Andrew Hudson - Fedora™ Unleashed, 2008 edition

Alternatively, you can use the at.allowfile; this does not exist by default. If you have a blank at.allowfile, no one except root is allowed to schedule jobs. As with at.deny, you can add usernames to at.allowone per line, and those users can schedule jobs. You should use either at.denyor at.allow: When someone tries to run ator batch, Fedora checks for the username in at.allow. If it is in there, or if at.allowdoes not exist, Fedora checks for the username in at.deny. If the username is in at.denyor at.denydoes not exist, the user is not allowed to schedule jobs.

The atand batchcommands work well if you want to execute a just single task at a later date, but they are less useful if you want to run a task frequently. Instead, there is the cronddaemon for running tasks repeatedly based on system — and user — requests. cronhas a similar permissions system to at: Users listed in the cron.denyfile are not allowed to use cron, and users listed in the cron.allowfile are. An empty cron.denyfile — the default — means everyone can set jobs. An empty cron.allowfile means that no one (except root) can set jobs.

There are two types of jobs: system jobs and user jobs. Only root can edit system jobs, whereas any user whose name appears in cron.allowor does not appear in cron.denycan run user jobs. System jobs are controlled through the /etc/ crontabfile, which by default looks like this:

SHELL=/bin/bash

PATH=/sbin:/bin:/usr/sbin:/usr/bin

MAILTO=root

HOME=/

# run-parts

1 * * * * root run-parts /etc/cron.hourly

2 4 * * * root run-parts /etc/cron.daily

22 4 * * 0 root run-parts /etc/cron.weekly

42 4 1 * * root run-parts /etc/cron.monthly

The first four lines are optional: SHELLspecifies which shell should be used to execute the job (defaults to the shell of the user who owns the crontabfile, usually /bin/bash), PATHspecifies the search path for executables to use, and you should avoid using environment variables there. MAILTOdefines to whom mail should be sent. If this is not set, it uses the owner of the crontab, but if you do not want to receive mail when your job runs, just set it to "". Finally, HOMEspecifies the home directory of the user; again, this defaults to the user's home directory if left unspecified.

The next line, # run-parts, starts with a pound sign (#) and so is treated as a comment and ignored. The next four lines are the important parts: They are the jobs themselves.

Each job is specified in seven fields that define the time to run, owner, and command. The first five commands specify the execution time in quite a quirky order: minute (0-59), hour (0-23), day of the month (1-31), month of the year (1-12), and day of the week (0-7). For day of the week, both 0 and 7 are Sunday, which means that 1 is Monday, 3 is Wednesday, and so on. If you want to specify "all values" (that is, every minute, every hour, every day, and so on), use an asterisk, *.

The next field specifies the username of the owner of the job. When a job is executed, it uses the username specified here. The last field is the command to execute.

So, the first job runs at minute 1, every hour of every day of every month and executes the command run-parts /etc/cron.hourly. The run-partscommand is a simple script that runs all programs inside a given directory — in this case, /etc/cron.hourly. So, in this case, the job executes at 00:01 (1 minute past midnight), 01:01, 02:01, 03:01, and so on, and uses all the programs listed in the cron.hourlydirectory.

The next job runs at minute 2 and hour 4 of every day of every month, running run-parts /etc/cron.daily. Because of the hour limitation, this script runs only once per day, at 4:02 a.m. Note that it uses minute 2 rather than minute 1 so that daily jobs do not clash with hourly jobs. You should be able to guess what the next two jobs do, simply by looking at the commands they run!

Inside each of those four directories ( cron.hourly, cron.daily, cron.weekly, and cron.monthly) is a collection of shell scripts that are run by run- parts. For example, in cron.dailyare scripts such as rpm, which saves a list of your packages in /var/log/rpmpkgsevery day; logrotate, which handles backing up of log files; and makewhatis, which updates the whatisdatabase. You can add other system tasks to these directories if you want to, but you should be careful to ensure your scripts are correct.

The crondaemon reads all the system crontabfiles and all user crontabfiles once a minute (on the minute; that is, at 6:00:00, 6:01:00, and so on) to check for changes. However, any new jobs it finds are not executed until at least one minute has passed.

For example, if it is 6:01:49 (that is, 49 seconds past one minute past 6 a.m.) and you set a cronjob to run at 6:02, it does not execute. At 6:02, the crondaemon rereads its configuration files and sees the new job, but is not able to execute it. If you set the job to run at 6:02 a.m. every day, it is executed the following morning and every subsequent morning.

This same situation exists when deleting jobs. If it is 6:01:49 and you have a job scheduled to run at 6:02, deleting the job makes no difference: cronruns it before it rereads the crontabfiles for changes. However, after it has reread the crontabfile and noticed the job is no longer there, it is not executed on subsequent days.

There are alternative ways of specifying dates. For example, you can use sets of dates and times by using hyphens or commas, such as hours 9-15 would execute at 9, 10, 11, 12, 13, 14, and 15 (from 9 a.m. to 3 p.m.), whereas 9, 11, 13, 15 would miss out at the even hours. Note that it is important that you do not put spaces into these sets because the crondaemon would interpret them as the next field. You can define a step value with a slash (/) to show time division: */4for hours means "every four hours all day," and 0 12/3means "every three hours from midnight to noon." You can also specify day and month names rather than numbers, using three-character abbreviations: Sun, Mon, Tue, Fri, Satfor days, or Jan, Feb, Mar, Oct, Nov, Decfor months.

As well as system jobs, there are also user jobs for those users who have the correct permissions. User jobs are stored in the /var/spool/crondirectory, with each user having his own file named after his username — for instance, /var/spool/cron/paulor /var/spool/cron/root. The contents of these files contain the jobs the user wants to run and take roughly the same format as the /etc/crontabfile, with the exception that the owner of the job should not be specified because it is always the same as the filename.

To edit your own crontabfile, type crontab -e. This brings up a text editor ( vimby default, but you can set the EDITOR environment variable to change that) in which you can enter your entries. The format of this file is a little different from the format for the main crontabbecause this time there is no need to specify the owner of the job — it is always you.