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The following example backs up the projects directory at 2:00 AM every weekday:
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The same entry is listed here again using prefixes for the month and weekday:
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To specify particular months, days, weeks, or hours, you can list them individually, separated by commas For example, to perform the previous task on Sunday, Wednesday, and Friday, you can use 0,3,5 in the day-of-week field, or their prefix equivalents, Sun,Wed,Fri
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cron also supports comments A comment is any line beginning with a # sign
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Environment Variables for cron
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The cron service also lets you define environment variables for use with tasks performed Linux defines variables for SHELL, PATH, HOME, and MAILTO SHELL designates the shell to use tasks, in this case the BASH shell PATH lists the directories where programs and scripts can be found This example lists the standard directories, /usr/bin and /bin, as well as the system directories reserved for system applications, /usr/sbin and /sbin MAILTO designates to whom the results of a task are to be mailed By default, these are mailed to the user who schedules it, but you can have the results sent to a specific user, such as the administrator s email address, or an account on another system in a network HOME is the home directory for a task, in this case the top directory
SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/
The crond Directory
On a heavily used system, the /etc/crontab file can become crowded easily There may also be instances where certain entries require different variables For example, you may need to run some task under a different shell To help better organize your crontab tasks, you can place crontab entries in files within the crond directory The files in the crond directory all
Basic System Administration
contain crontab entries of the same format as /etc/crontab They may be given any name They are treated as added crontab files, with cron checking them for tasks to run For example, Linux installs a sysstat file in the crond that contains crontab entries to run tools to gather system statistics
PART VII PART I PART I PART I PART I PART I PART I
The crontab Command
You use the crontab command to install your entries into a crontab file To do this, first create a text file and type your crontab entries Save this file with any name you want, such as mycronfile Then, to install these entries, enter crontab and the name of the text file The crontab command takes the contents of the text file and creates a crontab file in the /var/spool/cron directory, adding the name of the user who issued the command In the following example, the root user installs the contents of mycronfile as the root s crontab file This creates a file called /var/spool/cron/root If a user named justin installs a crontab file, it creates a file called /var/spool/cron/justin You can control use of the crontab command by regular users with the /etc/cronallow file Only users with their names in this file can create crontab files of their own Conversely, the /etc/crondeny file lists those users denied use of the cron tool, preventing them from scheduling tasks If neither file exists, access is denied to all users If a user is not in an /etc/cronallow file, access is denied However, if the /etc/cronallow file does not exist, and the /etc/crondeny file does, then all users not listed in /etc/crondeny are automatically allowed access
# crontab mycronfile
Editing in cron
Never try to edit your crontab file directly Instead, use the crontab command with the -e option This opens your crontab file in the /var/spool/cron directory with the standard text editor, such as Vi (crontab uses the default editor as specified by the EDITOR shell environment variable) To use a different editor for crontab, change the default editor by assigning the editor s program name to the EDITOR variable and exporting that variable Normally, the EDITOR variable is set in the /etc/profile script Running crontab with the -l option displays the contents of your crontab file, and the -r option deletes the entire file Invoking crontab with another text file of crontab entries overwrites your current crontab file, replacing it with the contents of the text file