Cron/ru

This article describes how to setup and use cron daemons in Gentoo Linux.

Что делает cron?
Cron is a daemon that runs scheduled tasks based on input from the command crontab. It accomplishes this task by waking up every minute and checking to see if there are any cron-jobs to run in any of the user crontabs.

Cron на практике
There are a few cron implementations to choose from in Portage. All of them offer a similar interface, namely the use of crontab or a similar command. There is also a related utility called Anacron which is meant to work with cron on systems that are not continuously running.

Также следует заметить, что не все доступные пакеты cron зависят от. Этот пакет не зависит технически от какого-либо из пакетов cron, но он предлагает сходную с cron-ом функциональность, которую оценят многие из пользователей.

Перед тем, как начать работу с cron-ом, Вам следует выбрать какую из его реализаций Вы хотите использовать. Для Вашего удобства, ниже мы собрали информацию о каждой из них.

vixie cron
Vixie cron - это полноценная реализация программы cron, основанная на SysV cron. Каждый пользователь имеет свой собственный файл заданий crontab, и ему разрешается указывать переменные среды внутри этого файла. В отличие от других видов cron, он также предлагает поддержку SELinux и PAM. Он поддерживает меньшее количество архитектур чем Dcron, но большее по сравнению с Fcron.

Особенности :


 * Support for SELinux;
 * Support for PAM ;
 * Setting of environment variables in crontabs (PATH, SHELL, HOME, etc.);
 * Each user can have a personal crontab; access is controlled by and

cronie
Cronie is a fork of vixie-cron done by Fedora. Because of it being a fork it has the same feature set the original vixie-cron provides. Additionally cronie comes with an anacron implementation which must be enabled through the  USE flag.

dcron (Dillon's Cron)
Dcron стремится быть простой, элегантной и безопасной реализацией программы cron. Он не разрешает установку переменных среды в файлах crontabs и все задания cron запускаются из. Как и vixie-cron, каждый пользователь имеет свой собственный файл заданий crontab.

Особенности :


 * Fast, simple and free of unnecessary features;
 * Access to crontab is limited to the cron group, i.e. it doesn't rely on any external faculties.

fcron
Fcron ориентирован на то, чтобы заменить собой vixie-cron и anacron. Он разработан для работы на системах, которые не работают непрерывно и включает в себя дополнительные особенности. Он имеет ограничения на запуск заданий, управление сериализацией работ, возможность назначить работам приятные в обращении значения и возможность запланировать запуск работ при загрузке системы. Просмотрите главную страницу fcron, чтобы получить больше информации.

Особенности :


 * Designed to work on systems that are not continuously running, i.e. it can run a job after restarting if it was missed;
 * Setting of environment variables and many other options in crontabs;
 * Enhanced crontab syntax with support for many new features;
 * Each user can have a personal crontab, access is controlled by and

bcron
Bcron - это новая система cron, разработанная с учетом безопасности ее работы. Чтобы добиться этого, система поделена на несколько отдельных программ, каждая из которых отвечает за отдельное задание, со строго контролируемым сообщением между ними. Пользовательский интерфейс представляет собой несущественное изменение интерфейсов для подобных систем (таких как vixie-cron), но внутренние части программы сильно отличаются. Чтобы получить больше информации, проконсультируйтесь с главной страницей bcron по адресу http://untroubled.org/bcron.

Особенности :


 * Drop-in replacement for vixie-cron;
 * Multiprocess design;
 * Native daylight savings time support.

anacron
Anacron - это не демон cron, это то, что работает в объединении с ним. Он выполняет команды по интервалам, указанным по дням, и не предполагает непрерывную работу системы; он запускает работы, которые были пропущены, пока система была отключена. Anacron обычно полагается на демон cron, чтобы запускаться каждый день.

Установка
Выберите реализацию cron, которая Вам больше нравится, и установите ее командой emerge.

Make sure the cron daemon of choice has been added to the system's init process; without this step the cron daemon will not perform it's job.

Optionally, if Fcron has not been installed, installing Anacron as a helper to the cron daemon might be a wise choice.

Again, do not forget to add anacron to the system's init process.

Системный файл crontab
The post install messages from some of these cron packages instruct the user to run crontab /etc/crontab. The file is the system crontab. A cron installation can use it in conjunction with to run the scripts in. Note that only vixie-cron and cronie schedule jobs in automatically. Dcron and fcron users will need to run crontab /etc/crontab every time they make changes to the file.

Please note that jobs scheduled in the system crontab might not show up in the list of cron-jobs displayed by running crontab -l.

Of course, users can choose not to use any system crontab at all. If dcron or fcron has been chosen, do not run crontab /etc/crontab. If vixie-cron, cronie or bcron has been chosen comment all lines in.

A quick and easy way to comment out all the lines in a file is by using the sed command. Run the following command to comment out all the lines in

Предоставление доступа к cron проверенным пользователям
For users other than root to have access to the cron daemon, read this section, otherwise proceed to the next section: Scheduling cron-jobs.

No matter which cron package has been chosen, to allow a user to use crontab he will first have to be in the cron group. As an example, to add the user wepy to the cron group run:

When using dcron, the above step is all that is needed to give a user access to crontab. Dcron users may proceed to the next section Scheduling cron-jobs, all others need to keep reading.

When using fcron, edit the and  files. The most secure way to run a system is to first deny all users in, and then explicitly allow users in.

If a user (wepy again for this example) should be able to schedule his own cron-jobs, then add him to as follows:

If vixie-cron or cronie has been chosen, then simply edit the file.

For example, to allow access to the user wepy, add him to as follows:

Планирование заданий cron
The process of editing crontabs is different for each package, but they all support the same basic set of commands: adding and replacing crontabs, editing crontabs, deleting crontabs, and listing cron-jobs in crontabs. The following list shows how to run various commands for each package.

Before any of these commands can be used, first understanding of the crontab itself is needed. Each line in a crontab specifies five time fields in the following order: the minutes (0-59), hours (0-23), days of the month (1-31), months (1-12), and days of the week (0-7, Monday is day 1, Sunday is day 0 and day 7). The days of the week and months can be specified by three-letter abbreviations like mon, tue, jan, feb, etc. Each field can also specify a range of values (e.g. 1-5 or mon-fri), a comma separated list of values (e.g. 1,2,3 or mon,tue,wed) or a range of values with a step (e.g. 1-6/2 as 1,3,5).

That sounds a little confusing, but with a few examples it is easy to see it is not as complicated as it sounds.

To test what was just covered go through the steps of actually inputting a few cron-jobs. First, create a file called and make it look like the this:

Now add that crontab to the system with the "new command" from the table above.

To verify the scheduled cron-jobs, use the proper list command from the table above.

A list resembling should be displayed; if not maybe the wrong command was issued to input the crontab.

This crontab should echo "I really like cron" every minute of every hour of every day every other month. Obviously a user would only do that if they really liked cron. The crontab will also echo "I like cron a little" at 16:30 every day in January and February. It will also echo "I don't really like cron" at 3:10 on the January 1st.

If using anacron keep reading this section. Otherwise, proceed to the next section on Editing crontabs.

Пользователи Anacron, возможно, захотят отредактировать. Этот файл имеет четыре поля: количество дней перед каждым запуском, задержка в минутах после которой он запускает задания, имя задания, и команда для запуска.

For example, to have it run echo "I like anacron" every 5 days, 10 minutes after anacron is started, enter the following:

Anacron exits after all of the jobs in anacrontab have finished. To check to see if these jobs should be performed every day, a cron daemon will be used. The instructions at the end of the next section explain how this should be handled.

Редактирование crontabs
Being realistic, no user would want their system telling them how much they like cron every minute. As a step forward, remove the previous example crontab using the corresponding remove command from the table above. Use the corresponding list command to view the cron-jobs afterward to make sure it worked.

No cron-jobs should be displayed in the output from crontab -l</tt>. If cron jobs are listed, then the remove command failed to remove the crontab; verify the correct remove command for the system's cron package.

Now that we have a clean state, let's put something useful into the root crontab. Most people will want to run updatedb</tt> on a weekly basis to make sure that mlocate works properly. To add that to the system's crontab, first edit again so that it looks like the following:

That would make cron run updatedb at 2:22 A.M. on Monday morning every week. Now input the crontab with the proper new command from the table above, and check the list again.

Now let's say emerge --sync</tt> should be ran on a daily schedule in order to keep the Portage tree up to date. This could be done by first editing and then using crontab crons.cron</tt> as was done in the example above, or by using the proper edit command from the table above. This provides a way to edit the user's crontab in situ, without depending on external files like.

The above command should open the user's crontab with an editor. For example, if emerge --sync</tt> is to be run every day at 6:30 A.M., make the crontab look something like this:

Again, check the cron-jobs list as done in the previous examples to make sure the jobs are scheduled. If they are all there, then the system is ready to rock and roll.

Использование cronbase
As mentioned earlier, all of the available cron packages depend on. The cronbase package creates, and a script called. Notice the default file contains something like this:

To avoid going into much detail, assume these commands will effectively run hourly, daily, weekly and monthly scripts. This method of scheduling cron-jobs has some important advantages:


 * They will run even if the computer was off when they were scheduled to run;
 * It is easy for package maintainers to place scripts in those well defined places;
 * The administrators know exactly where the cron-jobs and crontab are stored, making it easy to backup and restore these parts of their systems.

Использование anacron
As mentioned earlier, anacron is used on systems not meant to be run continuously (like most of the desktop installations). Its default configuration file,, is usually similar to the following:

The main difference between this and other common crontabs is that with anacron there is no fixed date/hour for the job scheduling, but only the period between every run. When anacron is started, it will check the contents of a set of files in and calculate if the corresponding entry in the configuration file has expired since the last run. If it has, then the command is invoked again.

В заключение, важно закомментировать какие-либо совпадающие записи в любом другом cron, установленном на системе, так, как в следующем примере с файлом crontab программы vixie-cron:

Без этого, части daily, weekly и monthly будут выполняться - в разное время - как демоном cron, так и anacron, приводя к возможным повторениям выполнения работ.

Завершающие заметки
Remember, each cron package is different and the range of features varies greatly. Be sure to consult the man pages for crontab, fcrontab or anacrontab, depending on which cron daemon has been used.

Удачи!

Устранение проблем
When having problems getting cron to work properly, this quick checklist might be helpful.

Cron запущен?
To verify that cron is running, see if it shows up in the process list:

Cron работает?
Попробуйте следующее:

Затем проверьте модифицируется ли файл периодически.

Работает ли Ваша команда?
То же самое, что и раньше, но также перенаправьте стандартный вывод ошибок:

Может ли cron запустить Ваше задание
Проверьте лог-файл cron, обычно или, на ошибки.

Появляются ли какие-нибудь файлы
cron usually sends mail when there is a problem; check for mail and look for the creation of a file.

Альтернативы заданиям Cron
Some hosting companies do not allow access to cron, but many cron jobs alternatives can be found which are free or commercially available:


 * EasyCron