CRONTAB Linux

DerMeldos

change crontab entrys:

crontab -e

running as a specific user:

Only /etc/crontab and the files in /etc/cron.d/ have a username field. In that file you can do this:

1 1 * * * username /path/to/your/script.sh

From root's crontab sudo crontab -e you can use:

1 1 * * * su username -c "/path/to/your/script.sh"

Or you can use the user's actual crontab like this:

sudo crontab -u username -e

DerMeldos

Anther option is to us the @reboot argument to perform an action after the reboot happened and place a sleep interval to make sure everything is really up.

@reboot sleep 10; sh /usr/local/scripts/pull_script.sh

DerMeldos

DerMeldos

"/etc/crontab" is for systemwide cronjob, where you need to specify a user for the job.
updates for example can override these.
It is recommended to use "crontab -e" to edit and create cronjobs. crontab -e will edit the cronjobs of the user who executes this command.
A User field is no longer required with "crontab -e".

Examples:
/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

# 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
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || { cd / && run-parts --report /etc/cron.daily; }
47 6    * * 7   root    test -x /usr/sbin/anacron || { cd / && run-parts --report /etc/cron.weekly; }
52 6    1 * *   root    test -x /usr/sbin/anacron || { cd / && run-parts --report /etc/cron.monthly; }

crontab -e:

# Edit this file to introduce tasks to be run by cron.
# 
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
# 
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').
# 
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
# 
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
# 
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
# 
# For more information see the manual pages of crontab(5) and cron(8)
# 
# m h  dom mon dow   command
PATH=/usr/bin:/usr/local/bin
*/5 * * * * /bin/bash /usr/local/bin/scripts/update-route53.sh
*/5 * * * * /bin/bash /usr/local/bin/scripts/update-securitygroups.sh

DerMeldos

Some value modifiers:

* = match all possible values (i.e., every hour)
, = match multiple values (i.e., 15,45)
- = range of values (i.e., 2-4)
/ = specifies steps (i.e., */4)

DerMeldos

*crontab -r removes the crontab