   #Fcron documentation Using fcron ... fcrondyn GNU GENERAL PUBLIC
   LICENSE

   Copyright © 2000-2025 Thibault Godouet Fcron 3.4.0 Web page :
   http://fcron.free.fr
   _______________________________________________________________________

           Fcron documentation
   Prev Chapter 2. Using fcron ... Next
   _______________________________________________________________________

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. How does fcron handle hardware suspend (e.g. suspend to memory,
          or disk -- a.k.a. hibernate)?

   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 email with the exit status even if the
          command had no output. How can I avoid the email?

   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?
   2.2.12. How can I emulate a Vixie cron @reboot 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. How does fcron handle hardware suspend (e.g. suspend to memory, or
disk -- a.k.a. hibernate)?

   fcron now fully supports suspend (to memory or disk). On resume it will
   adjust the task schedules accordingly, run runatresume tasks if any,
   and report non-execution of noticenotrun tasks.

   fcron will try to notice suspends by itself without external help. On
   Linux it can do so reliably via system APIs, but on other OSes it will
   instead check if it wakes up later than it expected after a sleep. This
   is far from bullet-proof, as fcron may not notice the computer was
   suspended or under-estimate the suspend duration. This is because fcron
   can be woken up by external events such as receiving a signal or
   fcrondyn interation. There is also a risk fcron may be woken up before
   the suspend code has set the computer clock correctly after resume. If
   so, it may compute an incorrect the time and date of the execution of a
   job (please see the entry about system clock adjustment in the present
   FAQ).

   Because of this, on non-Linux systems it is recommended to explicitly
   'tell' fcron about suspends and the precise suspend duration:

    1. send a SIGSTOP signal to fcron when going into suspend:

$ kill -STOP $(cat /var/run/fcron.pid)

       . This is to make sure it will read reschedule jobs before running
       any when the computer resumes.
    2. Write the suspend duration as a number of seconds (written as a
       string) in the suspend file, e.g.

$ echo 2342 > /var/run/fcron.suspend

    3. Wake up fcron and tell it to process the suspend file and
       reschedule tasks accordingly:

$ kill -CONT $(cat /var/run/fcron.pid)

   An example script implementing this for pm-utils and systemd is
   included: fcron.suspend.sh. Note that it is however not needed on
   Linux. Contribution of equivalent scripts for other systems is welcome.

   Alternatively you could have fcron stop on suspend and restart on
   resume. However the main drawback would then be that tasks running at
   the time of suspend would be run again at resume, even though they may
   not have been stopped and finish on resume (i.e. they would run twice,
   with a second instance started before the first one finishes).

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 avoid 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 email with the exit status even if the command had no
output. How can I avoid the email?

   You could disable email entirely by setting the "mail" option to "no".
   But, if you still want to receive the standard output as email, 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" | fcr
ontab -

   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 emails 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 command line arguments --once,
   --nosyslog, --sleeptime 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 Vixie cron's crontab syntax fully. You
   should be able to use a crontab with fcron with no modification (if not
   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
   does not do that by itself. Fcron does not support the /etc/cron.d/
   directory either, as it is just an extension to the /etc/crontab file.
   However /etc/cron.{daily,weekly,monthly} directories will work in fcron
   just fine: they are supported through the run-parts program, just as
   Vixie cron).

   So if you want to replace Vixie cron by fcron transparently, all you
   have to do is create a /usr/bin/crontab link to /usr/bin/fcrontab, and
   reinstall the system (f)crontab with 'fcrontab /etc/crontab' every time
   you modify it (if you needed some more work than that, please let me
   know!).

   You can also use the script script/check_system_crontabs to monitor for
   system (f)crontab changes, i.e. changes to /etc/(f)crontab and
   /etc/cron.d/. When it detects a change, it will generate a new system
   fcrontab, and install it automatically. Should you choose to use that
   script, please 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 fcron's behavior should be very
   similar to Vixie cron regarding /etc/crontab and /etc/cron.d/.

2.2.11. How can I emulate an anacron entry?

   As anacron, fcron does not assume that the system runs permanently.
   Thus, fcron features similar functionalities to anacron, but it has
   different means to achieve it, in other words other ways to define when
   a job should run. Fcron is in general much more flexible than anacron.
   The best thing to do is to have look at fcrontab(5), and choose the
   type of line which is the most appropriate for your needs (this is
   likely to be a @-line or a %-line).

   On a Debian/Ubuntu, the default /etc/anacrontab looks like:
1       5       cron.daily       nice run-parts --report /etc/cron.daily
7       10      cron.weekly      nice run-parts --report /etc/cron.weekly
@monthly        15      cron.monthly nice run-parts --report /etc/cron.monthly

   A close fcron equivalent would be (see serial):
# Run once a day/week/month, and ensure only one of those tasks is run at a time
:
!reset
!serial,nice(10)
%daily * * run-parts --report /etc/cron.daily
%weekly * * run-parts --report /etc/cron.weekly
%monthly * * * run-parts --report /etc/cron.monthly

   Or you could go for something a bit more specific (see lavg,
   noticenotrun):
# Only run the tasks out of office-hours when the system load is low
# and send an email to the user if fcron couldn't run the job:
!reset
!serial,lavg(0.5,1,1.5),noticenotrun,nice(10),mailto(admin@yourdomain.com)
%daily * 0-9,18-23 run-parts --report /etc/cron.daily
%weekly * 0-9,18-23 run-parts --report /etc/cron.weekly
%monthly * 0-9,18-23 * run-parts --report /etc/cron.monthly

   Also, if you do want to emulate an anacron entry of the form:
   0 delay job-identity /your/command

   then you can use something as:
   @runonce delay /your/command

2.2.12. How can I emulate a Vixie cron @reboot entry?

   No need to emulate any more, as Vixie cron shortcuts, including
   @reboot, are now supported by fcron!
   _______________________________________________________________________

   Prev     Home                       Next
   fcrondyn  Up  GNU GENERAL PUBLIC LICENSE
