cheatsheet on cron and backups

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1 Cron files:

Every user has a set of cron tables (files) that are stored in /var/spool/cron /var/log/cron # for CentOS

You do not edit them directly (but you can view them).

To edit them use the command crontab -e

To list them use crontab -l

To remove the table use =crontab -r=** Syntax. There are five numeric fields (or special keyword shortcuts like @yearly) followed by the command you want to run.

  • * * * * command to execute
  • minute (0 - 59)
  • hour (0 - 23)
  • day of month (1 - 31)
  • monthly (1 - 12)
  • day of week (0 - 6) Sunday = 0 or 7 
*    *    *   *    *    <command to be executed>
-    -    -   -    -   
|    |    |   |    |  
|    |    |   |    +------day of the week (0 - 6)  (Sunday=0
|    |    |   |
|    |    |   +-------month (1 - 12)
|    |    | 
|    |    +------ day of month ( 1 - 31 )
|    |   
|    +------hour (0 -23)
|
+----+ min (0 - 59 )

1.0.1 Each time field can be:

  • * for all possible times for that field
  • - a separated range (e.g. 2-5)
  • , a separated list (e.g. 1,3,5)

1.0.2 Special keyword shortcuts:

  • @reboot
  • @yearly
  • @monthly
  • @weekly
  • @daily
  • @hourly

To remotely sync backups every hour:

0 * * * *

2 My cron examples

2.1 Every night, send an email with a disk free report to an admin.

15 23 * * *   df -h --total | mail -s "Disk Free Report" admin@earth.continents.ops

2.2 Every Sunday at 11:30 pm

30 23 * * 0 my-start-of-the-week-backup-job

2.3 Every 15th of the month at 3 am

0  3 15 *  *   my-montly-maint-job

2.4 Every hour on the hour, on the quarter hour and on the half hour

cat /var/spool/cron root@c8host /var/spool/cron [196]$ cat root 

0 * * * * rsync -avz 192.168.111.11:/etc /backup/incremental/vm1/
15 * * * * rsync -avz 192.168.111.12:/etc /backup/incremental/vm2/
30 * * * * rsync -avz 192.168.111.13:/etc /backup/incremental/vm3/
root@c8host /var/spool/cron

2.4.1 Reminder of rsync syntax

  • rsync -vz from to
  • rsync -vz fromdirectory todirectory
  • rsync -vz source destination

if [destination] is omitted, rsync acts like ls -l on the source.

The source or destination may be local or remote, i.e. rsync works both ways. So source or destination can be:

- user@10.1.1.1:/home/zintis/folder

2.5 As a user, run date every 10 minutes (with output to a file)

0,10,20,30,40,50 * * * * date 1>> /home/zintis/cron-output.txt 2>> /home/zintis/cron-output.txt That is becuase 1 is stdout and 2 is stderr But I can combine that with & which is used for BOTH stdout and stderr so: 

0, 10, 20, 30, 40, 50 * * * * date &>> /home/zintis/cron-output.txt

You could also specify the minutes as:

  • */5 for every 5 minutes
  • */10 for every 10 minutes etc.

Or every 3 hours on the 15 minutes past the hour:

  • 15 */3 * * *

could also just add >> /home/myuser/myscript.log 2>&1 to the end of your crontab entry.

2.6 Another issue if running as a bash or as a script

I solved the problem. There are two ways:

M1

Change the redirection from &>> to 2>&1. So now crontab -e looks like 

*/1 * * * * /home/ranveer/vimbackup.sh >> /home/ranveer/vimbackup.log 2>&1

I believe the above works because by default cron is using sh to run the task instead of bash so &>> is not supported by sh.

M2

Change the default shell by adding SHELL=/bin/bash in the crontab -e file.

2.7 Also, consistency with /bin/sh

Make sure your shell redirection syntax conforms to /bin/sh. If you try to use shell redirection syntax that is not valid with /bin/sh then your command will fail and your cron job will not ever run.

In your /etc/cron.d/example1 config files if you specify a user other than root and that user's login shell is not /bin/bash… you must still use /bin/sh syntax in /etc/cron.d/example1 command.

For example If your user has shell csh or zsh or ksh set for his login shell. In your /etc/cron.d/example1 config file, the command must use /bin/sh syntax. Specifically any shell redirection must be /bin/sh syntax.

If you try to use for example csh shell redirect syntax in your /etc/cron.d/example1, then your cron job will never run. The logfile for crond located at /var/log/cron shall say that the command is run but the command will error out with a syntax error before your command gets run.

Good discussion of ubuntu options for cron are here

3 crontab Best Practices

Good link is available in sanctum.geek.nz

3.1 Home