cron(8) — AIX PS/2 1.2.1



CRON(8,C)                   AIX Commands Reference                    CRON(8,C)



-------------------------------------------------------------------------------
cron



PURPOSE

Runs commands automatically.

SYNTAX


    1   +--------------+   +---------------+
cron ---|              |---|               |---|
        +- -s logsize -+   +- -f openfreq -+


-----------------
1 Not usually run from the command line, but included in /etc/rc.


DESCRIPTION

The cron command runs shell commands at specified dates and times.  Regularly
scheduled commands can be specified according to instructions contained in
crontab files.  You can submit your crontab file via the crontab command (see
page crontab-1).  Use the at command (see page at-1) to submit commands
that are to be run only once.  Because the cron command never exits, it should
be run only once.  This is best done by running the cron command from the
initialization process through the /etc/rc command file (see page rc-1).

The cron command examines crontab files and at command files only during
process initialization and when a file changes.  This reduces the overhead of
checking for new or changed files at regularly scheduled intervals.

The cron command also executes a sync system call approximately once a minute
to ensure that all information in memory that should be on disk (buffered
output) is written out.  These periodic updates minimize the possibility of
file system damage in the event of a crash.  In addition, the cron command
keeps a number of frequently used system directories open to keep their inodes
in kernel memory for faster access.

The cron command creates a log of its activities in the file /usr/adm/cron/log.

For a discussion of how to schedule commands, see "crontab."

Note:  To increase system performance, the cron command will open, and hold
       open, very frequently used system files and directories.  This ensures
       that these frequently used files and directories are open when they are
       needed by other commands, and thus the other commands will often run
       faster.  The system administrator can specify which files should be held
       open by putting a list of file names into the file /etc/openfiles.  At



Processed November 8, 1990         CRON(8,C)                                  1



CRON(8,C)                   AIX Commands Reference                    CRON(8,C)



       periodic intervals (default is 20 minutes), cron will open the files
       named in this file.  The default time interval may be changed with the
       -f option described below.

FLAGS

-s logsize  Specifies the maximum size, in kilobytes, for the log file
            /usr/adm/cron/log.  When cron is started, if /usr/adm/cron/log is
            more than logsize kilobytes in length, it is renamed to
            /usr/adm/cron/log.old and a new log file /usr/adm/cron/log is
            created.

-f openfreq Specifies the number of minutes between readings of the file
            /etc/openfiles.  Changes to this file will go undetected for up to
            openfreq minutes (default 20 minutes).

FILES

/usr/adm/cron         Main cron directory.
/usr/adm/cron/log     Accounting information.
/usr/spool/cron       Spool area.
/etc/openfiles        List of files to be held open.
/usr/adm/cron/log.old Saved accounting log.

RELATED INFORMATION

See the following commands:  "at, batch,"  "crontab," and  "rc."

See the sync system call file and the openfiles special file in AIX Operating
System Technical Reference.

























Processed November 8, 1990         CRON(8,C)                                  2

Museum

Related Articles