writing
unit files blows. I do it, but I try not to.

Back to cron though, the only solution to this I have found is to
install a program called cronitor, which is like
a freemium tool that really isn't that scriptable either due to the
(very nice) terminal UI. This is useful, but I don't really see this as
something I can expect to have going into the future on all my systems.

Observability & Logging Utilities

I was able to get through all the issues above through sheer force of
will, but ultimately I was done in by the necessary interaction of cron
to two even more obtuse systems in *nix: email and logging.

Email is sort of baked into cron and from what I can tell is the main
way of getting notifications that running things had issues.

For instance if your trying to run a script and you got the path wrong
you'll need a working email server for cron to send a message. And if
you give a mouse a cookie…

Then you'll need to make sure your mail boxes for the users on your
system are working, you know where they are and you have a client for
reading them.

This seems like a huge dependency to have to just get a message about a
job having an error in it?

Shouldn't there just be a log file I can pop open in a text editor (like
Ritchie and Thompson intended goddamit)?

For me and I'm sure many others, I've never had a need to interact with
intra-system email. Even on the HPCC systems I worked in for my PhD this
is basically an unused feature.

When you do get jobs running without error (and thus no reason to email
you), you'll want to see their logs too.

In cron, its completely up to you to create and manage the lifecycle of
logs in your system. While in theory this is a good thing since it
allows you to not be locked into something you hate, in practice for
non-neckbeards it involves you having detailed knowledge of yet another
complex subsystem.

I simply don't know where to even start here. The cron docs say to use
rsyslogd, which I don't know how to use. Furthermore, my system is
using systemd which has nice commands that show you the latest in the
logs. Is this subsystem disjoint? It wouldn't suprise me that they would
jump through different calcified hoops to keep 40 year old things
running.

Again there is probably a simple answer to all this, but its one that
reading the man page can't get you. I don't log a lot of stuff, but
mistakes happen and logs can fill up a / partition scarily easy.

Runit tutorial

I've contemplated and even tried a few alternatives to cron. This one
was discovered while throwing stones at other birds. I was pleasantly
surprised.

Basically it boils down to using a process supervisor called
runit and a fancy big brother to the unix
sleep command called
snooze.

Runit was originally meant to be a replacement for older init systems
like sysvinit that is cross-platform as well as very simple and not
requiring many libraries. This was to suite it as part of the core
system to bootstrap everything else (think cron and sshd etc.).

While these are nice characteristics they aren't killer for us here.
However, nowadays its pretty popular with the anti-systemd crowd. Its an
option for init systems in Gentoo and others and is the default in Void
linux. Besides its simplicity it is pretty easy to get up and configured
and you just run a few shell scripts to get everything going.

I have a few complaints

the documentation is kind of nonlinear and doesn't give you a walkthrough of how to use the whole system [3].
commands are disjointed and spread between a number of executables and use of standard unix commands like ln and rm.

The second point is actually a feature where each of the little
components can be used standalone. However, this makes it a little more
confusing to wrap your head around and I found myself constantly
reviewing my notes to know which command to use.

I solved this with a few shell functions, but I would like to see a
wrapper CLI to make it a little centralized conceptually (and to add a
few convenience features) for those that would want it.

Reading these articles also helped in understanding it:

Even after reading these I had to muck around and figure a bunch of
little details out so I thought I would throw my own little tutorial on
the pile to hopefully save some people's time and make runit a little
more approachable.

Luckily on Ubuntu 20.04 its really easy to get runit installed and
running as a systemd service. Just install using apt:

sudo apt install runit

This even starts the service:

sudo systemctl status runit
sudo journalctl -u runit

Normally there are 3 stages (i.e. states) runit has:

Single run tasks on startup
Process supervision: starting, stopping, restarting services
Shutting services down as the system goes down

Because we aren't using runit as a PID 0 init system, we only care about
stage 2 & 3. The apt installation takes care of this for us
thankfully.

So you should see the following directories appear:

/etc/runit stages and runlevel stuff, ignore this (for now).
/etc/sv The services directory, this is where you author things.
/etc/service This is where "enabled" services are put.

I'll call these different directories by the environment variable I
refer to them as. I put this in my ~/.profile:

## runit known env variables

# the active service directory to query for state, this is what it is
# default, but I like to set so its easier for me to disable services
export SVDIR="/etc/service"

# SNIPPET: the normal wait time
# export SVWAIT=7

## my vars, not recognized by any runit tools

# this is the standard directory of where services are put for the
# system. The SerVice LIBrary
export SVLIB="/etc/sv"

# log dir
export SVLOG_SYS_DIR="/var/local/log/runit"

To define a service you make a directory in SVLIB with some
specially named shell scripts. Mine has these directories in it:

--> ls $SVLIB
backup  hello  printer_live  recollindex  test_env

Each one is a specific service. Lets first look at hello to get a
simple picture of what these are:

--> ls $SVLIB/hello
finish  log  run  supervise

The most important one is run which is a shell script:

#!/bin/sh

# run the service
while :
do
    echo "Hello"
    sleep 2
done

This just prints "Hello" to stdout and then waits 2 seconds.

This service isn't being run yet. For that you need to put it into the
SVDIR:

sudo ln -s -f "$SVLIB/hello" "$SVDIR/"

You can check the status of the service with the sv command:

--> sudo sv status $SVDIR/hello
run: /etc/service/hello: (pid 664634) 193s

You can check the status of all services similarly:

sudo sv status $SVDIR/*

If you see this:

--> sudo sv status $SVDIR/hello
down: /etc/service/hello: 1s, normally up, want up

There is something wrong with your run script.

Looking at sudo systemctl status runit and sudo journalctl -u runit could usually help me figure the issue out (no email!!!).

Once its working you should see the "Hello"s on the log for runit if you
aren't logging this service:

--> sudo journalctl -u runit | tail
Oct 23 16:49:30 ostrich 2[664634]: Hello
Oct 23 16:49:32 ostrich 2[664634]: Hello
Oct 23 16:49:34 ostrich 2[664634]: Hello
Oct 23 16:49:36 ostrich 2[664634]: Hello
Oct 23 16:49:38 ostrich 2[664634]: Hello
Oct 23 16:49:40 ostrich 2[664634]: Hello
Oct 23 16:49:42 ostrich 2[664634]: Hello
Oct 23 16:49:44 ostrich 2[664634]: Hello
Oct 23 16:49:46 ostrich 2[664634]: Hello
Oct 23 16:49:48 ostrich 2[664634]: Hello

Next you'll have the finish script which is just what should be run at
the end of the script:

#!/bin/sh

echo "Shutting Down"

We don't have anything to do really so we just write a message. But you
could do cleanup stuff here if you want.

Last the logging spec. This is a subdirectory called log:

--> tree $SVLIB/hello/log
/etc/sv/hello/log
├── run
└── supervise [error opening dir]

1 directory, 1 file

Where again the run is a shell script:

#!/bin/sh

exec svlogd -tt /var/local/log/runit/hello

To keep things simple this is what you want. In general you could swap
out different logging daemons other than svlogd (which comes with
runit), but I don't see a reason to and this Just Works™. Basically
runit will create this as a separate service, but just knows how to pipe
around outputs now.

If you add these and then reload the services:

sudo sv reload $SVDIR/hello

You'll stop seeing "Hello" in the runit system log, and start seeing it
in the log file we configured:

sudo less +F "$SVLOG_SYS_DIR/hello/current"
# or
sudo tail -f "$SVLOG_SYS_DIR/hello/current"

Before we go over configuring the logging daemon (that sysvlogd thing
we ran in log/run) I should mention all those supervise dirs that
were laying around.

These basically are the locks and other control data that runit uses to
manage the services. Don't mess with them. They are owned by root
anyways. One thing you can do if you think you messed things up is to
disable the service and remove them all to start fresh:

sudo rm -rf $SVDIR/hello
rm -rf $SVLIB/hello/supervise
rm -rf $SVLIB/hello/log/supervise

Now one last thing is to configure the log. This file doesn't go in the
service directory (SVLIB) but the directory where the logs are. So
make this file SVLOG_SYS_DIR/hello/config and it should have something
like this:

# max size in bytes
s100000

# keep max of 10 files
n10

# minimum of 5 files
N5

# rotate every number of seconds
t86400

# prepend each log message with the characters
pHELLO::

This lets you rotate logs and control file sizes. Its a really not nice
file format but I will forgive them considering they aren't using any
libraries for TOML or YAML parsing or such things. Again something I
would improve on for non PID 0 usage.

With this all in place you'll see something like this in
SVLOG_SYS_DIR/hello:

--> sudo tree $SVLOG_SYS_DIR/hello
/var/local/log/runit/hello
├── @400000005f929f2c12d2041c.s
├── @400000005f92b3043a84c42c.s
├── @400000005f92c6dd22d71b04.s
├── @400000005f92dab60d0ce77c.s
├── @400000005f92ee8f37dec1dc.s
├── @400000005f93026921e78ae4.s
├── @400000005f931643127f5bf4.s
├── @400000005f932a1d27cdd3b4.s
├── @400000005f933df70f77ffc4.s
├── @400000005f933e0a239542b4.s
├── config
├── current
└── lock

Where those ID named files are the rotated logs.

Now that we're done with the runit tutorial lets show you how to make a
timer service that acts like a cron job.

Timer Services With Runit and Snooze

snooze was also available in the Ubuntu package index so we just
install with apt:

sudo apt install snooze

Otherwise its very basic C so should be trivial to compile and install.

First lets just right a timer script that runs a command every 40
seconds seconds with sleep and then we can just replace sleep with
snooze. Our run script is then:

#!/bin/sh

echo "Sleeping until time to run"
sleep 40

/home/user/.local/bin/run_thing

echo "Finished with task"

Because we are running this script as a service and the job of a service
manager is to restart failing services, this script will just run over
and over. There really wasn't a need for the while loop in our hello
example.

Now to make this "cron-like" we replace sleep with snooze:

#!/bin/sh

echo "Sleeping until time to run"
snooze

/home/user/.local/bin/run_thing

echo "Finished with task"

Where the default of snooze is just to block until midnight every
night (that is hour zero). So basically its just calculating how long it
is until midnight and sleeping until then. Pretty simple right? Once
snoozing is over the command will run the task will terminate, get
restarted and then will snooze again until next time.

You can see all the options for configuring when snooze will sleep
until in its docs and man page (this one is actually readable). But for
instance you can set it sleep until 3 AM on Mondays and Thursdays:

snooze -w1,4 -H3

Something else I like to do is have it run a number of jobs in serial as
a cycle. Where you would have to have multiple cron jobs to achieve this
you can do it in one script with this method.

I have a two print jobs every week to keep the heads from drying out.
Black and white on Monday and color on Thursday at noon.

#!/bin/sh

echo "Snoozing until time to print"
snooze -w1 -H12

echo "Printing black and white test page"

# black and white
lp /home/salotz/testpage_bw.pdf


echo "Snoozing until next print job"
snooze -w4 -H12

echo "Printing RBG color test page"

# color: RBG
lp /home/salotz/testpage_color_rbg.pdf

echo "End of run script job cycle"

With cron you'd have something like [4]:

0 12 * * 1 salotz lp /home/salotz/testpage_bw.pdf
0 12 * * 4 salotz lp /home/salotz/testpage_color_rbg.pdf

So the syntax for snooze is actually pretty similar and perhaps a little
cleaner and semantic.

So just to review: By avoiding cron we've avoided some of the most
difficult and finnicky sysadmin tasks in linux, namely email and
logging, and traded them for runit. I think its a fair trade and as
we've seen setting up runit is trivial in Ubuntu (and likely other
distros).

I can definitely see now why runit has fierce supporters. I'll
definitely be using it for situations I would normally use systemd as
well. While I'm not going to be running Void linux on my desktop any
time soon its a good candidate for inside of containers.

Final Notes

As I mentioned I am not a grey-neckbeard and so if I've made any gross
oversights in my claims, please let me know in the comments. I would
love to like cron more.

Comments

Comments are implemented as a mailing list hosted on sourcehut:
https://lists.sr.ht/~salotz/salotz.info_comments-section

Comments are for the whole website so start your subject line with:
[runit_snooze] for this post.

Footnotes

Well, the idea was hinted at in the snooze README but I put it
together.

Every shell script is a scar:

master_tab="$(cat << EOF
$(cat $CONFIGS_LIB_DIR/cron/header.crontab)
$(cat $CONFIGS_LIB_DIR/cron/misc.crontab)
$(cat $CONFIGS_LIB_DIR/cron/my.crontab)
EOF
)"

# then write to the crontab file after cleaning it out
(crontab -r && crontab -l 2>/dev/null; echo "$master_tab") | crontab -

It only has How-To Guides and Reference according to this

classification: https://documentation.divio.com/
No I didn't do that myself, of course I used this

https://crontab.guru/.

DEV Community: salotz