Version : 2021.01
Updated : 2021/10/27 04:19
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:
[root@centos8 ~]# 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
A crontab file has five fields for specifying day, date and time:
| 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 :
For example, the /etc/cron.d/0hourly is as follows:
[root@centos8 ~]# cat /etc/cron.d/0hourly # Run the hourly jobs SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root 01 * * * * root run-parts /etc/cron.hourly
As already stated, each authorized user can create a crontab. To check if a crontab exists, the user needs to execute the following command:
[root@centos8 ~]# exit logout [trainee@centos8 ~]$ crontab -l no crontab for trainee
In order to create a crontab, the user should use the following command:
[trainee@centos8 ~]$ crontab -e
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@centos8 ~]$ 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@centos8 ~]$ su - Password: fenestros [root@centos8 ~]# 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.
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:
[root@centos8 ~]# 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 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 |
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@centos8 ~]# ls /var/spool/anacron/ cron.daily cron.monthly cron.weekly [root@centos8 ~]# cat /var/spool/anacron/cron.daily 20210602 [root@centos8 ~]# cat /var/spool/anacron/cron.weekly 20210602 [root@centos8 ~]# cat /var/spool/anacron/cron.monthly 20210526
Anacron consults the /var/spool/anacron/cron.daily file every hour as specified in the /etc/cron.hourly/0anacron script. Anacron then decides whether to execute the /usr/sbin/anacron -s based upon the current date:
[root@centos8 ~]# cat /etc/cron.hourly/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` = "$day" ]; then
exit 0
fi
# Do not run jobs when on battery power
online=1
for psupply in AC ADP0 ; do
sysfile="/sys/class/power_supply/$psupply/online"
if [ -f $sysfile ] ; then
if [ `cat $sysfile 2>/dev/null`x = 1x ]; then
online=1
break
else
online=0
fi
fi
done
if [ $online = 0 ]; then
exit 0
fi
/usr/sbin/anacron -s
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.
As root, create two jobs for the 31/12/2042 at 13h00 and 14h00 respectively:
[root@centos8 ~]# at 13:00 12/31/2042 warning: commands will be executed using /bin/sh at> pwd > /tmp/test13.atd [^D] at> <EOT> job 1 at Wed Dec 31 13:00:00 2042 [root@centos8 ~]# at 14:00 12/31/2042 warning: commands will be executed using /bin/sh at> free > /tmp/test14.atd [^D] at> <EOT> job 2 at Wed Dec 31 14:00:00 2042
The at files created can be found in /var/spool/at/ :
[root@centos8 ~]# ls /var/spool/at a000010249d9f8 a000020249da34 spool
Viewing the contents of the second file you will see something similar to the following example:
[root@centos8 ~]# at -c 2
#!/bin/sh
# atrun uid=0 gid=0
# mail trainee 0
umask 22
LS_COLORS=rs=0:di=38\;5\;33:ln=38\;5\;51:mh=00:pi=40\;38\;5\;11:so=38\;5\;13:do=38\;5\;5:bd=48\;5\;232\;38\;5\;11:cd=48\;5\;232\;38\;5\;3:or=48\;5\;232\;38\;5\;9:mi=01\;05\;37\;41:su=48\;5\;196\;38\;5\;15:sg=48\;5\;11\;38\;5\;16:ca=48\;5\;196\;38\;5\;226:tw=48\;5\;10\;38\;5\;16:ow=48\;5\;10\;38\;5\;21:st=48\;5\;21\;38\;5\;15:ex=38\;5\;40:\*.tar=38\;5\;9:\*.tgz=38\;5\;9:\*.arc=38\;5\;9:\*.arj=38\;5\;9:\*.taz=38\;5\;9:\*.lha=38\;5\;9:\*.lz4=38\;5\;9:\*.lzh=38\;5\;9:\*.lzma=38\;5\;9:\*.tlz=38\;5\;9:\*.txz=38\;5\;9:\*.tzo=38\;5\;9:\*.t7z=38\;5\;9:\*.zip=38\;5\;9:\*.z=38\;5\;9:\*.dz=38\;5\;9:\*.gz=38\;5\;9:\*.lrz=38\;5\;9:\*.lz=38\;5\;9:\*.lzo=38\;5\;9:\*.xz=38\;5\;9:\*.zst=38\;5\;9:\*.tzst=38\;5\;9:\*.bz2=38\;5\;9:\*.bz=38\;5\;9:\*.tbz=38\;5\;9:\*.tbz2=38\;5\;9:\*.tz=38\;5\;9:\*.deb=38\;5\;9:\*.rpm=38\;5\;9:\*.jar=38\;5\;9:\*.war=38\;5\;9:\*.ear=38\;5\;9:\*.sar=38\;5\;9:\*.rar=38\;5\;9:\*.alz=38\;5\;9:\*.ace=38\;5\;9:\*.zoo=38\;5\;9:\*.cpio=38\;5\;9:\*.7z=38\;5\;9:\*.rz=38\;5\;9:\*.cab=38\;5\;9:\*.wim=38\;5\;9:\*.swm=38\;5\;9:\*.dwm=38\;5\;9:\*.esd=38\;5\;9:\*.jpg=38\;5\;13:\*.jpeg=38\;5\;13:\*.mjpg=38\;5\;13:\*.mjpeg=38\;5\;13:\*.gif=38\;5\;13:\*.bmp=38\;5\;13:\*.pbm=38\;5\;13:\*.pgm=38\;5\;13:\*.ppm=38\;5\;13:\*.tga=38\;5\;13:\*.xbm=38\;5\;13:\*.xpm=38\;5\;13:\*.tif=38\;5\;13:\*.tiff=38\;5\;13:\*.png=38\;5\;13:\*.svg=38\;5\;13:\*.svgz=38\;5\;13:\*.mng=38\;5\;13:\*.pcx=38\;5\;13:\*.mov=38\;5\;13:\*.mpg=38\;5\;13:\*.mpeg=38\;5\;13:\*.m2v=38\;5\;13:\*.mkv=38\;5\;13:\*.webm=38\;5\;13:\*.ogm=38\;5\;13:\*.mp4=38\;5\;13:\*.m4v=38\;5\;13:\*.mp4v=38\;5\;13:\*.vob=38\;5\;13:\*.qt=38\;5\;13:\*.nuv=38\;5\;13:\*.wmv=38\;5\;13:\*.asf=38\;5\;13:\*.rm=38\;5\;13:\*.rmvb=38\;5\;13:\*.flc=38\;5\;13:\*.avi=38\;5\;13:\*.fli=38\;5\;13:\*.flv=38\;5\;13:\*.gl=38\;5\;13:\*.dl=38\;5\;13:\*.xcf=38\;5\;13:\*.xwd=38\;5\;13:\*.yuv=38\;5\;13:\*.cgm=38\;5\;13:\*.emf=38\;5\;13:\*.ogv=38\;5\;13:\*.ogx=38\;5\;13:\*.aac=38\;5\;45:\*.au=38\;5\;45:\*.flac=38\;5\;45:\*.m4a=38\;5\;45:\*.mid=38\;5\;45:\*.midi=38\;5\;45:\*.mka=38\;5\;45:\*.mp3=38\;5\;45:\*.mpc=38\;5\;45:\*.ogg=38\;5\;45:\*.ra=38\;5\;45:\*.wav=38\;5\;45:\*.oga=38\;5\;45:\*.opus=38\;5\;45:\*.spx=38\;5\;45:\*.xspf=38\;5\;45:; export LS_COLORS
LANG=en_US.UTF-8; export LANG
HISTCONTROL=ignoredups; export HISTCONTROL
GUESTFISH_RESTORE=\\e[0m; export GUESTFISH_RESTORE
HOSTNAME=centos8.ittraining.loc; export HOSTNAME
GUESTFISH_INIT=\\e[1\;34m; export GUESTFISH_INIT
USER=root; export USER
GUESTFISH_PS1=\\[\\e[1\;32m\\]\>\<fs\>\\[\\e[0\;31m\\]\ ; export GUESTFISH_PS1
PWD=/root; export PWD
HOME=/root; export HOME
MAIL=/var/spool/mail/root; export MAIL
SHELL=/bin/bash; export SHELL
SHLVL=1; export SHLVL
LOGNAME=root; export LOGNAME
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin; export PATH
GUESTFISH_OUTPUT=\\e[0m; export GUESTFISH_OUTPUT
HISTSIZE=1000; export HISTSIZE
LESSOPEN=\|\|/usr/bin/lesspipe.sh\ %s; export LESSOPEN
cd /root || {
echo 'Execution directory inaccessible' >&2
exit 1
}
${SHELL:-/bin/sh} << 'marcinDELIMITER4c5fdfe0'
free > /tmp/test14.atd
marcinDELIMITER4c5fdfe0
To delete that job you can use the at command:
[root@centos8 ~]# at -l 1 Wed Dec 31 13:00:00 2042 a root 2 Wed Dec 31 14:00:00 2042 a root [root@centos8 ~]# atq 1 Wed Dec 31 13:00:00 2042 a root 2 Wed Dec 31 14:00:00 2042 a root [root@centos8 ~]# at -d 2 [root@centos8 ~]# atq 1 Wed Dec 31 13:00:00 2042 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@centos8 ~]# touch todo.list [root@centos8 ~]# echo pwd > todo.list [root@centos8 ~]# echo free >> todo.list [root@centos8 ~]# echo who >> todo.list [root@centos8 ~]# cat todo.list pwd free who [root@centos8 ~]# at 14:30 12/31/2042 < todo.list warning: commands will be executed using /bin/sh job 3 at Wed Dec 31 14:30:00 2042
Important - The batch command or it's alias at -b is used to execute commands when the system load is under a certain level. By default the value is set to 1.5. This value can be modified by the -l switch of the atd command.
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