Copyright 2003-2025 Nicholas J. Kain. See LICENSE for licensing information.
ncron runs programs at user specified times. It works somewhat differently than traditional cron, which wakes up at regular intervals to see if it should run jobs at that current moment.
ncron instead has a run interval specified for every job that tells it the minimum time to wait between invocations of a given program. Then constraints can be specified for jobs that restrict when a program can be run. For example, a job might be restricted to only run between 1AM and 5AM.
The most recent time that a program has been run is persistently stored to disk, so system downtimes cause minimal disruptions to run schedules. ncron also has the ability to simply run a given job one time and never again.
A nice advantage of this scheme is that ncron can precalculate when the next job will run and sleep until that time. It does not wake at regular intervals just to check to see if it should run jobs. This approach allows for better power efficiency from fewer context switches.
- Linux kernel
- GCC or Clang
- GNU Make
- For developers: Ragel
ncron should work on most POSIX platforms, but the primary development platform is x64 Linux with recent glibc.
Compile and install ncron.
- Build ncron:
make - Install the
ncronexecutable in a normal place:
$ su
# install -o root -g root -m 755 ncron /usr/local/bin/ncron
# mkdir -m 755 -p /usr/local/share/man/man1
# mkdir -m 755 -p /usr/local/share/man/man5
# install -m 644 ncron.1 /usr/local/share/man/man1/
# install -m 644 crontab.5 /usr/local/share/man/man5/
# exit
Read the manpages for information on crontab format and paths.
I provide a sample init file for the OpenRC init system in 'init/openrc/ncron'.
$ man 5 crontab
$ man 1 ncron
A system-wide ncron will have its crontab at /var/lib/ncron/crontab.
The directory will look much like:
# ls -laF /var/lib/ncron
total 12
drwx------ 2 root root 36 May 13 2017 ./
drwxr-xr-x 3 root root 4096 May 13 2017 ../
-rw------- 1 root root 252 May 13 2017 crontab
-rw-r--r-- 1 root root 131 May 13 2017 exectimes
A simple crontab file (remove the leading spaces if you copy/paste):
!1
command=/bin/echo Hello world!
interval=5m
The exectimes file must be readable and writable by ncron; create
it with touch and set proper ownership and permissions on it. exectimes
stores the history of previously run jobs (denoted by the !NUMBER
markers in the crontab).
Read the man pages for more info! But what I've written here should be enough to get you started.
The most recent (Feb2024) ncron makes some changes to the crontab format:
- The dummy '!0' job is no longer required to terminate the crontab.
- Single line comments starting with '#' or ';' are now allowed.
- The syntax for specifying constraints uses '-' as a pair delimiter rather than ','.
- The semantics and syntax for specifying time constraints has changed. 'hour=' and 'minute=' are replaced with 'time=', which allows for continuous blocks of hh:mm time to be specified as a constraint.
- Constraint ranges must be low-high. If the old high-low behavior is needed, write two constraints in the form of low-high instead. This restriction may be relaxed in the future.
Legacy cron woke up at regular intervals to run jobs because precalculating a run time subject to constraints is an instance of solving the Boolean satisfiability problem, which is NP-complete in the general case. It may seem like solving this problem ahead of time rather than by statistical sampling (as legacy cron essentially does) would be computationally costly. But ncron is in practice very fast, as it can simply handle calendar day constraints by 'crossing out' days from a year. Constraint times for a given job are not allowed to vary dependently on other constraints (for example, constraint times can't vary by the day of the week). Given these chosen limitations, ncron is able to calculate the nearest valid runtime very quickly.