Scheduling conditional statements with crontab

 

Recently while trying to auto-restart a daemon which is down by identifying with ps aux and if condition, it is not worked as intended with crontab.

I used following command which is perfectly running in from command line but not through crontab.

if [ `ps aux | grep nrpe | grep -v grep | wc -l` -eq 0 ]; then service nagios-nrpe-server restart ;fi

After trying with different commands following thing worked for me:

pgrep nrpe; [ $? != 0 ] && /etc/init.d/nagios-nrpe-server restart

where pgrep returns non zero exit code if the process nrep is not running and with $? is used to get the exit code of previous command (in this case pgrep) and start the process.

Crontab Hack – run every 5th, 15th, 25th … mins

To set crontab to run on every 5th, 15th, 25th, 35th, 45th, and 55th use following command:

5-55/10 * * * * command

This will execute your command on every 10th minute between 5 mins to 55 mins.

Crontab every 30 seconds

 

To run a cron every 30 seconds check following example:

* * * * * /path/to/your/script arg1 arg2
* * * * * (sleep 30; /path/to/your/script arg1 arg2)

In above example both commands will start at 0th second of every minute, but the second command will sleep for 30 seconds and then start.

Linux view all users crontab

 

In general we will use crontab – e or crontab -l to view crontab.

Both commands crontab -e and crontab -l will show only current users(where we logged in) crontab.

To view other user crontabs use following command:

$ crontab -u <userName> -l

To list guest user crontab use following command:

$ crontab -u guest -l

Before running above command you need to login into root.

In Ubuntu based operating systems we need to use sudo with above command.

$ sudo crontab -u guest -l

To view all users crontab user following command:

$ for i in $(grep -P ":[0-9]{4}:[0-9]{4}" /etc/passwd | cut -d: -f1)
do 
    crontab -u $i -l; 
done 

Default crontab file location

 

Crontab configuration files of all users are available in path “/var/spool/cron/crontabs/”

If we list directory “/var/spool/cron/crontabs”, we can see root file and some files with usernames.

There will a crontab file for each user.

root file is for root user, user1 file is for user1, user2 file is for user2 and etc….

Depending on our requirement we can update these files.

To list available command in crontab use following command:

$ crontab -l

If we run crontab -l command from root user we can list commands configured in root users crontab and when they are going to execute, similarly we can list commands configured in other users crontab by logging into corresponding account.

To edit crontab use following command:

$ crontab -e

If you want to edit root user crontab login into root user and run crontab -e from root user.

Ubuntu change crontab editor permanently

 

By mistake if an uncomfortable editor is set for crontab, we can change it with following command:

$ select-editor

The output of above command will asks you to select any editor from list as shown below:

Output:

Select an editor. To change later, run ‘select-editor’.
1. /bin/ed
2. /bin/nano <—- easiest
3. /usr/bin/vim.basic
4. /usr/bin/vim.tiny

Choose 1-4 [2]: 4

Since I am comfortable with vim chosen option 4, which will set my crontab editor as vim.tiny.

After choosing an option just press enter and open crontab, it should opened with the editor you have selected. In my case it opened with vim.tiny.

There is one more post in my blog which will set the crontab editor only for that session.

Here is the link to change crontab editor for current session only.

-Sany