2004.08_Charly's Column-Fcron.pdf

Charly’s column

SYSADMIN

On Schedule

Cron executes the jobs you hand to it reliably and on schedule.

This legacy tool just ticks away like a Swiss watch, completely

unfazed by external influences – unfortunately that is not always

a good thing. BY CHARLY KÜHNAST

files, deleting temp files, or

performing backups in the early

hours of the morning are best left to

cron. This is fine until one job takes

longer than expected and overlaps the

time of another. In the past, I have found

myself yearning for a tool that could

gracefully handle that kind of situation,

and now I’ve found exactly that tool:

Fcron [1].

Fcron version 2.0.1. has just gone

stable. Fcron’s author. Thibault Godouet,

looks set to put the good old vixie cron

out of a job. To do so, Thibault has

equipped the tool with all kinds of extra

goodies. Installing fcron is trivial. After

unpacking the 142KBytes source files,

simply enter:

the similarities between the old and new

versions stop. Although the structure

and syntax of fcrontab are based on the

legacy role model, there are some differ-

ences.

To get started, let’s tell fcron to do

something that cron can do: I want fcron

to run the /bin/date command at five

minutes to every full hour. The fcrontab

entry looks like this:

days, another slight change to the com-

mand is required:

0 14-22/2 * * *~0 /bin/date

In other words, fcron treats elements

with the tilde character (~) as excep-

tions. Fcron’s ability to check the system

load is one of its neatest features. The

following line launches a command

called /bin/heavyjob/ once an hour – but

only if the mean system load (loadavg)

over a five minute period is below a

threshold of 1.5:

50***/bin/date

What’s new with fcron is its ability to

launch commands relative to a com-

puter’s uptime. I want to run /bin/date

every two and a half hours, starting at

system boot time:

@lavg5(1.5) 1h /bin/heavyjob

The useful manpage installs along with

fcron or fcrontab. It contains detailed ref-

erences for fcron options, and examples.

In my case at least, the good old cron

tool is out of a job.

@ 2h30m /bin/date

./configure && make && U

make install

Be Nice!

You can assign a nice value to a program

for jobs that put a heavy load on the sys-

tem:

■

If so desired, the installation routine will

add the program to your startup scripts.

Following the pattern typical of cron,

fcron uses an fcrontab file. You can enter

fcrontab -e to edit the file. This is where

INFO

[1] Fcron: http://fcron.free.fr

&nice(10) 0 1 * * * /usr/local U

/shellscripts/MyBackupScript.sh

Charly Kühnast is a

Unix System Man-

ager at the data-

center in Moers, near

Germany’s famous

River Rhine. His tasks

include ensuring fire-

wall security and availability and

taking care of the DMZ (demilitarized

zone). Although Charly started out on

IBM mainframes, he has been working

almost exclusively with Linux since

1995. To stay in shape he tries to get in

some karate training on his leisure

time.

SYSADMIN

The following line runs a command on

the hour between 2.00 pm and 10 pm:

SWIG Programming .............. 58

It is not always possible to write every bit of

your code in your programming language of

choice. SWIG lets you make C wrappers and

scripting easier.

Admin Workshop: DNS ....... 62

Dynamic DNS assignment by your ISP has

always meant you cannot run web services.

That is until now. We explore how you can

make changing IP addresses automatic.

0 14-22 ***/bin/date

To launch date every second(!) hour, I

need to modify the entry as follows:

0 14-22/2 * * * /bin/date

Fcron also supports exceptions: to run a

command on weekdays but not on Sun-

www.linux-magazine.com

August 2004

The Sysadmin’s Daily Grind: Fcron

R outine tasks, such as rotating log-

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