Crontab - Quick reference

Crontab - Quick reference

Setting up cronjobs in Unix and Solaris

cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.
Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times.

Following points sum up the crontab functionality :

1. Crontab Restrictions
2. Crontab Commands
3. Crontab file - syntax
4. Crontab Example
5. Crontab Environment
6. Disable Email
7. Generate log file for crontab activity

8. Next Steps

1. Crontab Restrictions
____________
You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file /usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.

2. Crontab Commands
__________
export EDITOR=vi ;to specify a editor to open crontab file.

crontab -e Edit your crontab file, or create one if it doesn't already exist.
crontab -l Display your crontab file.
crontab -r Remove your crontab file.
crontab -v Display the last time you edited your crontab file. (This option is only available on a few systems.)

3. Crontab file
___________
Crontab syntax :-
A crontab file has five fields for specifying day , date and time followed by the command to be run at that interval.

* * * * * command to be executed - - - - - | | | | | | | | | +----- day of week (0 - 6) (Sunday=0) | | | +------- month (1 - 12) | | +--------- day of month (1 - 31) | +----------- hour (0 - 23) +------------- min (0 - 59)

* in the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

Note: The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed .

4. Crontab Example
_______

A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30 18 * * * rm /home/someuser/tmp/*

Changing the parameter values as below will cause this command to run at different time schedule below :

min	hour	day/month	month	day/week	Execution time
30	0	1	1,6,12	*	-- 00:30 Hrs on 1st of Jan, June & Dec.
:
0	20	*	10	1-5	--8.00 PM every weekday (Mon-Fri) only in Oct.
:
0	0	1,10,15	*	*	-- midnight on 1st ,10th & 15th of month
:
5,10	0	10	*	1	-- At 12.05,12.10 every Monday & on 10th of every month
:

Note : If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.

5. Crontab Environment
___________
cron invokes the command from the user's HOME directory with the shell, (/usr/bin/sh).
cron supplies a default environment for every shell, defining:
HOME=user's-home-directory
LOGNAME=user's-login-id
PATH=/usr/bin:/usr/sbin:.
SHELL=/usr/bin/sh

Users who desire to have their .profile executed must explicitly do so in the crontab entry or in a script called by the entry.

6. Disable Email
____________

By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command At the end of the cron job line .

>/dev/null 2>&1

7. Generate log file
________________

To collect the cron execution execution log in a file :

30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log

8. Next Steps

This article covered a significant aspect of system administration of setting up cronjobs . Unix administration involves lots of different tasks and some of these tasks are covered in this website but still there are many areas not covered here .

Backup your MySQL databases automatically with AutoMySQLBackup

If you site relies on MySQL and stores its sensitive data in a MySQL database, you will most definitely want to backup that information so that it can be restored in case of any disaster (manual mistake to delete some data, software errors, hardware errors, server compromise, etc.).

With mysqldump anyone can write a small shell script and running it from cron, it will achieve an automatic backup solution. There are many such scripts already available freely and also many commercial solutions also (I assume as I have not tested any really ;)). The script that I liked the most is AutoMySQLBackup, because it doesn’t have any real requirements (mysqldump of course is needed - in any mysql client package - and gzip or bzip2 to compress the resulting file) and has all the features I was looking for in such a script.

AutoMySQLBackup has all the features I needed: it can backup a single database, multiple databases, or all the databases on the server; each database is saved in a separate file that can be compressed (with gzip or bzip2); it will rotate the backups and not keep them filling your hard drive (as normal in the daily backup you will have only the last 7 days of backups, the weekly if enabled will have one for each week, etc.). It has also some other features (check the project homepage for full details), that I am not using myself (like email logs for example), but other peoples might find interesting.

The installation is very simple: just download the one file bash script and save it somewhere, customize it to fit your setup (only some basic changes are needed: like the MySQL user and password, backup location), make it executable and activate it in cron as needed (daily for example).

Here are the variables that I usually setup:

# Username to access the MySQL server e.g. dbuser
USERNAME=dbuser
# Username to access the MySQL server e.g. password
PASSWORD=password
# Host name (or IP address) of MySQL server e.g localhost
DBHOST=localhost
# List of DBNAMES for Daily/Weekly Backup e.g. "DB1 DB2 DB3"
DBNAMES="all"
# Backup directory location e.g /backups
BACKUPDIR="/var/backup/mysql"
# Mail setup
MAILCONTENT="quiet"

You can also run the script manually:

./automysqlbackup.sh.2.5

but probably you will want to enable it in cron and run daily. You can just dump it in the /etc/cron.daily/ folder and you should be done. :).
Myself I like to place it in /opt and create a symlink to the file (to help with future updates of the script):

ln -s automysqlbackup.sh.2.5 automysqlbackup.sh

and run it from cron by placing in /etc/crontab something like:

#MySQL Daily backup
45 5   * * *   root    /opt/automysqlbackup.sh >/dev/null 2>&1

If needed, after the script is finished, you can save the backup folder where you placed the databases to a remote location (ftp, nfs, smb, or whatever) or another local medium (like tape for ex.). Saving to remote locations is not supported in the script itself so you will have to either mount the remote space locally (if it is possible… for nfs, smb, for ex.) and save directly onto it, or you will have to use another method to upload the files remotely (maybe you just have to include the backup folder used above in your regular backup script).

Are you using a different scrip to backup your databases? I would like to hear of it… Why did you like it? What features were you looking for? Or you had to write your own script because you have not found your needed features in any existing script available freely? Let me know…