Tom Ordonez

Data Science, Analytics, Growing Teams

Automating Tasks with Crontab


Automating tasks with crontab can save you hours of work. This is a short tutorial about cron jobs.

Automating Tasks with Crontab

Automating tasks

These are a few tasks that need automating:

  • Every day I have to launch Chrome and open the exact same 20 URLs.
  • Every other day I have to empty my Recycle bin.
  • Every Monday morning I have to send a tweet that says "Happy Monday".
  • Some days I have to open a file. Filter the contents and send the result to another file.
  • I also would like to tweet a random quote from a list of quotes every day at noon.
  • I have my left Ctrl key remapped to my Caps Lock key. When I reboot, it goes back to the default setting. I need to change this every time.

These are repetitive tasks that have a formula:

  • Start
  • Step 1
  • Step 2
  • Step 3
  • Finish

These tasks can be automated.

Automating tasks with Crontab

As seen here. Cron is a system daemon. Not a demon. A daemon is used to execute tasks in the background at specific times. Not in an evil way.

Cron is like Wall-E

Cron is like having a robot do your routine tasks.

Crontab is a text file

Cron is a daemon. A process that runs tasks. Which tasks?

Any task that you add to a text file call the crontab.

Each user has a crontab. You don't have to login as root to run this text file.

Edit the Crontab

In a terminal run this:

crontab -e

This will open the crontab. The file has a few comments to help you get started:

1 # Edit this file to introduce tasks to be run by cron.
2 # 
3 # Each task to run has to be defined through a single line
4 # indicating with different fields when the task will be run
5 # and what command to run for the task
6 # 
7 # To define the time you can provide concrete values for
8 # minute (m), hour (h), day of month (dom), month (mon),
9 # and day of week (dow) or use '*' in these fields (for 'any').# 
10 # Notice that tasks will be started based on the cron's system
11 # daemon's notion of time and timezones.
12 # 
13 # Output of the crontab jobs (including errors) is sent through
14 # email to the user the crontab file belongs to (unless redirected).
15 # 
16 # For example, you can run a backup of all your user accounts
17 # at 5 a.m every week with:
18 # 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
19 # 
20 # For more information see the manual pages of crontab(5) and cron(8)
21 # 
22 # m h  dom mon dow   command
23

Crontab syntax

Follow this syntax:

m h  dom mon dow   command

Where:

  • m = minute (0-59)
  • h = hour (0-23, 0=midnight)
  • dom = day of month (1-31)
  • mon = month (1-12)
  • dow = day of week (0-6, 0=Sunday)

You can use a * for any minute or any hour or any day of month or any month or any day of week such as:

* * * * *

Run at 7:45 am. any day of month, any month, any day of week:

45 7 * * *

Run at 7:45 am. The 1st day of month, any month, on Monday:

45 7 1 * 1
  • 45 = minutes
  • 7 = hour
  • 1 = day of month
  • * = any month
  • 1 = day of week (Monday)

It becomes more interesting...

Run at 7:45 am. Any day of the month, any month, on Monday, Wednesday and Friday:

45 7 * * 1,3,5

Run at 7:45 am. From the 1st to the 15 of November and December:

45 7 1-15 11,12 *

Run every 30 days at 8:30am:

30 8 */30 * *

Fun right?

Crontab command

I know what you are thinking...automate everything right?

Here are some guidelines:

  • Use full paths to files
  • Environment variables are not loaded
  • Crontab doesn't behave like bash
  • Leave a new (empty) line at the end of crontab
  • Follow troubleshooting guidelines below
  • Automate E.V.E.R.Y.T.H.I.N.G

Remap Ctl key to Capslock at reboot

@reboot setxkbmap -option caps:ctrl_modifier

OK so cron is not so greedy...got the joke?

...Cron has some special strings:

  • @reboot: Run once at startup
  • @daily: Run once a day aka 0 0 * * *
  • @hourly: Ronceour...I made up that word. Run once an hour aka 0 * *
  • @weekly: Run once a week aka 0 0 * * 0

You also have @yearly and @monthly

Launch Python Script with Selenium and Chromedriver

Follow this tutorial: crontab with selenium and Chrome Driver for all the geek details.

It runs a python script at 7:45am every day and send stdout and stderr to a log file.

45 7 * * * export DISPLAY=:0 && cd /home/your-username/Documents/scripts/ && /usr/bin/python awesome.py > awesome.log 2>&1

See Troubleshooting guide below to read more about "stdout and stderr to a log file".

Empty the Trash every day at 6pm

In Linux you can empty the trash using the command line.

sudo apt install trash-cli

To read the manual page use:

man trash-cli

To empty trash use:

trash-empty

In the crontab use:

0 18 * * * trash-empty

Every Monday morning I have to send a tweet that says "Happy Monday".

I use a Ruby gem to connect to the Twitter API from the command line.

15 8 * * 1 /usr/local/bin/t update "Happy Monday"

Troubleshooting Cron

I use 2 steps to troubleshoot the cron jobs:

  1. Review /var/log/syslog
  2. Redirect stdout and stderr

Review /var/log/syslog

Every time you edit your crontab you will see something like this in the syslog file:

Dec  5 16:32:43 your-computer crontab[6475]: (your-user) BEGIN EDIT (your-user)

Dec  5 16:33:59 your-computer crontab[6475]: (your-user) END EDIT (your-user)

For my trash-empty I will get something like:

Dec  5 18:00:01 my-computer CRON[28030]: (my-user) CMD (trash-empty)

Inside syslog you can at least see if the cron job is running.

Redirect stdout and stderr

It might happen that you schedule the cron job but it doesn't do anything. If you go to syslog you might see that the task ran but yet it didn't do anything.

Get more details of input/output redirection here.

You should know the basics:

One greater than sign >:

  • Redirect stdout to a file
  • If the file doesn't exist then create it. Otherwise overwrite current file

Two greater than signs >>

  • Redirect stdout to a file
  • If the file doesn't exist then create it. Otherwise append to current file

Redirect stdout to a file

1>file.txt

Redirect stderr to a file

2>file.txt

Redirect stdout and stderr to a file

&>file.txt

Redirect stderr to stdout and add to a file

2>&1

Every Monday send a Tweet v2

If I want to troubleshoot this cron job. I need to add redirection of stdout and stderr to a file. Then I can open the file to see if the command generated an error.

Every Monday morning send a tweet that says "Happy Monday".

15 8 * * 1 /usr/local/bin/t update "Happy Monday" >> /home/tom/Documents/crontom.log 2>&1
  • 15 = minutes
  • 8 = hour
  • * = any day of month
  • * = any month
  • 1 = day of week (Monday)

I could open the file crontom.log and see if it redirected a correct stdout or if it generated a stderr.

Backup your crontab

  • Your crontab is located at /var/spool/cron/crontabs/your-username
  • You can create a cron job to backup your crontab. Say that 3 times fast.
  • You cannot just add this job to your crontab because this directory requires root access.

You need to add this job to your root crontab:

sudo crontab -e

And then add a line such as backing up the file at 2:05am.

5 2 * * * cat /var/spool/cron/crontabs/your-username > /home/your-username/Documents/crontab_backup.txt 2>&1

If you open the file crontab_backup.txt.

You will see that a new line is added at the top that says:

# DO NOT EDIT THIS FILE - edit the master and reinstall.

If you look at the permissions of this file it will show:

-rw-r--r-- 1 root root

Which means that the file is owned by root and it's set to read only to your-username

How to kill a cron job

Find the PID with:

ps aux

In this example I scheduled a cron job for 7:45 am to run Python

tom      20979  0.0  0.0  43092 15168 ?        S    07:45   0:00 /usr/bin/python awesome.py

If you already knew the PID you could also do

ps -p PID -f

Which would give something like:

UID        PID  PPID  C STIME TTY          TIME CMD
tom      20979 20978  0 07:45 ?        00:00:00 /usr/bin/python awesome.py

Then you can kill the cron job with:

kill PID

In my case I would do: kill 20979

If you have questions or comments please add them below

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