⚠️ WARNING: For tasks to work, it's important that the user credentials have not expired. Use 'chage -l [user]' to check.
[root@rhel8 system]# chage -l root
Last password change : Nov 11, 2020
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7
Commands:
You might need to install 'at'
# dnf install -y at
Start and enable the service
# systemctl enable --now atd.service
You can feed at:
When you run 'at' by itself (with a timespec) it will prompt you for the commands with an interactive prompt. Use 'Ctrl+d' to exit.
Common commands:
at.allow, at.deny - determine who can submit jobs via at or batch
The /etc/at.allow and /etc/at.deny files determine which user can submit commands for later execution via 'at' or 'batch'
Commands:
Cron/crond - daemon to execute scheduled commands
Crontab - Crontab is the program used to install a crontab table file, remove or list the existing tables used to serve the cron(8) daemon
Anacron - Anacron is used to execute commands periodically, with a frequency specified in days. Unlike cron, it does not assume that the machine is running continuously. Hence, it can be used on machines that are not running 24 hours a day to control regular jobs as daily, weekly, and monthly jobs.
Cron checks these files and directories:
/etc/crontab - System crontab. Nowadays the file is empty by default. Originally it was usually used to run daily, weekly, monthly jobs. By default these jobs are now run through anacron which reads /etc/anacrontab configuration file. See anacrontab(5) for more details.
/etc/cron.d/ - Directory that contains system cronjobs stored for different users.
/var/spool/cron - Directory that contains user crontables created by the crontab command.
/etc/cron.hourly - This is where cron executed Anacron from
cron (crond.service)
├─> /etc/crond.hourly/*
│ └─> 0anacron
│ └─> /usr/sbin/anacron -s
│ └─> /etc/anacrontab
│ ├─> run-parts -> /etc/cron.daily/*
│ ├─> run-parts -> /etc/cron.weekly/*
│ └─> run-parts -> /etc/cron.monthly/*
│
├─> /etc/crontab (system crontab - obsolete)
│
├─> /etc/cron.d/* (system cronjobs for users)
│
└─> /var/spool/cron/ (user crontabs)
├─> user1
└─> user2
Commands:
📝 NOTES
Example crontab template
#SHELL=/bin/bash
#PATH=/sbin:/bin:/usr/sbin:/usr/bin
#MAILTO=root
#===============================================================================
# +--------- Minute (0-59) | Output Dumper: >/dev/null 2>&1
# | +------- Hour (0-23) | Multiple Values Use Commas: 3,12,47
# | | +----- Day Of Month (1-31) | Do every X intervals: */X -> Example: */15 * * * * Is every 15 minutes
# | | | +--- Month (1 -12 or Jan-Dec) | Aliases: @reboot -> Run once at startup; @hourly -> 0 * * * *;
# | | | | +- Day Of Week (0-6) (Sunday = 0) | @daily -> 0 0 * * *; @weekly -> 0 0 * * 0; @monthly ->0 0 1 * *;
# | | | | | | @yearly -> 0 0 1 1 *;
# * * * * * COMMAND |
#===============================================================================
To show the contents of a the crontab without editing
# crontab -l
Edit crontab
# crontab -e
To edit the crontab of another user
# crontab -u [user] -e
To list the crontab of another user
# crontab -u [user] -l
You can use '/etc/cron.deny' and '/etc/cron.allow'
Commands:
Anacron is used to execute commands periodically, with a frequency specified in days. Unlike cron(8), it does not assume that the machine is running continuously. Hence, it can be used on machines that are not running 24 hours a day to control regular jobs as daily, weekly, and monthly jobs.
Anacron reads a list of jobs from the /etc/anacrontab configuration file (see anacrontab(5)). This file contains the list of jobs that Anacron controls. Each job entry specifies a period in days, a delay in minutes, a unique job identifier, and a shell command
# 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
Run status:
# ll /var/spool/anacron/
total 12
-rw-------. 1 root root 9 Mar 11 14:28 cron.daily
-rw-------. 1 root root 9 Mar 10 18:02 cron.monthly
-rw-------. 1 root root 9 Mar 10 17:42 cron.weekly
In case you want to disable Anacron, add a line with 0anacron which is the name of the script running the Anacron into the /etc/cron.hourly/jobs.deny file.