Cron

The cron daemon is a background process that runs particular programs at particular times.

ls /etc/cron*

/etc/crontab

/etc/cron.d:
popularity-contest

/etc/cron.daily:
apt-compat    dpkg       man-db   passwd
bsdmainutils  logrotate  mlocate  popularity-contest

/etc/cron.hourly:

/etc/cron.monthly:
vrms

/etc/cron.weekly:
fstrim  man-db  tor

• Root put (His/Other Users) jobs here: /etc/crontab (system-wide crontab)

• extension of crontab: /etc/cron.d

• Not in cron format. These are scripts that run once a dail, hour, etc.

  • cron.daily/
  • cron.hourly/
  • cron.weekly/
  • cron.monthly/

head -11 /etc/crontab

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user	command
17 *	* * *	root    cd / && run-parts --report /etc/cron.hourly

echo "30 8-16 * * * milad updtime >> /home/milad/test" >> /etc/crontab 
# system wide, root access, will run by milad

Every 30 Min From 8 Am to 4 PM Every day of month, day of week

crontab -e # user specific cron file

# in top of crontab file will email the command output to milad
MAILTO=milad

crontab -l # shows usere cron file
crontab -r # delete crontab file

/etc/cron.deny  # users in this file can't use cron
/etc/cron.allow # if created only users in this file can use cron (NO ONE ELSE)

if both cron.allow and cron.deny files exist the cron.deny is ignored.

At

queue, examine or delete jobs for later execution

systemctl is-active atd.service

active

at HH:MM If that time is already past, the next day is assumed.
at midnight, noon, or teatime (4pm)
at now + count time-units

• Time-units:
  • minutes, hours, days, or weeks
  • today by suffixing the time with today
  • tomorrow by suffixing the time with tomorrow

Examples:

• at 1am tomorrow
• at 17:04 sep 23

at -l     : atq
at -d, -r : atrm
at -b     : batch

/etc/at.deny  # users in this file can't use at
/etc/at.allow # if created only users in this file can use at (NO ONE ELSE)

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Lecture notes

• All lecture notes are available at github in ipynb format: https://github.com/ravexina/linux-notes.

• There are html exports of project available at: https://ravexina.github.io/linux-notes.

License

Linux Notes by Milad As (Ravexina) is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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