Crontab in GNU/Linux

0. Intro

Crontab is a Unix-like system utility to help us to execute jobs periodical and automatically. We present quick configuration for executing periodic task with an Ubuntu based Linux distro.

1. Set up

# Set up
sudo apt-get update
sudo apt-get install -y cron nano

Optional:

# Crontab uses variable EDITOR to define a shell text editor
# and since Vim is ugly as hell (╯°□°）╯︵ ┻━┻, so we use nano
export EDITOR="nano"

Note: You can make it permanent adding the last command to .bashrc file of your profile and then use command "source .bashrc".

2. Basic use and sintaxis

2.1 Basics

Crontab works by editing some text file from our machine (typically /etc/crontab, but can also uses /etc/cron.d). Basically, we have to create a bash script for Crontab using command:

crontab -e

This will open our text editor in terminal and let us introduce command by following the next structure that specifies at what moment the job will be executed:

.---------------- 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

Note:

• After saving changes in you text editor and cron will begin with the automatic executions.
• Crontab it depends on internal clock of your Unix-like machine to execute the jobs periodically.

More fun with Crontab

• Asterisk (*) in a field signifies the entire range for that field (e.g. 0-59 for the minute field).
• A comma (,) can be used to specify a list e.g 1,4,6,8 (run job at minutes 1,4,6,8)
• Ranges are specified with a dash (-), for example run a job between 1 and 3.
• Ranges can be combined with lists e.g. 1-3,9-12 which means run a job in minutes between 1 and 3 then between 9 and 12.
• The / character can be used to introduce a step e.g. 2/5 which means starting at 2 then every 5 (2,7,12,17,22...). They do not wrap past the end.
• Ranges and steps can be combined e.g. */2 signifies starting at the minimum for the relevant field then every 2 e.g. 0 for minutes( 0,2...58), 1 for months (1,3 ... 11) etc.

2.1.1 Examples

Run each minute to write a bash command output to a text

* * * * * echo "I was ran in this date: $(date)" >> /my_path/example.txt

Run each 10 minutes

*/10 * * * * echo "I was ran in this date: $(date)" >> /my_path/example.txt

Run all days at 4:30 pm

30 4 * * * echo "I was ran in this date: $(date)" >> /my_path/example.txt

Run all days at 4:30 pm but in business days (not weekends)

30 4 * * 1-5 echo "I was ran in this date: $(date)" >> /my_path/example.txt

2.2 More features

Access crontab configuration of and specific user

crontab -u my_username -e

List all current crontab jobs

crontab -l

Eliminate current script of crontab jobs (be careful)

contrab -r

2.3 Paths and execution on bash terminal

Please note that in Unix systems we can locate a program file in the user's path using which command. For example, we can find related to Python and Rscript (a bash utility to run R commands and scripts):

which python
# /Users/my_user/.pyenv/shims/python

which Rscript
# /Users/my_user/opt/anaconda3/bin/Rscript

With this information, we can execute Python adn R file from terminal in two ways:

Route 1: specifing paths on terminal

You can help to execute a Python/R script by specifying the path where they are located,

/my_path_to_python hello_world.py # python

/my_path_to_Rscript hello_world.R # R files, using Rscript

Example:

/Users/my_user/.pyenv/shims/python hello_world.py

Route 2: shebang notation #!

We can add a header to out script to tell bash where to find the program that will execute a script, as follows:

#!/my_path_to_Rscript
...
# Some cool code
# More of that cool code

After that, we have to gave the script permision of execution:

chmod +x my_script.R

And run the script:

./my_script.R

2.4 Python/R scripts and Crontab

To execute a Crontab job for Python o R, we have to combine the previous knowledge:

Example 1

my_example.py

#!/my_path_to_python

print("Hello Brenda")

Open crontab job editor

crontab -e

Specify the right sintaxis of crontab

*/10 * * * * /my_path_to_python my_example.py # Execute the script each 10 minutes

Example 2

my_example.R

#!/my_path_to_Rscript
HelloBrenda <- function(){
   print('hello')
}

HelloBrenda()

chmod +x my_example.R

Open crontab job editor

crontab -e

Specify the right sintaxis of crontab

*/10 * * * * ./my_example.R # Execute the script each 10 minutes

3. References

• https://help.ubuntu.com/community/CronHowto

• Consulting crontab manual:

  man crontab