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Next: 38. postgres SQL Server Up: rute Previous: 36. httpd Contents
Subsections
37.
crond and
atd
crond and
atd
are two very simple and important
services that everyone should be familiar with.
crond does
the job of running commands periodically (daily, weekly), and
atd's main feature is to run a command once at some future time.
These two services are so basic that we are not going to detail their package contents and invocation.
37.1
/etc/crontab Configuration File
The
/etc/crontab file
dictates a list of periodic jobs to be
run--like updating the
locate (see page
![[*]](crossref.png)
) and
whatis
(see page ) databases, rotating logs
(see Section 21.4.9), and possibly performing backup tasks. If
anything needs to be done periodically, you can
schedule that
job in this file.
/etc/crontab is read by
crond on
startup.
crond will already be running on all but the most
broken of UNIX systems.
After modifying
/etc/crontab, you should restart
crond with
/etc/rc.d/init.d/crond restart
(or
/etc/init.d/crond restart, or
/etc/init.d/cron restart).
/etc/crontab consists of single line definitions for the time of
the day/week/month at which a particular command should be run. Each
line has the form,
|
<time> <user> <executable> |
where
<time> is a time pattern that the current time must match for
the command to be executed,
<user> tells under what user
the command is to be executed, and
<executable> is the
command to be run.
The time pattern gives the minute, hour, day of the month,
month, and weekday that the current time is compared. The
comparison is done at the start of every single minute. If
crond gets a match, it will execute the command. A simple
time pattern is as follows.
|
50 13 2 9 6 root /usr/bin/play /etc/theetone.wav |
which will
playen WAV
Sat Sep 2 13:50:00
every year, and
|
50 13 2 * * root /usr/bin/play /etc/theetone.wav |
will
play it at
13:50:00 on the 2nd of every month, and
|
50 13 * * 6 root /usr/bin/play /etc/theetone.wav |
will do the same on every Saturday. Further,
|
50 13,14 * * 5,6,7 root /usr/bin/play /etc/theetone.wav |
will
play at
13:50:00 and
at
14:50:00 on Friday, Saturday, and Sunday, while
|
*/10 * * * 6 root /usr/bin/play /etc/theetone.wav |
will
play every 10 minutes the whole of Saturday. The
/ is a special notation meaning ``in steps of''.
Note that in the above examples, the
play command is executed as
root.
The following is an actual
/etc/crontab
file:
5 10 |
# Environment variables firstSHELL=/bin/bashPATH=/sbin:/bin:/usr/sbin:/usr/binMAILTO=rootHOME=/ # Time specs30 20 * * * root /etc/cron-alarm.sh35 19 * * * root /etc/cron-alarm.sh58 18 * * * root /etc/cron-alarm.sh01 * * * * root run-parts /etc/cron.hourly02 4 * * * root run-parts /etc/cron.daily22 4 * * 0 root run-parts /etc/cron.weekly42 4 1 * * root run-parts /etc/cron.monthly |
Note that the
# character is used
for comments as usual.
crond also allows you to specify
environment variables under which commands are to be run.
Your time additions should come like mine have, to remind me of the last three Metro trains of the day.
The last four entries are vendor supplied. The
run-parts command is a simple script to run all
the commands listed under
/etc/cron.hourly,
/etc/cron.daily, etc. Hence, if you
have a script that needs to be run every day but not at a specific
time, you needn't edit your
crontab file: rather just place the
script with the others in
/etc/cron.<interval>.
My own
/etc/cron.daily/ directory contains:
5 10 |
total 14drwxr-xr-x 2 root root 1024 Sep 2 13:22 .drwxr-xr-x 59 root root 6144 Aug 31 13:11 ..-rwxr-xr-x 1 root root 140 Aug 13 16:16 backup-rwxr-xr-x 1 root root 51 Jun 16 1999 logrotate-rwxr-xr-x 1 root root 390 Sep 14 1999 makewhatis.cron-rwxr-xr-x 1 root root 459 Mar 25 1999 radiusd.cron.daily-rwxr-xr-x 1 root root 99 Jul 23 23:48 slocate.cron-rwxr-xr-x 1 root root 103 Sep 25 1999 tetex.cron-rwxr-xr-x 1 root root 104 Aug 30 1999 tmpwatch |
It is advisable to go through each of these now to see what your system is doing to itself behind your back.
37.2 The
at Command
at will execute a command at some future time, and only once. I
suppose it is essential to know, although I never used it myself until
writing this chapter.
at is the front end to the
atd daemon
which, like
crond will almost definitely be running.
Try our wave file example, remembering to press
-
to get the
<EOT> (End Of Text):
5 |
[root@cericon /etc]# at 14:19at> /usr/bin/play /etc/theetone.wavat> <EOT>warning: commands will be executed using /bin/shjob 3 at 2000-09-02 14:19 |
You can type
atq to get a list of current jobs:
|
3 2000-09-02 14:19 a |
a means is the queue name,
3 is the
job number, and
2000-09-02 14:19 is the scheduled time of
execution. While
play is executing,
atq will display:
|
3 2000-09-02 14:19 = |
The
at and
atd man pages contain
additional information.
Note that
atd should generally be disabled for
security.
37.3 Other
cron Packages
There are many
crond implementations. Some have more flexible
config files, and others have functionality cope with job schedules
that run when the machine is typically switched off (like home PCs). Your
distribution may have chosen one of these packages instead.
Next: 38. postgres SQL Server Up: rute Previous: 36. httpd Contents