The crond service, launched at boot time, is responsible for executing certain scripts and commands at specific intervals. The crond service assumes that the system is online permanently. In the case of a downtime coinciding with a cronjob, the cronjob is simply not executed until the following period.
Under Red Hat/CentOS, every 60 seconds crond reads the system crontab file, /etc/crontab, any crontabs in /etc/cron.d/ and all the user crontabs.
User crontabs are files named after the user who created them and can be found in /var/spool/cron/.
The crond service writes to a log file - /var/log/cron.
If a command or script produces an output, that output is sent to root by mail.
The root user can establish lists of users that can or cannot create their own crontabs by editing either the /etc/cron.allow or /etc/cron.deny files.
A typical system crontab under Red Hat/CentOS looks as follows:
SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/ # 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
Each line in a crontab starts with five columns:
| Minutes | Hours | Day of the month | Month | Day of the week |
|---|---|---|---|---|
| (0-59) | (0-23) | (1-31) | (1-12) | (0|7-6)* |
The following examples of values in those columns help explain how versatile a crontab can be:
| Example | Description |
|---|---|
| An absolute value such as 10 | In the Minutes column = 10 minutes after each hour |
| A series of values such as 2,6,8 | In the Month column = February, June and August |
| A range such as 1-5 | In the Day of the week column = From Monday through to Friday |
| The wildcard * | In the Day of the month column = Every day of the month |
| A regular interval such as 0-23/2 | In the Hour column = Every two hours |
Names of months or days of the week can be specified by name. Environment variables can be set in the crontab.
Special nicknames are supported :
As already stated, each authorized user can create a crontab. To check if a crontab exists, the user needs to execute the following command:
[trainee@centos ~]$ crontab -l no crontab for trainee
In order to create a crontab, the user should use the following command:
$ crontab -e [Enter]
This command launches VI. Edit your crontab as follows:
* * * * * /bin/pwd > pwd.txt
Save and quit VI.
You will obtain a result similar to the following example:
[trainee@centos ~]$ crontab -e no crontab for trainee - using an empty one crontab: installing new crontab
The crontab that you have just created has been saved to disk in /var/spool/cron/.
You cannot view the contents of the file as a normal user. To do so you must become root:
[trainee@centos ~]$ su - Password: [root@centos ~]# cat /var/spool/cron/trainee * * * * * /bin/pwd > pwd.txt
In order to allow or deny the right to a user to edit their crontab, root can edit either the cron.allow or cron.deny files. However, if root puts a user in the cron.deny file, any existing cronjobs will continue unless they are manually deleted:
The major drawback with cron is that it assumes the machine is up and running at all times. For this reason, anacron is now responsable for executing the contents of the following directories :
Anacron reads a list of jobs from its configuration file /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 5 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
This file contains two important columns at the beginning of each line:
| Interval | Delay |
|---|---|
| period in days | delay in minutes |
The period is the number of days that anacron waits before checking that the job has been executed. The delay in minutes is the delay from the moment that anacron checks after which the job will be run. Anacron is called upon boot.
Anacron records the date of execution in a special timestamp file for that job, so it can know when to execute it again. Only the date is used for the time calculations. The hour is not used :
[root@centos ~]# cat /var/spool/anacron/cron.daily 20131025
The files are stored in /var/spool/anacron/:
[root@centos ~]# ls -l /var/spool/anacron total 12 -rw-------. 1 root root 9 Oct 25 18:08 cron.daily -rw-------. 1 root root 9 Oct 25 18:46 cron.monthly -rw-------. 1 root root 9 Oct 25 18:26 cron.weekly
As in the case of cron, root has the ability to control who can and cannot use the at command by editing one of the following:
If the at.allow file exists, only users in that file can use the at command.
Take the example of root creating two at jobs for the 31/12/2015 at 1pm and 2pm respectively:
[root@centos ~]# at 13:00 12/31/2015 at> pwd > /tmp/test1.atd at> <EOT> job 1 at 2015-12-31 13:00 [root@centos ~]# at 14:00 12/31/2015 at> free > /tmp/test2.atd at> <EOT> job 2 at 2015-12-31 14:00
The at files created can be found in /var/spool/at/ :
[root@centos ~]# ls -l /var/spool/at total 12 -rwx------. 1 root root 2540 Oct 27 14:16 a00001017126d0 -rwx------. 1 root root 2541 Oct 27 14:16 a000020171270c drwx------. 2 daemon daemon 4096 Jan 30 2012 spool
Viewing the contents of the first file you will see something similar to the following example:
#!/bin/sh
# atrun uid=0 gid=0
# mail trainee 0
umask 22
HOSTNAME=centos.fenestros.loc; export HOSTNAME
SHELL=/bin/bash; export SHELL
HISTSIZE=1000; export HISTSIZE
QTDIR=/usr/lib/qt-3.3; export QTDIR
QTINC=/usr/lib/qt-3.3/include; export QTINC
USER=root; export USER
LS_COLORS=rs=0:di=01\;34:ln=01\;36:mh=00:pi=40\;33:so=01\;35:do=01\;35:bd=40\;33\;01:cd=40\;33\;01:or=40\;31\;01:mi=01\;05\;37\;41:su=37\;41:sg=30\;43:ca=30\;41:tw=30\;42:ow=34\;42:st=37\;44:ex=01\;32:\*.tar=01\;31:\*.tgz=01\;31:\*.arj=01\;31:\*.taz=01\;31:\*.lzh=01\;31:\*.lzma=01\;31:\*.tlz=01\;31:\*.txz=01\;31:\*.zip=01\;31:\*.z=01\;31:\*.Z=01\;31:\*.dz=01\;31:\*.gz=01\;31:\*.lz=01\;31:\*.xz=01\;31:\*.bz2=01\;31:\*.tbz=01\;31:\*.tbz2=01\;31:\*.bz=01\;31:\*.tz=01\;31:\*.deb=01\;31:\*.rpm=01\;31:\*.jar=01\;31:\*.rar=01\;31:\*.ace=01\;31:\*.zoo=01\;31:\*.cpio=01\;31:\*.7z=01\;31:\*.rz=01\;31:\*.jpg=01\;35:\*.jpeg=01\;35:\*.gif=01\;35:\*.bmp=01\;35:\*.pbm=01\;35:\*.pgm=01\;35:\*.ppm=01\;35:\*.tga=01\;35:\*.xbm=01\;35:\*.xpm=01\;35:\*.tif=01\;35:\*.tiff=01\;35:\*.png=01\;35:\*.svg=01\;35:\*.svgz=01\;35:\*.mng=01\;35:\*.pcx=01\;35:\*.mov=01\;35:\*.mpg=01\;35:\*.mpeg=01\;35:\*.m2v=01\;35:\*.mkv=01\;35:\*.ogm=01\;35:\*.mp4=01\;35:\*.m4v=01\;35:\*.mp4v=01\;35:\*.vob=01\;35:\*.qt=01\;35:\*.nuv=01\;35:\*.wmv=01\;35:\*.asf=01\;35:\*.rm=01\;35:\*.rmvb=01\;35:\*.flc=01\;35:\*.avi=01\;35:\*.fli=01\;35:\*.flv=01\;35:\*.gl=01\;35:\*.dl=01\;35:\*.xcf=01\;35:\*.xwd=01\;35:\*.yuv=01\;35:\*.cgm=01\;35:\*.emf=01\;35:\*.axv=01\;35:\*.anx=01\;35:\*.ogv=01\;35:\*.ogx=01\;35:\*.aac=01\;36:\*.au=01\;36:\*.flac=01\;36:\*.mid=01\;36:\*.midi=01\;36:\*.mka=01\;36:\*.mp3=01\;36:\*.mpc=01\;36:\*.ogg=01\;36:\*.ra=01\;36:\*.wav=01\;36:\*.axa=01\;36:\*.oga=01\;36:\*.spx=01\;36:\*.xspf=01\;36:; export LS_COLORS
MAIL=/var/spool/mail/root; export MAIL
PATH=/usr/lib/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin; export PATH
PWD=/root; export PWD
LANG=en_US.UTF-8; export LANG
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass; export SSH_ASKPASS
HISTCONTROL=ignoredups; export HISTCONTROL
SHLVL=1; export SHLVL
HOME=/root; export HOME
LOGNAME=root; export LOGNAME
QTLIB=/usr/lib/qt-3.3/lib; export QTLIB
CVS_RSH=ssh; export CVS_RSH
LESSOPEN=\|/usr/bin/lesspipe.sh\ %s; export LESSOPEN
G_BROKEN_FILENAMES=1; export G_BROKEN_FILENAMES
XAUTHORITY=/root/.xauthuwdZK4; export XAUTHORITY
cd /root || {
echo 'Execution directory inaccessible' >&2
exit 1
}
${SHELL:-/bin/sh} << 'marcinDELIMITER14b80a14'
pwd > /tmp/test1.atd
marcinDELIMITER14b80a14
<note important> Notez que tous les jobs AT sont exécutés par /bin/sh. </note>
To delete that job you can use the at command:
[root@centos ~]# at -l 1 2015-12-31 13:00 a root 2 2015-12-31 14:00 a root [root@centos ~]# at -d 1 [root@centos ~]# at -l 2 2015-12-31 14:00 a root
To execute several commands at the same time, it is simple to create a text file containing the commands and then to redirect the contents of the file to at's standard input:
[root@centos ~]# touch todo.list [root@centos ~]# echo pwd > todo.list [root@centos ~]# echo free >> todo.list [root@centos ~]# echo who >> todo.list [root@centos ~]# cat todo.list pwd free who [root@centos ~]# at 14:30 12/31/2015 < todo.list job 3 at 2015-12-31 14:30
~~DISCUSSION:off~~
<html> <center> Copyright © 2011-2014 Hugh Norris.<br><br> <a rel=“license” href=“http://creativecommons.org/licenses/by-nc-nd/3.0/”><img alt=“Creative Commons License” style=“border-width:0” src=“https://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png” /></a><br />This work is licensed under a <a rel=“license” href=“http://creativecommons.org/licenses/by-nc-nd/3.0/”>Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License</a> </center> </html>