intro: timer(1) - run command at a specific interval

timer [-w] [-c clockid] interval command [arguments...]

At some point I got a bit annoyed with how cron would always mean sending an email on command output, even for things like munin-cron which typically runs every 5 minutes.
Plus what seems to be rather awkward error handling, specially for directories like /etc/cron.daily containing multiple scripts where if one fails the others wouldn't be ran.

So wrote a timer command, which simply launches a single command at a specified internal relative to itself, rather than at a human time. It is very basic, and not meant to be a cron replacement, see bottom of article for those, but more like an alternative model.

Also in the effects that might be interesting to some when compared to other similar tools:

• Can specify a clock other than realtime, which can make it more predictable. And on systems with *_ALARM clocks like Linux, allowing the machine to wake up.
• Doesn't interacts with the filesystem, so works well for embedded devices or immutable systems
• Unlike systemd timers it isn't linked to an ecosystem, so can be used with your favorite supervisor (say OpenRC, s6, runit, …) or even none for throwaway timers
• Exits when the command wasn't successful, so failures can be easily supervised

Links

• timer(1) manual
• Git repository
• Tarballs

Examples

munin-cron

#!/sbin/openrc-run
supervisor=supervise-daemon
command="timer"
command_user="munin"
command_args="5m munin-cron"

export SSD_NICELEVEL="20"
export SSD_IONICELEVEL="3:7" # I7

mirroring

Note: uses foreground and execlineb commands from execline instead of using shell

#!/sbin/openrc-run
supervisor=supervise-daemon
command="timer"
command_user="haelwenn"
# Better pass -w to not munch I/Os right when system is probably still somewhat booting up
command_args="-w 1d foreground execlineb -c /git/mirror/update.eb '' sh /tank/kopimi/mirror/update.sh"

export SSD_NICELEVEL="20"
export SSD_IONICELEVEL="3:4" # I4

Similar cron-alternative tools

• Leah Neukirchen's snooze: Sleeps until matching scheduled human-time, where it then execs into a command instead of spawning it
• Paul Jarc's runwhen: Sleeps until time computed with few supporting utilities, allows both machine-time and human-time, where it then execs into a command instead of spawning it
• Uwe Ohse's uschedule: Central per-user daemon, very cron-like (Or it's maintained version by JdeBP)