Techie May 2022
Introduction
In Linux, each user has their own crontab that can be used to schedule jobs that will be run as that user. Though crontabs are files in /var/spool/cron/crontabs, they are not intended to be edited directly. You’re therefore required to view, update and delete a user crontab file using the crontab command.
1 . View current user’s crontab.
$ crontab -l
2 . View root user’s crontab.
$ sudo crontab -l
3 . View another user’s crontab.
$ crontab -u user_name_here -l
4 . Edit a user’s crontab.
NB: Always use crontab -e and never modify crontab files directly
$ crontab -u user_name_here -e
5 . Edit the root user’s crontab.
NB: Jobs scheduled in the root user crontab will be executed as root with all of its privileges.
$ sudo crontab -e
6 . Delete a user’s crontab.
NB: If you dont use the -i directive, you will not be prompted to confirm deletion.
$ crontab -u user_name_here -r -i
7 . To set a bash file to run in crontab, normally you’d make the file executable by doing chmod u+x your_bash_file.sh (executable for current user) or chmod a+x your_bash_file.sh (executable for all users). And then you’d open the crontab for editing and add this:
# m h dom mon dow command
00 13 * * * your_bash_file.sh
So that bash file will be executed at 13:00 PM everyday.
Suppose you haven’t made the bash file executable, crontab could still execute it as long as you provide the path to the binary like so:
# m h dom mon dow command
00 13 * * * /bin/sh your_bash_file.sh
8 . Run crontab command that reboots your machine at 22:00 on every day of the month on every month only from Monday through Friday.
# m h dom mon dow command
0 22 * * 1-5 reboot
9 . Suppose you want to run crontab command that executes a bash script at minute 23 past every 2nd hour between 0 (12am) and 20 (8pm), on every day of the month on every month and on every week day.
# m h dom mon dow command
23 0-20/2 * * * /bin/sh your_bash_file.sh
10 . Set a time zone by adding a CRON_TZ declaration in your crontab.
# CRON_TZ=America/Los_Angeles
CRON_TZ=your_time_zone
If that doesn’t work add this instead:
# TZ=America/Los_Angeles
TZ=your_time_zone
Or you can set the time zone on the server all together via the terminal. First view the list of time zones available:
timedatectl list-timezones
Then set your desired time zone:
sudo timedatectl set-timezone America/Los_Angeles
Thanks for reading, see you in the next one!
