Frequently Asked Questions

Copyright © 2000-2024 Thibault Godouet | Fcron 3.3.2 (dev release) | Web page : http://fcron.free.fr

Documentation de fcron
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# 2.2. Frequently Asked Questions

This FAQ intends to complement the man pages by following a more practical approach.

If you think a QA should be added, please mail me it!

2.2.1. How does fcron handle system clock adjustments?
2.2.2. How does fcron handle daylight saving time changes?
2.2.3. What about fcron and software suspend (aka. suspend to RAM, to disk)?
2.2.4. How can I prevent fcrontab from considering the first "word" of my command line as a user name i.e. "runas(word)"?
2.2.5. I have a job which usually terminates with a non-zero status. When it does, I receive a mail with the exit status even if the command had no output. How can I avoid the mail?
2.2.6. What does "loadavg" mean anyway?
2.2.7. How can I use fcrontab in scripts?
2.2.8. Can I use fcron from time to time, for instance in a script?
2.2.9. Can I run fcron without root's privileges?
2.2.10. Has fcron some incompatibilities with Vixie cron?
2.2.11. How can I emulate an anacron entry?

### 2.2.1. How does fcron handle system clock adjustments?

First, you must understand that fcron determines, for each job, its next time and date of execution. It then determines which of those jobs would be the next to run and then, sleeps until that job should be run. In other words, fcron doesn't wake up like Vixie cron each minute to check all job in case one should be run ... and it avoids some problems associated with clock adjusts.

This means that if the new time value is set into the past, fcron won't run a particular job again. For instance, suppose the real time and system clock are 3:00, so the next job cannot be scheduled to run before 3:00, as it would have already been run and re-scheduled.

First, suppose you set your system clock into the past, say to 2:00, Presuming that the last run was shortly before 3:00. then fcron will sleep until the next job should be executed. The execution time for a job is determined by identifying the last time that the job ran and computing the next scheduled time. This means that the next scheduled time must be on or after 3:01. Therefore, in this example, fcron will not run a job for at least one hour.

Next, if you set the system time into the future, say to 4:00, fcron will run every job scheduled between the old and the new time value once, regardless of how many times it would have been scheduled. When fcron wakes up to run a job after the time value has changed, it runs all the jobs which should have run during the interval because they are scheduled to run in a past time.

As special case is when "@xxx" style scheduling rules are involved, you must consider the "adjustment-interval". The "adjustment-interval" is the time difference between the original system time and the new system time. The concerned jobs will run at "adjust-interval" too early or too late depending of the nature of the adjust.

To conclude, fcron behaves quite well for small clock adjusts. Each job which should have run does so once, but not exactly at the correct time as if the job were scheduled within the adjustment interval. But, if you have to make a big change in the time and date, you should probably reset all the scheduled "nextexe" by running "fcrontab -z" on all the fcrontabs.

### 2.2.2. How does fcron handle daylight saving time changes?

There are two types of daylight saving time change: the remove-one-hour change (for instance, "at 3:00, it will be 2:00"), and the add-one-hour one (for instance, "at 2:00, it will be 3:00"). In both cases, fcron will run the job only once (more precisely, it won't run the job twice as many as it should have).

In the first case, the official time changes as follow (in order of real chronological time): [x, 2:00] -> i1: [2:00, 3:00] -> i2: [2:00, 3:00] -> [3:00, y]. i1 and i2 are the names of the two intervals [2:00, 3:00]. For this kind of DST change, a job which should run between 2:00 and 3:00 will run either in i1 or in i2, but not both. Consequently, a job scheduled to run every day at 2:30 (for instance) will be run only once. There is no way for a user to know if the job will be run in i1 or i2.

In the second case, there is a gap in time: the official time changes as follow (in order of real chronological time): [x, 2:00] -> [3:00, y]. A job scheduled between 2:00 and 3:00 will get run once, and only once, even if it should have been run several times. For instance, a job which should have run every 10 minutes will run only once, not 6 times, between 1:59 and 3:01. A job scheduled to run at 2:xx will run at 3:xx. For instance, if a job is scheduled to run every day at 2:30, it will run at 3:30 the day of this kind of DST change.

### 2.2.3. What about fcron and software suspend (aka. suspend to RAM, to disk)?

We suppose here that you are using swsusp and the hibernate script to do a "suspend to disk", but it should be similar with other methods.

When you switch on your computer after a suspend to disk, the system time will be incorrect, and will then be corrected by the hibernate script. Before it is corrected, fcron may compute the time and date of the next execution of a job: the computation would then be incorrect (please see the entry about system clock adjustment in the present FAQ).

So you should have the hibernate script stop fcron before the suspend, and then restart it when the system is switched on, ie. put a line like "RestartServices fcron" in your hibernate.conf file. That way, the system time will always be correct when fcron runs (assuming that fcron is started after the system time is corrected).

### 2.2.4. How can I prevent fcrontab from considering the first "word" of my command line as a user name i.e. "runas(word)"?

Suppose you have an user called "echo" (weird idea ... :)) ). If you use the line '* * * * * echo "Hello!"' in root's fcrontab, "echo" will be interpreted as "runas(echo)".

To suppress that, put your command in quotes:

    * * * * * 'echo "Hello!"'

will work as expected as quotes are allowed for the shell command but not for the user name.

### 2.2.5. I have a job which usually terminates with a non-zero status. When it does, I receive a mail with the exit status even if the command had no output. How can I avoid the mail?

You could disable mail entirely by setting the "mail" option to "no". But, if you still want to receive the standard output as mail, you can add an command which always evaluates to 0, like "/bin/true", after your primary command. This will not affect your job nor create additional output. For example:

    * * * * * /a/non/zero/status/job ; /bin/true

### 2.2.6. What does "loadavg" mean anyway?

The "/proc/loadavg" file provides loadavg values. These values are (in order): the system load averages for the past 1, 5, and 15 minutes; a count of the (active tasks/active processes); the pid of last process run;

The active task count includes those processes marked as running or uninterruptable. A load average is an estimate of the average number of processes running within a specified period. The load averages are computed from active task count. They are updated each time active task counts are taken.

The load average formula is:

    loadavg(d,n) = active_task_cnt - (active_task_cnt - old_loadavg)*(1/exp(d/n)

where: d is the time interval between active task count readings, typically every 5 seconds; n is the time over which the readings are averaged.

### 2.2.7. How can I use fcrontab in scripts?

You can use pipes with "fcrontab -l" (list the fcrontab) and "fcrontab -" (read the new fcrontab from input). For example:

     echo -e "`fcrontab -l | grep -v exim`\n0 * * * *	/usr/sbin/exim -q" | fcrontab -

can be used to add a line. Another way to do it would be to: list the fcrontab settings into a temporary file ("fcrontab -l > tmpfile"); modify the temporary file ("echo $LINE >> tmpfile"); replace the original fcrontab by the temporary; and finally, remove the temporary file ("fcrontab tmpfile ; rm -f tmpfile").

### 2.2.8. Can I use fcron from time to time, for instance in a script?

Let's suppose you use fcron in your ppp-up script. Fcron can permit you to run some jobs at connection startup, but not at each connection, like it would be if the job was run directly by the ppp-up script: for instance, only once every week.

Example 2-5. Using fcron in a script: running a job once every day, week, etc, at dialup connection

You can use a script like:

    # A ppp-up script ...
    # run the jobs pending, then returns:
    fcron -f -y -o

in conjunction with a fcrontab file like:

    # a fcrontab file ...
    %random(no),weekly,stdout * * /a/command/to/download/latest/mozilla/release
    %monthly,stdout * * * /update/junkbuster/config/files

You can also use fcron to run some jobs until the end of the connection. For instance, you can make fetchmail retrieve mails more often during connection: we suppose that it is configured to retrieve mails every hour, which launches a dialup connection if necessary, and we want it to check for mails every 5 minutes while connected.

Example 2-6. Using fcron in a script: running a job until the end of the connection

ppp-up script:

    # A ppp-up script ...
    # run fcron at the beginning of the connection:
    fcron -b

ppp-down script:

    # A ppp-down script ...
    # stop fcron at the end of the connection:
    # -- Warning: killall may have unwanted effect on non linux systems --
    killall -TERM fcron

the fcrontab:

    # a fcrontab file ...
    @volatile,first(0) 5 fetchmail

If you run fcron in several scripts, or if you run fcron as a daemon and want also to run fcron in scripts, then you should use fcron, fcrontab and fcrondyn's `--configfile`.

For more details, see fcron's options `--once`, `--nosyslog`, `--firstsleep` and `--configfile` in fcron(8), and fcrontab's options `volatile`, `stdout`, `first` in fcrontab(5)

### 2.2.9. Can I run fcron without root's privileges?

Yes, you can. To do that, see the following instructions, but please note that fcrondyn currently does *not* work without root privileges.

1. First, create a directory where you'll install fcron, and some subdirectories:

       bash$ mkdir /home/thib/fcron
       bash$ cd /home/thib/fcron
       bash$ mkdir doc man spool

2. Then, run configure with option `--with-run-non-privileged`, set all user names and groups to yours, and set appropriately the paths:

   Warning
   This option allows a non privileged user to run fcron. When used, fcron does not change its rights before running a job (i.e., if joe runs fcron, every job will run as joe). It means that YOU SHOULD NOT RUN FCRON AS A PRIVILEGED USER WHEN COMPILED WITH THIS OPTION or you'll have a serious security hole.

       bash$ ./configure --with-run-non-privileged --with-rootname=thib
       --with-rootgroup=thib --with-username=thib --with-groupname=thib
       --with-etcdir=/home/thib/fcron --with-piddir=/home/thib/fcron
       --with-fifodir=/home/thib/fcron --with-spooldir=/home/thib/fcron/spool
       --with-docdir=/home/thib/fcron/doc --prefix=/home/thib/fcron

The rest of the installation is described in the install file.

### 2.2.10. Has fcron some incompatibilities with Vixie cron?

As far as I know, fcron supports completely the syntax of Vixie cron's crontab, excepted the @* syntax (@annually, @weekly, etc: if you use that, you will have to replace it with the explicit equivalent given in crontab(5)). So you should not have much problem with that (otherwise, please contact me at `<fcron@free.fr>`).

The main problem is about the management of the system (f)crontab. Vixie cron monitors the changes on /etc/crontab every minute, and automatically takes into account the changes if any. As for now, fcron do not do that. Fcron do not support the /etc/cron.d/ dir too, as it is just an extension of the /etc/crontab file. But be reassured: /etc/cron.{daily,weekly,monthly} are supported by fcron (in fact, those dirs are not managed by fcron directly, but by run-parts, which is independent from fcron).

So if you want to replace transparently Vixie cron by fcron, all you have to do is creating a /usr/bin/crontab link to /usr/bin/crontab, and to reinstall the system (f)crontab with fcrontab /etc/crontab each time you modify it (if you find something else to do, please tell me!).

You can also use the script script/check_system_crontabs to generate a system fcrontab from /etc/(f)crontab and /etc/cron.d/, and install it automatically. If you choose to do that, take a look at the beginning of the script: you will find insctructions on how to use it -- and a few warnings you should pay attention to. With this script, the behavior of fcron should be very similar to Vixie cron's one concerning /etc/crontab and /etc/cron.d/.

### 2.2.11. How can I emulate an anacron entry?

You should use a line similar to the following one:

@volatile,first(xx) BIG-period /your/command

where xx is the time you would have put in your anacron line.

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