The cron daemon schedules system tasks according to commands found within each crontab file. A crontab file consists of commands, one command per line, that will be executed at regular intervals. The beginning of each line contains date and time information that tells the cron daemon when to execute the command.
10 3 * * * /usr/sbin/logadm (1) 15 3 * * 0 /usr/lib/fs/nfs/nfsfind (2) 1 2 * * * [ -x /usr/sbin/rtc ] && /usr/sbin/rtc -c > /dev/null 2>&1 (3) 30 3 * * * [ -x /usr/lib/gss/gsscred_clean ] && /usr/lib/gss/gsscred_clean (4)
The following describes the output for each of these command lines:
The first line runs the logadm command at 3:10 a.m. every day.
The second line executes the nfsfind script every Sunday at 3:15 a.m.
The third line runs a script that checks for daylight savings time (and make corrections, if necessary) at 2:10 a.m. daily.
If there is no RTC time zone, nor an /etc/rtc_config file, this entry does nothing.
x86 only - The /usr/sbin/rtc script can only be run on an x86 based system.
The fourth line checks for (and removes) duplicate entries in the Generic Security Service table, /etc/gss/gsscred_db, at 3:30 a.m. daily.
For more information about the syntax of lines within a crontab file, see Syntax of crontab File Entries.
Table 8-2 Default crontab Files
Besides the default crontab files, users can create crontab files to schedule their own system tasks. Other crontab files are named after the user accounts in which they are created, such as bob, mary, smith, or jones.
To access crontab files that belong to root or other users, superuser privileges are required.
The cron daemon performs the following tasks at startup:
Checks for new crontab files.
Reads the execution times that are listed within the files.
Submits the commands for execution at the proper times.
Listens for notifications from the crontab commands regarding updated crontab files.
In much the same way, the cron daemon controls the scheduling of at files. These files are stored in the /var/spool/cron/atjobs directory. The cron daemon also listens for notifications from the crontab commands regarding submitted at jobs.
A crontab file consists of commands, one command per line, that execute automatically at the time specified by the first five fields of each command line. These five fields, described in the following table, are separated by spaces.
Table 8-3 Acceptable Values for crontab Time Fields
Follow these guidelines for using special characters in crontab time fields:
Use a space to separate each field.
Use a comma to separate multiple values.
Use a hyphen to designate a range of values.
Use an asterisk as a wildcard to include all possible values.
Use a comment mark (#) at the beginning of a line to indicate a comment or a blank line.
For example, the following crontab command entry displays a reminder in the user's console window at 4 p.m. on the first and fifteenth days of every month.
0 16 1,15 * * echo Timesheets Due > /dev/console
Each command within a crontab file must consist of one line, even if that line is very long. The crontab file does not recognize extra carriage returns. For more detailed information about crontab entries and command options, refer to the crontab(1) man page.