% CRONTAB(1) % % 20 Nov 2019
crontab - manipulate per-user crontabs (dillon's lightweight cron daemon)
crontab file [-u user] - replace crontab from file
crontab - [-u user] - replace crontab from stdin
crontab -l [-u user] - list crontab for user
crontab -e [-u user] - edit crontab for user
crontab -d [-u user] - delete crontab for user
crontab -c dir - specify crontab directory
crontab manipulates the per-user crontabs.
Generally the -e option is used to edit your crontab. crontab will use the editor specified by your EDITOR or VISUAL environment variable (or /usr/bin/vi) to edit the crontab.
crontab doesn't provide the kinds of protections that programs like visudo do against syntax errors and simultaneous edits. Errors won't be detected until crond reads the crontab file. What crontab does is provide a mechanism for users who may not themselves have write privileges to the crontab folder to nonetheless install or edit their crontabs. It also notifies a running crond daemon of any changes to these files.
Only users who belong to the same group as the crontab binary will be able to install or edit crontabs. However it'll be possible for the superuser to install crontabs even for users who don't have the privileges to install them themselves. (Even for users who don't have a login shell.) Only the superuser may use the -u or -c switches to specify a different user and/or crontab directory.
The superuser also has his or her own per-user crontab, saved as /var/spool/cron/crontabs/root.
Unlike other cron daemons, this crond/crontab package doesn't try to do
everything under the sun. It doesn't try to keep track of user's preferred
shells; that would require special-casing users with no login shell. Instead,
it just runs all commands using /bin/sh. (Commands can of course be script
files written in any shell you like.)
Nor does it do any special environment handling. A shell script is better-suited to doing that than a cron daemon. This cron daemon sets up only four environment variables: USER, LOGNAME, HOME, and SHELL.
Our crontab format is roughly similar to that used by vixiecron. Individual fields may contain a time, a time range, a time range with a skip factor, a symbolic range for the day of week and month in year, and additional subranges delimited with commas. Blank lines in the crontab or lines that begin with a hash (#) are ignored. If you specify both a day in the month and a day of week, it will be interpreted as the Nth such day in the month.
Some examples:
# MIN HOUR DAY MONTH DAYOFWEEK COMMAND
# run `date` at 6:10 am every day
10 6 * * * date
# run every two hours at the top of the hour
0 */2 * * * date
# run every two hours between 11 pm and 7 am, and again at 8 am
0 23-7/2,8 * * * date
# run at 4:00 am on January 1st
0 4 1 jan * date
# run every day at 11 am, appending all output to a file
0 11 * * * date >> /var/log/date-output 2>&1
To request the last Monday, etc. in a month, ask for the "5th" one. This will always match the last Monday, etc., even if there are only four Mondays in the month:
# run at 11 am on the first and last Mon, Tue, Wed of each month
0 11 1,5 * mon-wed date
When the fourth Monday in a month is the last, it will match against both the "4th" and the "5th" (it will only run once if both are specified).
The following formats are also recognized:
# schedule this job only once, when crond starts up
@reboot date
# schedule this job whenever crond is running, and sees that at least one
# hour has elapsed since it last ran
@hourly ID=job1 date
The formats @hourly, @daily, @weekly, @monthly, and @yearly need to update
timestamp files when their jobs have been run. The timestamp files are saved as
/var/spool/cron/cronstamps/user.jobname. So for all of these formats, the cron
command needs a jobname, given by prefixing the command with ID=jobname.
(This syntax was chosen to maximize the chance that our crontab files will be
readable by other cron daemons as well. They might just interpret the
ID=jobname as a command-line environment variable assignment.)
There's also this esoteric option, whose usefulness will be explained later:
# don't ever schedule this job on its own; only run it when it's triggered
# as a "dependency" of another job (see below), or when the user explicitly
# requests it through the "cron.update" file (see crond(8))
@noauto ID=namedjob date
There's also a format available for finer-grained control of frequencies:
# run whenever it's between 2-4 am, and at least one day (1d)
# has elapsed since this job ran
* 2-4 * * * ID=job2 FREQ=1d date
# as before, but re-try every 10 minutes (10m) if my_command
# exits with code 11 (EAGAIN)
* 2-4 * * * ID=job3 FREQ=1d/10m my_command
These formats also update timestamp files, and so also require their jobs to be assigned IDs.
Notice the technique used in the second example: jobs can exit with code 11 to indicate they lacked the resources to run (for example, no network was available), and so should be tried again after a brief delay. This works for jobs using either @freq or FREQ=... formats; but the FREQ=.../10m syntax is the only way to customize the length of the delay before re-trying.
Jobs can be made to "depend" on, or wait until AFTER other jobs have successfully completed. Consider the following crontab:
* * * * * ID=job4 FREQ=1d first_command
* * * * * ID=job5 FREQ=1h AFTER=job4/30m second_command
Here, whenever job5 is up to be run, if job4 is scheduled to run within the next 30 minutes (30m), job5 will first wait for it to successfully complete.
(What if job4 doesn't successfully complete? If job4 returns with exit code EAGAIN, job5 will continue to wait until job4 is retried---even if that won't be within the hour. If job4 returns with any other non-zero exit code, job5 will be removed from the queue without running.)
Jobs can be told to wait for multiple other jobs, as follows:
10 * * * * ID=job6 AFTER=job4/1h,job7 third_command
The waiting job6 doesn't care what order job4 and job7 complete in. If job6 comes up to be re-scheduled (an hour later) while an earlier instance is still waiting, only a single instance of job6 will remain in the queue. It will have all of its "waiting flags" reset: so each of job7 and job4 (supposing again that job4 would run within the next 1h) will again have to complete before job6 will run.
If a job waits on a @reboot or @noauto job, the target job being waited on will also be scheduled to run. This technique can be used to have a common job scheduled as @noauto that several other jobs depend on (and so call as a subroutine).
The command portion of a cron job is run with /bin/sh -c ... and may
therefore contain any valid Bourne shell command. A common practice is to
prefix your command with exec to keep the process table uncluttered. It is
also common to redirect job output to a file or to /dev/null. If you do not,
and the command generates output on stdout or stderr, that output will be
mailed to the local user whose crontab the job comes from. If you have crontabs
for special users, such as uucp, who can't receive local mail, you may want to
create mail aliases for them or adjust this behavior. (See crond(8) for details
how to adjust it.)
Whenever jobs return an exit code that's neither 0 nor 11 (EAGAIN), that event will be logged, regardless of whether any stdout or stderr is generated. The job's timestamp will also be updated, and it won't be run again until it would next be normally scheduled. Any jobs waiting on the failed job will be canceled; they won't be run until they're next scheduled.
Ought to be able to have several crontab files for any given user, as an organizational tool.
crond(8)
Matthew Dillon (dillon@apollo.backplane.com): original developer
James Pryor (dubiousjim@gmail.com): current developer