Cron hints

Cron is the default scheduler in linux distributions. Here’s a couple of easy helpers

Background

Cron tasks are added by adding rows at the bottom of the crontab file using the following syntax

1 2 3 4 5 /path/to/command arg1 arg2

or

1 2 3 4 5 /path/to/script.sh

Where,

1. Minute (0-59)
2. Hours (0-23)
3. Day (0-31)
4. Month (0-12 [12 == December])
5. Day of the week(0-7 [7 or 0 == sunday]) /path/to/command – Script or command name to schedule

* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Each value can be made using

Examples

There are some alias helpers which are not universally supported

Cron with Seconds resolution

CRON has 60 second resolution, so to get sub minute events, you need to add a cron task more than once.

i.e. Every 30 seconds

* * * * * command
* * * * * sleep 30 && command

Cron location

Troubleshooting

1. Checking Start

   When a Cron task starts it is logged in the syslog found at /var/log/syslog also accessible using dmesg for recent events only.

   A cron task executing a script such as 0 * * * * /path/to/script.sh will appear in the log as script.sh, and as a result you can find the task executing using grep.

   grep <pattern> /var/log/syslog which should list every time the pattern is matched in the syslog, and therefore listing every time cron started the task.

   i.e. grep my_script.sh /var/log/syslog

2. Checking simultaneous execution

   ps aux | grep <pattern> can be used to show all current processes which match a pattern.

   i.e. ps aux | grep my_script.sh

   Sometimes it is worth adding this check into the Cron script to determine whether to bail out preventing multiple copies of a script to execute in parallel

   Example: */2 * * * * /path/to/myscript.sh

    #!/bin/sh
    if ps -ef | grep -v grep | grep doctype.php ; then
            exit 0
    else
            /home/user/bin/doctype.php >> /home/user/bin/spooler.log &
            #mailing program
            /home/user/bin/simplemail.php "Print spooler was not running...  Restarted."
            exit 0
    fi
   

   Another alternative to prevent multiple execution can be achieved via flock

   i.e. * * * * * /usr/bin/flock -n /tmp/fcj.lockfile /usr/local/bin/frequent_cron_job

3. Logging Errors

   42 1 * * * /path/to/real/job || echo $? will write an error code to the syslog, if the first script fails

   /var/log/auth.log records when authentication occurs, and can therefore be used to determine when processes such as the cron script starts and stops.

   A script which doesn’t report a consistent error already, can be corrected

   • Add set -e -u to top of the script file, under the #! line
   • Then change the cron task script to sh -x main_script.sh || echo Cron task failed: $? and note -x causes each line to be printed out before it is executed
   • Then grep the syslog to find the task grep 'Cron' /var/log/syslog