In this tutorial, you'll learn the importance of cron jobs and why you need them. You'll have a look at
python-crontab, a Python module to interact with the
crontab. You'll learn how to manipulate cron jobs from a Python program using the
What Is Cron?
During system administration, it's necessary to run background jobs on a server to execute routine tasks. Cron is a system process which is used to execute background tasks on a routine basis. Cron requires a file called
crontab which contains the list of tasks to be executed at a particular time. All these jobs are executed in the background at the specified time.
To view cron jobs running on your system, navigate to your terminal and type in:
The above command displays the list of jobs in the
crontab file. To add a new cron job to the
crontab, type in:
The above command will display the
crontab file where you can schedule a job. Let's say you have a file called
hello.py which looks like:
print "Hello World"
Now, to schedule a cron job to execute the above script to output to another file, you need to add the following line of code:
50 19 * * * python hello.py >> a.txt
The above line of code schedules the execution of the file with output to a file called
a.txt. The numbers before the command to execute define the time of execution of the job. The timing syntax has five parts:
- day of month
- day of week
Asterisks(*) in the timing syntax indicate it will run every time.
python-crontab is a Python module which provides access to cron jobs and enables us to manipulate the
crontab file from the Python program. It automates the process of modifying the
crontab file manually. To get started with
python-crontab, you need to install the module using pip:
pip install python-crontab
Once you have
python-crontab installed, import it into the python program.
from crontab import CronTab
Writing Your First Cron Job
Let's use the
python-crontab module to write our first cron job. Create a Python program called
writeDate.py, add the code to print the current date and time to a file. Here is how
writeDate.py would look:
import datetime with open('dateInfo.txt','a') as outFile: outFile.write('\n' + str(datetime.datetime.now()))
Save the above changes.
Let's create another Python program which will schedule the
writeDate.py Python program to run at every minute. Create a file called
CronTab module into the
from crontab import CronTab
CronTab module, let's access the system
my_cron = CronTab(user='your username')
The above command creates an access to the system
crontab of the user. Let's iterate through the cron jobs and you should be able to see any manually created cron jobs for the particular username.
for job in my_cron: print job
Save the changes and try executing the
scheduleCron.py and you should have the list of cron jobs, if any, for the particular user. You should be able to see something similar on execution of the above program:
50 19 * * * python hello.py >> a.txt # at 5 a.m every week with:
Let's move on with creating a new cron job using the
CronTab module. You can create a new cron by using the new method and specifying the command to be executed.
job = my_cron.new(command='python /home/jay/writeDate.py')
As you can see in the above line of code, I have specified the command to be executed when the cron job is executed. Once you have the new cron job, you need to schedule the cron job.
Let's schedule the cron job to run every minute. So, in an interval of one minute, the current date and time would be appended to the
dateInfo.txt file. To schedule the job for every minute, add the following line of code:
Once you have scheduled the job, you need to write the job to the cron tab.
Here is the
from crontab import CronTab my_cron = CronTab(user='roy') job = my_cron.new(command='python /home/roy/writeDate.py') job.minute.every(1) my_cron.write()
Save the above changes and execute the Python program.
Once it gets executed, check the
crontab file using the following command:
The above command should display the newly added cron job.
* * * * * python /home/roy/writeDate.py
Wait for a minute and check your home directory and you should be able to see the
dateInfo.txt file with the current date and time. This file will get updated each minute, and the current date and time will get appended to the existing content.
Updating an Existing Cron Job
To update an existing cron job, you need to find the cron job using the command or by using an Id. You can have an Id set to a cron job in the form of a comment when creating a cron job using
python-crontab. Here is how you can create a cron job with a comment:
job = my_cron.new(command='python /home/roy/writeDate.py', comment='dateinfo')
As seen in the above line of code, a new cron job has been created using the comment as
dateinfo. The above comment can be used to find the cron job.
What you need to do is iterate through all the jobs in
crontab and check for the job with the comment
dateinfo. Here is the code:
my_cron = CronTab(user='roy') for job in my_cron: print job
Check for each job's comment using the
my_cron = CronTab(user='jay') for job in my_cron: if job.comment == 'dateinfo': print job
Once you have the job, reschedule the cron job and write to the cron. Here is the complete code:
from crontab import CronTab my_cron = CronTab(user='roy') for job in my_cron: if job.comment == 'dateinfo': job.hour.every(10) my_cron.write() print 'Cron job modified successfully'
Save the above changes and execute the
scheduleCron.py file. List the items in the
crontab file using the following command:
You should be able to see the cron job with updated schedule time.
* */10 * * * python /home/jay/writeDate.py # dateinfo
Clearing Jobs From Crontab
python-crontab provides methods to clear or remove jobs from
crontab. You can remove a cron job from the
crontab based on the schedule, comment, or command.
Let's say you want to clear the job with comment
dateinfo from the
crontab. The code would be:
from crontab import CronTab my_cron = CronTab(user='roy') for job in my_cron if job.comment == 'dateinfo': my_cron.remove(job) my_cron.write()
Similarly, to remove a job based on a comment, you can directly call the
remove method on the
my_cron without any iteration. Here is the code:
To remove all the jobs from the
crontab, you can call the
Once done with the changes, write it back to the cron using the following command:
Calculating Job Frequency
To check how many times your job gets executed using
python-crontab, you can use the
frequency method. Once you have the job, you can call the method called
frequency, which will return the number of times the job gets executed in a year.
from crontab import CronTab my_cron = CronTab(user='roy') for job in my_cron: print job.frequency()
To check the number of times the job gets executed in an hour, you can use the method
my_cron = CronTab(user='roy') for job in my_cron: print job.frequency_per_hour()
To check the job frequency in a day, you can use the method
Checking the Job Schedule
python-crontab provides the functionality to check the schedule of a particular job. For this to work, you'll need the
croniter module to be installed on your system. Install
croniter using pip:
pip install croniter
Once you have
croniter installed, call the schedule method on the job to get the job schedule.
sch = job.schedule(date_from=datetime.datetime.now())
Now you can get the next job schedule by using the
Here is the complete code:
import datetime from crontab import CronTab my_crons = CronTab(user='jay') for job in my_crons: sch = job.schedule(date_from=datetime.datetime.now()) print sch.get_next()
You can even get the previous schedule by using the
Wrapping It Up
In this tutorial, you saw how to get started with using
python-crontab for accessing system
crontab from a Python program. Using
python-crontab, you can automate the manual process of creating, updating, and scheduling cron jobs.
Have you used
python-crontab or any other libraries for accessing system
crontab? I would love to hear your thoughts. Do let us know your suggestions in the comments below.
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