cron/README.md

About

This package contains a small cron implementation called gcrond.

It was written due to a perceived lack of a proper, simple cron implementation. All other cron implementation I came across were either decade old, abandoned pieces of horror ("Cool, I didn't even know that C syntax allows this!") or hopelessly integrated into other, much larger projects (e.g. absorbed by SystemD or in the case of OpenBSD cron, married to special OpenBSD syscalls).

It was a fun little exercise and it seems to work so far. No idea about standards compliance tough, the implementation was mostly written against the Wikipedia article about Cron.

License

The source code in this package is provided under the OpenBSD flavored ISC license. So you can practically do as you wish, as long as you retain the original copyright notice. The software is provided "as is" (as usual) with no warranty whatsoever (e.g. it might actually do what it was designed for, but it could just as well set your carpet on fire).

The sub directory m4 contains third party macro files used by the build system which may be subject to their own, respective licenses.

Portability

The program in this package has been written for and tested on a GNU/Linux system, so there may be some GNU-isms in there in addition to Linux specific code. Depending on your target platform, some minor porting effort may be required.

Building and installing

This package uses autotools. If you downloaded a distribution tar ball, simply run the configure script and then make after the Makefile has been generated. A list of possible configure options can be viewed by running configure --help.

If you really wish to do so, run make install to install the program on your system.

When working with the git tree, run the autogen.sh script to generate the configure script and friends.

Crontab File Format

The cron daemon reads its configuration from all files it can find in /etc/crontab.d/ (exact path can be configured).

The files are read line by line. Empty lines or lines starting with '#' are skipped.

Each non-empty line consists of the typical cron fields:

1. The minute field. Legal values are from 0 to 59.
2. The hour field. Legal values are from 0 to 23.
3. The day of month field. Legal values are from 1 to 31 (or fewer, depending on the month.
4. The month field. Legal values are from 1 to 12 (January to December) or the mnemonics JAN, FEB, MAR, APR, ...
5. The day of week field. Legal values are from 0 to 6 (Sunday to Saturday) or the mnemonics SUN, MON, TUE, WED, ...
6. The command to execute.

The fields are separated by spaces. For the time matching fields, multiple comma separated values can be specified (e.g. MON,WED,FRI for a job that should run on Mondays, Wednesdays and Fridays).

The wild-card character * matches any legal value. An stepping can be specified by appending / and then a stepping (e.g. for the minute field, */5 would let a job run every five minutes).

A range of values can also be specified as <lower>-<upper>, for instance MON-FRI would match every day from Monday to Friday (equivalent to 1-5).

Intervals and specific values can be combined, for instance a day of month field */7,13,25 would trigger once a week, starting from the first of the month (1,7,14,21,28), but additionally include the 13th and the 25th. The same could be expressed as 1-31/7,13,25.

Instead of specifying a terse cron matching expression, the first five fields can be replaced with one of the following mnemonics:

• @yearly or @anually is equivalent to 0 0 1 1 *, i.e. 1st of January at midnight
• @monthly is equivalent to 0 0 1 * *, i.e. 1st of every month at midnight
• @weekly is equivalent to 0 0 * * 0, i.e. every Sunday at midnight
• @daily is equivalent to 0 0 * * *, i.e. every day at midnight
• @hourly is equivalent to 0 * * * *, i.e. every first minute of the hour

Lastly, the command field is not broken down but passed to /bin/sh -c as is.

Security Considerations

The cron daemon currently has no means of specifying a user to run the jobs as, so if cron runs as root, the jobs it starts do as well. Since by default it reads its configuration from /etc which by default is only writable by root, this shouldn't be too much of a problem when using cron for typical system administration tasks.

If a job should run as another user, tools such as su, runuser, setpriv et cetera need to be used.

Possible Future Directions

The following things would be nice to have:

• decent logging for cron and the output of the jobs.
• cron jobs per user, e.g. scan ~/.crontab.d or similar and run the collected jobs as the respective user.
• timezone handling
• some usable strategy for handling time jumps, e.g. caused by a job that syncs time with an NTP server on a system without RTC.