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n this book, you ve learned about various tasks you can perform to keep Ubuntu running smoothly. You may decide that you want some of these tasks to occur on a regular basis. For example, perhaps you want your /home folder to be backed up every day, or perhaps you want to clean the /tmp folder to ensure that you always have enough free disk space. You could carry out each task individually, but human nature would no doubt step in, and you would forget, or you might perform the action twice, because you ve forgotten that you ve already done it. As you might expect, Linux is able to automate the running of particular tasks. They can either be run periodically at scheduled times or as one-time tasks. Using Linux s scheduling features is explained in this chapter.
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Under Ubuntu, the main way of scheduling tasks is via the cron daemon. This works on behalf of the user in order to schedule individual tasks, and it is also used by the system to run vital system tasks, although a different way of working is used in each case. For cron to run user-scheduled tasks, it reads a file called crontab. Each user has her own version of this file, which is stored in the /var/spool/cron/crontabs directory. This file can be edited in a text editor, but a special command should be used to do so.
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Note System-wide tasks are handled by the /etc/crontab/ file. This runs scripts contained in /etc/
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cron.hourly, /etc/cron.daily, and so on, depending on when the tasks are meant to be run (every hour, day, week, or month). The average user never needs to bother with system-wide cron jobs. These are handled by the internal system, and programs create their own entries as and when necessary.
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The cron daemon starts at bootup and simply sits in the background while you work, checking every minute to see if a task is due. As soon as one comes up, it commences the task, and then returns to a waiting status.
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CHAPTER 33 SCHEDULING TASKS
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Adding a scheduled task is relatively easily and is done via the shell. Entering the following command will cause your personal crontab file to be loaded into the GNU nano text editor, ready for editing: crontab -e If this is the first time you ve edited your crontab file, it will most likely be completely empty (in fact, you might see a message about crontab having to create the file from scratch). However, don t be put off. Adding a new entry is relatively easy and normally takes the form of something like this: 01 12 15 * * tar -cjf /home/keir/mybackup.tar.bz2 /home/keir Let s examine the line piece by piece. The first part the numbers and asterisks refers to when the task should be run. From left to right, the fields refer to the following: Minutes, from 0 to 59 Hours, in 24-hour time, so from 0 to 23 Day dates, for the day of the month, from 1 to 31 (assuming the month has that many days) Months, from 1 to 12 Day, for a particular day, either from 0 to 6 (0 is Sunday), or specified as a three-letter abbreviation (mon, tue, wed, and so on) In the example, the task is set to run at the first minute at the twelfth hour (midday) on the fifteenth day of the month. But what do the asterisks stand for They re effectively wildcards and tell cron that every possible value applies. Because an asterisk appears in the month field, this task will be run every month. Because an asterisk appears in the day field, the task will be run every day. You might have noticed a logical contradiction here. How can you specify a day if you also specify a date in the month Wouldn t this seriously limit the chances of the task ever running Yes, it would. If you were to specify sat, for example, and put 15 in the date field, the task would run on only the fifteenth of the month if that happened to be a Saturday. This is why the two fields are rarely used in the same crontab entry, and an asterisk appears in one if the other is being used. After the time and date fields comes the command itself: tar. As you learned in the previous chapter, tar is designed to back up your personal data. Only standard BASH shell commands can be used in the command section. cron isn t clever enough to interpret symbols such as the tilde (~) as a way of referring to your home directory. For this reason, it s best to be very thorough when defining a cron job and always use absolute paths. Let s take a look at another example (shown in Figure 33-1): 59 23 * * 0-3 tar -cjf /home/keir/mybackup.tar.bz2 /home/keir
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