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Lesson 10 The crontab command
ObjectiveExplain the use of the crontab command to schedule a process.

crontab command

Configuring cron

The proper way to run cron is to configure the /etc/rc or /etc/rc.local file to run it. A typical rc.local entry would be /usr/sbin/crond. Therefore, you do not use cron as you would a standard application. For example, you do not type cron filename. You simply set it to run as a daemon.

crontab

After you have set crond to run, the next step is to place scripts that contain entries you want to execute into the spool directory.
You refer to the cron program as follows: /var/spool/cron. You can place a file into the /var/spool/cron directory; however, you do not run the file directly. To register the proper entries, you must use the crontab command. The crontab program allows you to create and manage the entries that inform the cron daemon about the applications and processes you want to automate.

Using crontab arguments

The crontab command allows you to create a crontab file, which you can then add to your crontab directory. The syntax for using crontab is
crontab arguments file
The arguments for crontab are as follows:
  1. -l displays crontab output as standard output
  2. -r deletes a current crontab entry
  3. -e edits a current crontab entry
Linux also adds a –u option, which specifies the user's crontab file. For example:
crontab –u root –l

would list all crontab entries belonging to root. On other versions of UNIX, you do not need to specify a username to list your crontab entries.

Creating crontab entries

In this file, you can create entries that will execute certain commands. A crontab entry contains six fields:
  1. Minute (0 to 59)
  2. Hour (0 to 23)
  3. Day of the month (1 to 31)
  4. Month of the year (1 to 12)
  5. Day of the week (0 to 6); note that 0 indicates Sunday
A sixth field is possible. It refers to the crontab schedule you are actually creating. However, you should focus on the five entries in the above bullet points. As with many other UNIX commands, you can use the wildcard command to indicate any and all times. The wildcard is indicated with an asterisk (*). For example, the following crontab entry would execute the nmap program every Sunday at 5:15 a.m:
15 5 * * 0 nmap localhost

You can enter multiple values within each entry, as long as you use a comma. For example, you can enter the following to run nmap at 5:15 and 6:15 a.m. every Friday and Saturday:
15 5,6 * * 5,6 nmap localhost

The following entry would display the system time every 20 minutes:
1,21,41 * * * * (echo -n " "; date; echo) > /dev/con1