An experienced Linux sysadmin knows
the importance of running the routine maintenance jobs in the background
automatically.
Linux Cron utility is an effective way to schedule a routine background job at a specific time and/or day on an on-going basis.
Linux Crontab Format
Linux Cron utility is an effective way to schedule a routine background job at a specific time and/or day on an on-going basis.
Linux Crontab Format
MIN
HOUR DOM MON DOW CMD
Table:
Crontab Fields and Allowed Ranges (Linux Crontab Syntax)
|
||
Field
|
Description
|
Allowed
Value
|
MIN
|
Minute field
|
0 to 59
|
HOUR
|
Hour field
|
0 to 23
|
DOM
|
Day of Month
|
1-31
|
MON
|
Month field
|
1-12
|
DOW
|
Day Of Week
|
0-6
|
CMD
|
Command
|
Any command to be executed.
|
1. Scheduling a Job For a Specific
Time
The basic usage of cron is to
execute a job in a specific time as shown below. This will execute the Full
backup shell script (full-backup) on 10th June 08:30 AM.
Please note that the time field uses 24 hours format. So, for 8 AM use 8, and for 8 PM use 20.
Please note that the time field uses 24 hours format. So, for 8 AM use 8, and for 8 PM use 20.
30
08 10 06 * /home/ramesh/full-backup
- 30 – 30th Minute
- 08 – 08 AM
- 10 – 10th Day
- 06 – 6th Month (June)
- * – Every day of the week
2. Schedule a Job For More Than One
Instance (e.g. Twice a Day)
The following script take a
incremental backup twice a day every day.
This example executes the specified incremental backup shell script (incremental-backup) at 11:00 and 16:00 on every day. The comma separated value in a field specifies that the command needs to be executed in all the mentioned time.
This example executes the specified incremental backup shell script (incremental-backup) at 11:00 and 16:00 on every day. The comma separated value in a field specifies that the command needs to be executed in all the mentioned time.
00
11,16 * * * /home/ramesh/bin/incremental-backup
- 00 – 0th Minute (Top of the hour)
- 11,16 – 11 AM and 4 PM
- * – Every day
- * – Every month
- * – Every day of the week
3. Schedule a Job for Specific Range
of Time (e.g. Only on Weekdays)
If you wanted a job to be scheduled
for every hour with in a specific range of time then use the following.
Cron Job everyday during working
hours
This example checks the status of
the database everyday (including weekends) during the working hours 9 a.m – 6
p.m
00
09-18 * * * /home/ramesh/bin/check-db-status
- 00 – 0th Minute (Top of the hour)
- 09-18 – 9 am, 10 am,11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
- * – Every day
- * – Every month
- * – Every day of the week
Cron Job every weekday during
working hours
This example checks the status of
the database every weekday (i.e excluding Sat and Sun) during the working hours
9 a.m – 6 p.m.
00
09-18 * * 1-5 /home/ramesh/bin/check-db-status
- 00 – 0th Minute (Top of the hour)
- 09-18 – 9 am, 10 am,11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
- * – Every day
- * – Every month
- 1-5 -Mon, Tue, Wed, Thu and Fri (Every Weekday)
4. How to View Crontab Entries?
View Current Logged-In User’s
Crontab entries
To view your crontab entries type
crontab -l from your unix account as shown below.
ramesh@dev-db$ crontab -l
@yearly
/home/ramesh/annual-maintenance
*/10 * * * *
/home/ramesh/check-disk-space
[Note: This displays crontab of the
current logged in user]
View Root Crontab entries
Login as root user (su – root) and
do crontab -l as shown below.
root@dev-db# crontab -l
no crontab for root
Crontab HowTo: View Other Linux
User’s Crontabs entries
To view crontab entries of other
Linux users, login to root and use -u {username} -l as shown below.
root@dev-db# crontab -u sathiya -l
@monthly
/home/sathiya/monthly-backup
00 09-18 * * *
/home/sathiya/check-db-status
5. How to Edit Crontab Entries?
Edit Current Logged-In User’s
Crontab entries
To edit a crontab entries, use
crontab -e as shown below. By default this will edit the current logged-in
users crontab.
ramesh@dev-db$ crontab -e
@yearly
/home/ramesh/centos/bin/annual-maintenance
*/10 * * * *
/home/ramesh/debian/bin/check-disk-space
~
"/tmp/crontab.XXXXyjWkHw"
2L, 83C
[Note: This will open the crontab
file in Vim editor for editing.
Please note cron created a temporary
/tmp/crontab.XX... ]
When you save the above temporary
file with :wq, it will save the crontab and display the following message
indicating the crontab is successfully modified.
~
"crontab.XXXXyjWkHw" 2L,
83C written
crontab: installing new crontab
Edit Root Crontab entries
Login as root user (su – root) and
do crontab -e as shown below.
root@dev-db# crontab -e
Edit Other Linux User’s Crontab File
entries
To edit crontab entries of other
Linux users, login to root and use -u {username} -e as shown below.
root@dev-db# crontab -u sathiya -e
@monthly
/home/sathiya/fedora/bin/monthly-backup
00 09-18 * * *
/home/sathiya/ubuntu/bin/check-db-status
~
~
~
"/tmp/crontab.XXXXyjWkHw"
2L, 83C
6. Schedule a Job for Every Minute
Using Cron.
Ideally you may not have a
requirement to schedule a job every minute. But understanding this example will
will help you understand the other examples mentioned below in this article.
* * * * * CMD
The * means all the possible unit —
i.e every minute of every hour through out the year. More than using this *
directly, you will find it very useful in the following cases.
• When
you specify */5 in minute field means every 5 minutes.
• When
you specify 0-10/2 in minute field mean every 2 minutes in the first 10 minute.
• Thus
the above convention can be used for all the other 4 fields.
7. Schedule a Background Cron Job
For Every 10 Minutes.
Use the following, if you want to
check the disk space every 10 minutes.
*/10 * * * *
/home/ramesh/check-disk-space
It executes the specified command
check-disk-space every 10 minutes through out the year. But you may have a
requirement of executing the command only during office hours or vice versa.
The above examples shows how to do those things.
Instead of specifying values in the
5 fields, we can specify it using a single keyword as mentioned below.
There are special cases in which
instead of the above 5 fields you can use @ followed by a keyword — such as
reboot, midnight, yearly, hourly.
Table: Cron special keywords and its
meaning
Keyword Equivalent
@yearly 0 0 1 1 *
@daily 0 0 * * *
@hourly 0 * * * *
@reboot Run at startup.
8. Schedule a Job For First Minute
of Every Year using @yearly
If you want a job to be executed on the
first minute of every year, then you can use the @yearly cron keyword as shown
below.
This will execute the system annual
maintenance using annual-maintenance shell script at 00:00 on Jan 1st for every
year.
@yearly
/home/ramesh/red-hat/bin/annual-maintenance
9. Schedule a Cron Job Beginning of
Every Month using @monthly
It is as similar as the @yearly as
above. But executes the command monthly once using @monthly cron keyword.
This will execute the shell script
tape-backup at 00:00 on 1st of every month.
@monthly
/home/ramesh/suse/bin/tape-backup
10. Schedule a Background Job Every
Day using @daily
Using the @daily cron keyword, this
will do a daily log file cleanup using cleanup-logs shell scriptat 00:00 on every
day.
@daily
/home/ramesh/arch-linux/bin/cleanup-logs "day started"
11. How to Execute a Linux Command
After Every Reboot using @reboot?
Using the @reboot cron keyword, this
will execute the specified command once after the machine got booted every time.
@reboot CMD
12. How to Disable/Redirect the
Crontab Mail Output using MAIL keyword?
By default crontab sends the job
output to the user who scheduled the job. If you want to redirect the output to
a specific user, add or update the MAIL variable in the crontab as shown below.
ramesh@dev-db$ crontab -l
MAIL="ramesh"
@yearly
/home/ramesh/annual-maintenance
*/10 * * * *
/home/ramesh/check-disk-space
[Note: Crontab of the current logged
in user with MAIL variable]
If you wanted the mail not to be
sent to anywhere, i.e to stop the crontab output to be emailed, add or update
the MAIL variable in the crontab as shown below.
MAIL=""
13. How to Execute a Linux Cron Jobs
Every Second Using Crontab.
You cannot schedule a every-second
cronjob. Because in cron the minimum unit you can specify is minute. In a
typical scenario, there is no reason for most of us to run any job every second
in the system.
14. Specify PATH Variable in the
Crontab
All the above examples we specified
absolute path of the Linux command or the shell-script that needs to be
executed.
For example, instead of specifying
/home/ramesh/tape-backup, if you want to just specify tape-backup, then add the
path /home/ramesh to the PATH variable in the crontab as shown below.
ramesh@dev-db$ crontab -l
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/home/ramesh
@yearly annual-maintenance
*/10 * * * * check-disk-space
[Note: Crontab of the current logged
in user with PATH variable]
15. Installing Crontab From a Cron
File
Instead of directly editing the
crontab file, you can also add all the entries to a cron-file first. Once you
have all thoese entries in the file, you can upload or install them to the cron
as shown below.
ramesh@dev-db$ crontab -l
no crontab for ramesh
$ cat cron-file.txt
@yearly /home/ramesh/annual-maintenance
*/10 * * * *
/home/ramesh/check-disk-space
ramesh@dev-db$ crontab cron-file.txt
ramesh@dev-db$ crontab -l
@yearly
/home/ramesh/annual-maintenance
*/10 * * * *
/home/ramesh/check-disk-space
Note: This will install the
cron-file.txt to your crontab, which will also remove your old cron entries.
So, please be careful while uploading cron entries from a cron-file.txt.
