Why anacron
Cron can be used to
run a task on regular basis. It cannot anyway run passed task.
Example i want a a task to be run at 10:20, but at this time the
computer was off and now it's 11:00, this means the task will wait
tell next time the condition is met again.
Anacron, on the
other hand, will run an action after a specified amount of time
beginning when the computer is on.
Adding Tasks
To add a job add it
to the file /etc/anacrontab, as root
Note:
You must have proper
permission for tasks to be run.
Cron and anacron job files
[root@localhost ~]#
cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# For details see
man 4 crontabs
# Example of job
definition:
# .----------------
minute (0 - 59)
# | .-------------
hour (0 - 23)
# | | .----------
day of month (1 - 31)
# | | | .-------
month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .----
day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * *
user-name command to be executed
[root@localhost ~]#
cat /etc/anacrontab
# /etc/anacrontab:
configuration file for anacron
# See anacron(8) and
anacrontab(5) for details.
SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random
delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be
started during the following hours only
START_HOURS_RANGE=3-22
#period in days
delay in minutes job-identifier command
1 2 cron.daily nice
run-parts /etc/cron.daily
7 25 cron.weekly nice
run-parts /etc/cron.weekly
@monthly
45 cron.monthly nice run-parts /etc/cron.monthly
Adding a task
I will add a custom
script, that will run as root.
Note here that I
also customize the anacrontab file to suit my needs
[root@localhost ~]#
cat /etc/anacrontab
# /etc/anacrontab:
configuration file for anacron
# See anacron(8) and
anacrontab(5) for details.
SHELL=/bin/bash #i
want to use bash instead of sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin:/root
#the /root folder contains my script
#MAILTO=root #I want
to send output to a file. Uncomment to mail it
# the maximal random
delay added to the base delay of the jobs
RANDOM_DELAY=0 #random 0 to 45 minutes added to job's time. want 0
# the jobs will be
started during the following hours only
START_HOURS_RANGE=3-22
#start of action in 24h
#period in days
delay in minutes job-identifier command
1 2 cron.daily nice
run-parts /etc/cron.daily
7 25 cron.weekly nice
run-parts /etc/cron.weekly
@monthly
45 cron.monthly nice run-parts /etc/cron.monthly
#Here I added my job
to be launched 5 minutes after power on of the computer every day
#you can give it any
name (here "ka"). The script or other command must be in
PATH above
1 5 ka
ka_update &>> /root/log"
Anacron log file
A file under the
/var/spool/anacron directory, with the same name as the job. This
file will contain a single line that indicates the last time the job
was executed.
[root@localhost ~]#
ls -ld /var/spool/anacron/*
-rw-------. 1 root
root 9 May 30 19:02 /var/spool/anacron/ka
[root@localhost ~]#
cat /var/spool/anacron/ka
20170530
[root@localhost ~]#
date
Wed May 31 07:30:05
CEST 2017
Some very useful options
Taken simply from man anacron
Testing anacrontab
-T
Anacrontab testing. Tests the /etc/anacrontab configuration file for
validity. If there is an error in the file, it is shown on the
standard output and Anacron returns the value of 1. Valid
anacrontabs return the value of 0.
Force execution.
You can force
execution
-f
Forces execution of all jobs, ignoring any timestamps.
-n Runs
jobs immediately and ignores the specified delays in the
/etc/anacrontab file. This options implies -s.
-s
Serializes execution of jobs. Anacron does not start a new job
before the previous one finished.
-d Does
not fork Anacron to the background. In this mode, Anacron will
output informational messages to standard error, as well as to
syslog. The output of any job is mailed by Anacron.
Custom files
and folders
You can use some
custom files instead of the standard files
-t
some_anacrontab Uses the specified anacrontab, rather than the
/etc/anacrontab default one.
-S spooldir
Uses the specified spooldir to store timestamps in. This option is
required for users who wish to run anacron themselves.
# anacron -S /mySpool -t /etc/myAnacron
Example
[root@localhost ~]#
cat ka_update
#!/bin/bash
#check
first if there is an update available
dnf check-update &>
/dev/null #suppress all output
#exit code 0 no update, 1
error and 100 there is updates
r=$? #this contains dnf
check-update exit code
echo "Exit code = $r"
if [[ $r -eq 100 ]]
then
dnf -y update &> /dev/null && echo
"Updated $(date)"
elif
[[ $r -eq 0 ]]
then echo "No updates"
elif [[
$r -eq 1 ]]
then echo "Error dnf update command"
fi
Execute the job
# anacron -d -f -n
ka #execute the job named "ka" immediately
Anacron
started on 2017-05-31
Will run job `ka'
Jobs will be
executed sequentially
Job `ka' started
Job `ka'
terminated
Normal exit (1 job run)
How things really work
crond is a service
that is normally enabled by default, but anacron is not a service.
cron executes hourly
a script located under /etc/cron.hourly, called 0anacron
[root@localhost ~]#
cd /etc/cron.hourly/
[root@localhost
cron.hourly]# ls
0anacron
This script is very
simple, and well documented
[root@localhost
cron.hourly]# cat 0anacron
#!/bin/sh
# Check whether
0anacron was run today already
if test -r
/var/spool/anacron/cron.daily; then
day=`cat
/var/spool/anacron/cron.daily`
fi
if [ `date +%Y%m%d`
= "$day" ]; then
exit 0
fi
# Do not run jobs
when on battery power
if [ `cat
/sys/class/power_supply/AC/online 2>/dev/null`x = 0x ]; then
exit 0
fi
/usr/sbin/anacron -s#when the 2 above conditions are false /sbin/anacron runs
Hope, this was helpful. Please enjoy
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