Linux Utility - Crontab (Cron) Scheduler

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1 - About

The cron scheduler daemon (in ISC Cron) gets this data from crontab files.

Each user can have their own crontab, and though these are files in /var/spool/ , they are not intended to be edited directly.

Crontab is the program used to install, deinstall or list this file (tables)

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3 - Crontab File

A crontab file consists of commands, one command per line, that execute automatically at the time specified by the first five fields of each command line.

[#] minute hour day-of-month month day-of-week command

where:

  • # is the comment mark to comment a line (Optional)
  • The time field minute hour day-of-month month day-of-week can be
Field Star One Number */n List of number Range of Value -
minute * means every hour 0 to 59 - 5 means on the 5th minute of the hour */5 means every 5 minutes 0,5,10,15,20,25,30,35,40,45,50,55 - Same as */5 (ie every 5 minutes) 10-15 means every minutes between 10 en 15
hour * means every hour 0 to 24 - 4 means at 4 hour */5 means every 5 hour 0,5,10,15,20 means at 0, 5,10,15 and 20 hour 5-10 means every hour between 5 and 10
day-of-month * means every day of the month 1 to 28/30 or 31 - 5 means the 5 of the month */5 means every 5 day 5,10,15,20 means at 5,10,15 and 20 day of the month 5-10 means every day between the 5th and 10th day of the month
month * means every month 1-12 - 5 means in May */5 means every 5 month 5,10 means in May and October 5-10 means in May, June, July, August, September, October
day of week * means every day 0-7 where 0 and 7 is Sunday - 5 means Friday */5 means every Friday 2,5 means Tuesday and Friday 0-3 means Sunday, Monday, Tuesday, Wednesday
  • command is the path the command

4 - Example

  • At 13h30 every Monday
30 13 * * 1
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5 - How to

5.1 - List Job

crontab -l
# weekly gather statistics 
00  23    *  *  3 /home/gerard/scripts/gather_statistics.sh > /dev/null
# run the script to load tables between 22:00 and 10:00
*/5 22-23 *  *  * /home/gerard/scripts/loadTables.sh > /dev/null
*/5 00-09 *  *  * /home/gerard/scripts/loadTables.sh > /dev/null
#the first load check just after 22:00
03  22-23 *  *  * /home/gerard/check_load_is_running.sh
03  00-09 *  *  * /home/gerard/check_load_is_running.sh
#the last load check just before 10:00
58  09    *  *  * /home/gerard/check_load_is_running.sh

It will lists the jobs for the user who ran the command.

To see the job of another user:

  • -u option
crontab -u userName -l
  • logged in as the other user
  • or run it with sudo.

5.2 - View (Hourly|Daily|Weekly|Monthly) Cronjobs

cd /etc/cron.(hourly|daily|weekly|monthly)/
ls -l
cat filename

5.3 - View Software Specific Cronjobs

cd /etc/cron.d/
ls -l
cat filename

5.4 - Create

  • User scope: call the command below and modify the file accordingly.
crontab -e
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5.5 - Test

You can test the scheduled script with the following command:

env -i  yourScript

The env -i will forget the environment variables as the cron do. See the environment variable section below.

5.6 - History: See log

cat /var/log/cron
# or
sudo cat /var/log/cron

Example:

May 11 18:25:01 euramsodw010 crond[29434]: (oracle) CMD (/home/oracle/bin/cancel_session_above_timeout.sh > /home/oracle/bin/cancel_session_above_timeout.log 2>&1)

6 - Configuration

6.1 - Environment Variable

Several environment variables are set up automatically by the cron(8) daemon:

  • SHELL is set to /bin/sh,
  • LOGNAME and HOME are set from the /etc/passwd line of the crontab´s owner.

HOME and SHELL may be overridden by settings in the crontab; LOGNAME may not.

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7 - Metadata

Each user can have their own crontab. They are files in /var/spool/cron that are not intended to be edited directly.

8 - Documentation / Reference