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Python Crontab API

Project description

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Description

Crontab module for read and writing crontab files and accessing the system cron automatically and simply using a direct API.

Comparing the below chart you will note that W, L, # and ? symbols are not supported.

Field Name Mandatory Allowed Values Allowed Special Characters
Minutes Yes 0-59 * / , -
Hours Yes 0-23 * / , -
Day of month Yes 1-31 * / , -
Month Yes 1-12 or JAN-DEC * / , -
Day of week Yes 0-6 or SUN-SAT * / , -

Supported special cases allow crontab lines to not use fields. These are the supported aliases:

Case Meaning
@reboot Every boot
@hourly 0 * * * *
@daily 0 0 * * *
@weekly 0 0 * * 0
@monthly 0 0 1 * *
@yearly 0 0 1 1 *
@annually 0 0 1 1 *
@midnight 0 0 * * *

How to Use the Module

Getting access to a crontab can happen in four ways, two system methods:

from crontab import CronTab

system_cron = CronTab()
user_cron = CronTab('root')

And two ways from non-system sources:

file_cron = CronTab(tabfile='filename.tab')
mem_cron = CronTab(tab="""
  * * * * * command
""")

Creating a new job is as simple as:

job  = cron.new(command='/usr/bin/echo')

And setting the job’s time restrictions:

job.minute.during(5,50).every(5)
job.hour.every(4)
job.day.on(4, 5, 6)

job.dow.on('SUN')
job.month.during('APR', 'NOV')

Each time restriction will clear the previous restriction:

job.hour.every(10) # Set to * */10 * * *
job.hour.on(2)     # Set to * 2 * * *

Appending restrictions is explicit:

job.hour.every(10)  # Set to * */10 * * *
job.hour.also.on(2) # Set to * 2,*/10 * * *

Creating a job with a comment:

job = cron.new(command='/foo/bar',comment='SomeID')

Disabled or Enable Job:

job.enable()
job.enable(False)
False == job.is_enabled()

Validity Check:

True == job.is_valid()

Use a special syntax:

job.every_reboot()

Find an existing job by command:

list = cron.find_command('bar')

Find an existing job by comment:

list = cron.find_comment('ID or some text')

Set and get the comment for a job:

comment = job.meta(['New Comment for job'])

Clean a job of all rules:

job.clear()

Iterate through all jobs:

for job in cron:
    print job

Iterate through all lines:

for line in cron.lines:
    print line

Remove Items:

cron.remove( job )
cron.remove_all('echo')

Write CronTab back to system or filename:

cron.write()

Write CronTab to new filename:

cron.write( 'output.tab' )

Proceeding Unit Confusion

It is sometimes logical to think that job.hour.every(2) will set all proceeding units to ‘0’ and thus result in “0 */2 * * *”. Instead you are controlling only the hours units and the minute column is unaffected. The real result would be “* */2 * * *” and maybe unexpected to those unfamiliar with crontabs.

There is a special ‘every’ method on a job to clear the job’s existing schedule and replace it with a simple single unit:

job.every(4).hours()  == '0 */4 * * *'
job.every().dom()     == '0 0 * * *'
job.every(2).months() == '0 0 0 * *'
job.every(2).dows()   == '0 0 * * */2'

This is a convenience method only, it does normal things with the existing api.

Log Functionality

The log functionality will read a cron log backwards to find you the last run instances of your crontab and cron jobs.

The crontab will limit the returned entries to the user the crontab is for:

cron = CronTab(user='root')

for d in cron.log:
    print d['pid'] + " - " + d['date']

Each job can return a log iterator too, these are filtered so you can see when the last execution was:

for d in cron.find_command('echo')[0].log:
    print d['pid'] + " - " + d['date']

Schedule Functionality

If you have the croniter python module installed, you will have access to a schedule on each job. For example if you want to know when a job will next run:

schedule = job.schedule(date_from=datetime.now())

This creates a schedule croniter based on the job from the time specified. The default date_from is the current date/time if not specified. Next we can get the datetime of the next job:

datetime = schedule.get_next()

Or the previous:

datetime = schedule.get_prev()

The get methods work in the same way as the default croniter, except that they will return datetime objects by default instead of floats. If you want the original functionality, pass float into the method when calling:

datetime = schedule.get_current(float)

If you don’t have the croniter module installed, you’ll get an ImportError when you first try using the schedule function on your cron job object.

Extra Support

  • Support for SunOS, AIX & HP with compatibility ‘SystemV’ mode.
  • Python 3.2 and Python 2.7/2.6 tested.
  • Windows support works for non-system crontabs only. ( see mem_cron and file_cron examples above for usage )

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