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diff --git a/chapter_06.xml b/chapter_06.xml index 99c44e1..a2d0426 100644 --- a/chapter_06.xml +++ b/chapter_06.xml @@ -239,4 +239,84 @@ right from within <application>top</application> itself. </section> +<section> +<title>cron</title> + +<para> +Ok, so we've learned many different ways of viewing the active +processes on our system and means of signalling them, but what if we +want to run a process periodically? Fortunately, Slackware includes +just the thing, <application>crond</application>(8). cron runs +processes for every user on the schedule that user demands. This makes +it very useful for processes that need to be run periodically, but +don't require full daemonization, such as backup scripts. Every user +gets their own entry in the cron database, so non-root users can +periodically run processes too. +</para> + +<para> +In order to run programs from cron, you'll need to use the +<application>crontab</application>(1). The man page lists a variety of +ways to do this, but the most common method is to pass the +<arg>-e</arg> argument. This will lock the user's entry in the cron +database (to prevent it from being overwritten by another program), +then open that entry with whatever text editor is specified by the +VISUAL environment variable. On Slackware systems, this is typically +the <application>vi</application> editor. You may need to refer to the +chapter on <application>vi</application> before continuing. +</para> + +<para> +The cron database entries may seem a little archaic at first, but they +are highly flexible. Each uncommented line is processed by +<application>crond</application> and the command specified is run if +all the time conditions match. +</para> + +<screen><prompt>darkstar:~$ </prompt><userinput>crontab -e</userinput> +# Keep current with slackware +30 02 * * * /usr/local/bin/rsync-slackware64.sh 1>/dev/null 2>&1 +</screen> + +<para> +As mentioned before, the syntax for cron entries is a little difficult +to understand at first, so let's look at each part individually. From +left to right, the different sections are: Minute, Hour, Day, Month, +Week Day, and Command. Any asterisk <keycap>*</keycap> entry matches +every minute, hour, day, and so on. So from the example above, the +command is "/usr/local/bin/rsync-slackware64.sh 1>/dev/null 2>&1", and +it runs every weekday or every week of every month at 2:30 a.m. +</para> + +<para> +<application>crond</application> will also e-mail the local user with +any output the command generates. For this reason, many tasks have +their output redirected to <filename>/dev/null</filename>, a special +device file that immediately discards everything it receives. In order +to make it easier for you to remember these rules, you might wish to +paste the following commented text at the top of your own cron entries. +</para> + +<screen> +# Redirect everything to /dev/null with: +# 1>/dev/null 2>&1 +# +# MIN HOUR DAY MONTH WEEKDAY COMMAND +</screen> + +<para> +By default, Slackware includes a number of entries and comments in +root's crontab. These entries make it easier to setup periodic system +tasks by creating a number of directories in <filename>/etc</filename> +corrosponding to how often the tasks should run. Any script placed +within these directories will be run hourly, daily, weekly, or monthly. +The names should be self-explainatory: +<filename>/etc/cron.hourly</filename>, +<filename>/etc/cron.daily</filename>, +<filename>/etc/cron.weekly</filename>, and +<filename>/etc/cron.monthly</filename>. +</para> + +</section> + </chapter> |