Cron

Cron can be useful.

Overview/Concepts

• cron is a daemon that executes programs at predetermined times or intervals
• it's controlled by a crontab file where each line specifies times and commands
• each person has their own crontab file (in addition to system crontabs)
• The command typically runs in a different context: you can't be sure what directory it is in, whether you have activated a virtual environment (you haven't) and so on: the command needs to be stand-alone
• any output to stdout is emailed to you

Crontab Syntax

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  *  command to be executed

Here's an example (/etc/crontab):

07 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 1 * * fri root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

# ================================================================
# backup strategy is all on old-tempest

# these files will always have new contents and so should be backed up afresh each day
1 0 * * * root date >| /home/sysadmin/today; date >| /students/gdome/today

# end of the day, check the backups
30 23 * * * root /root/bin/check-backup-logs

# on the weekend, check for disk hogs, during the cp-al
0 1 * * 6 root /root/bin/disk-hogs

# put this on new because of the python script
# this will run while the cp -al is happening
30 1 * * * root /home/sysadmin/bin/find_quiet_homedirs

Editing a Crontab File

The crontab files are stored in a special system directory (typically /var/spool/cron). The following command to allow you to edit your crontab in your favorite editor (the value of the EDITOR environment variable):

$ printenv
$ echo $EDITOR
$ EDITOR=emacs
$ crontab -e

To see your crontab, do

$ crontab -l

Example: sending email

We can schedule an email message to get sent every day at a particular time. To do so, we need:

1. A stand-alone python script

Stand-Alone Script

The following bash script sets up your Python program:

#!/bin/bash

source /home/cs304/public_html/downloads/venv/bin/activate
cd /home/cs304/public_html/downloads/cron/
python plainmail2.py
echo "email sent"

To make this executable, do:

$ chmod a+rx plainmail2.sh

Stand-Alone Email Script

The following sends the emails:

# Import smtplib for the actual sending function
import smtplib

# Import the email modules we'll need
from email.mime.text import MIMEText

def email(body,
          to='scott.anderson@wellesley.edu',
          from_addr='cs304@wellesley.edu',
          subject='Subject'):
    msg = MIMEText(body)
    msg['Subject']=subject
    msg['From']=from_addr
    msg['To']=to
    
    # Send the message via our own SMTP server.
    s = smtplib.SMTP('localhost')
    s.send_message(msg)
    s.quit()

if __name__ == '__main__':
    import pymysql
    import dbi
    dsn = dbi.read_cnf('/home/cs304/.my.cnf')
    conn = dbi.connect(dsn)
    dbi.select_db(conn,'webdb')
    curs = dbi.dictCursor(conn)
    curs.execute('select addr from emails')
    for row in curs.fetchall():
        print(row['addr'])
        email('Just a reminder ...',
              to=row['addr'],
              subject='confirmation')



        

Activity

Copy the cron folder and then do this activity.

$ cd ~/cs304
$ cp -r ~cs304/pub/downloads/cron cron
$ cd cron