freeCodeCamp/guide/english/linux/some-usages-for-linux/automating-tasks/index.md

title
Automating Tasks

Automating Tasks or Task Scheduling

There is a feature of Linux that is exclusively leveraged in a web server,known as CRON As we know that one of the most compelling features about any web server is that it's always powered on and always connected to the internet. This means that we can instruct our server to perform tasks automatically, without us needing to enter a command into a shell. One of the most commonly used programs for executing other programs with a regular frequency is called cron. Let's take a look at how to use cron to schedule a program to be run.

The cron program is part of a family of programs called daemons. A daemon is a program that is always running in the background of our computer. First, let's see if cron is running. We can get a list of all running programs with the ps command while using the -A flag:

ps -A

##  PID TTY          TIME CMD
##    1 ?        00:00:13 systemd
##    2 ?        00:00:00 kthreadd
##    3 ?        00:00:03 ksoftirqd/0
##    5 ?        00:00:00 kworker/0:0H
##    7 ?        00:00:11 rcu_sched
##    8 ?        00:00:00 rcu_bh
##    9 ?        00:00:00 migration/0
## ...

You probably have a huge list of programs in your terminal now! Instead of shifting through this listing line-by-line, let's pipe the output of this command to grep and we'll look for cron:

ps -A | grep "cron"

##  1273 ?        00:00:01 cron

You might or might not get a cron running .

Looks like the cron daemon is running! In order to assign programs to be executed with cron we need to edit a special text file called the cron table. Before we edit the cron table we need to select the default text editor. If you like using nano (the text editor we've been using throughout this book) then enter select-editor into the console, type in the number that corresponds to nano (usually 2) and then press enter:

select-editor

## Select an editor.  To change later, run 'select-editor'.
##   1. /bin/ed
##   2. /bin/nano        <---- easiest
##   3. /usr/bin/vim.basic
##   4. /usr/bin/vim.tiny
## 
## Choose 1-4 [2]:

Now that we've chosen a text editor we can edit the cron table using the command crontab -e (cron table edit) which will automatically open nano with the appropriate file.

crontab -e

# Edit this file to introduce tasks to be run by cron.
#
# m h  dom mon dow   command

Let's go over the layout of the cron table. First you should notice that any text after a pound sign (#) is a comment, so it's not seen by cron (just like bash comments). The cron table has six columns:

1. Minute (m)
2. Hour (h)
3. Day of Month (dom)
4. Month (mon)
5. Day of Week (dow)
6. Command to be run (command)

Each column is separated by a single space in the table. The first five columns allow you to specify when you want a particular command to be run. Only certain values are valid in each column:

1. Minute: 00 - 59 (A particular minute in an hour)
2. Hour: 00 - 23 (0 is the midnight hour)
3. Day of Month: 01 - 31 (1 is the first day of the month)
4. Month: 01 - 12 (1 is January)
5. Day of Week 0 - 6 (0 is Sunday)

There are also a few other characters that are valid in the cron table. The most commonly used character is a star (*) which represents all of the possible values in a column. So a star in the Minute column means "run every minute," and a star in the Hour column means "run during every hour." Knowing this let's make our first entry in the cron table. If we want a command to be executed every minute, during every hour, on every day of the month, during every month, on every day of the week, then we can put stars in all of the first five columns, followed by the command that we want to run. In this case the command that cron will run every minute will be date >> ~/date-file.txt, which will append the date and time when the command is executed to a file in our home directory called date-file.txt. This is what your cron table should look like before you save and exit from nano:

# Edit this file to introduce tasks to be run by cron.
#
# m h  dom mon dow   command
* * * * * date >> ~/date-file.txt

Save and exit nano just like you would for a regular text file and then wait a little bit! After a minute has gone by use cat to look at ~/date-file.txt:

cd
cat date-file.txt

## Thu Jun  8 18:50:01 UTC 2017

Look like our entry in the cron table is working! Wait another minute and then look at the file again:

cat date-file.txt

## Thu Jun  8 18:50:01 UTC 2017
## Thu Jun  8 18:51:01 UTC 2017

Unless we delete the line that we entered in the cron table, the output from date will be appended to date-file.txt every minute.

The single line of bash date >> ~/date-file.txt is a much simpler program than we would probably use in a cron table, though it's good for illustrating how a cron table works. If you want to do more complex tasks with cron it's better for cron to execute a bash script that you've written in advance. That way you can just specify bash /path/to/script.sh in the last column of the table.

Using stars in all columns is the simplest line of a cron table, so let's look at some examples of more complex table entries:

# m h  dom mon dow   command
00 * * * * bash /path/to/script.sh     # Runs every hour at the start of the hour
00 12 * * * bash /path/to/script.sh    # Runs every day at noon
* 12 * * * bash /path/to/script.sh     # Runs every minute between 12pm and 12:59pm
00 00 05 * * bash /path/to/script.sh   # Runs the 5th day of every month at midnight
00 00 * 07 * bash /path/to/script.sh   # Runs every day in the month of July at midnight
00 00 * * 2 bash /path/to/script.sh    # Runs every Tuesday at midnight

Besides numbers and the star there are a few other characters that you can use in cron table columns including a hyphen (-) for specifying ranges and a comma (,) for specifying lists of items. For example 00-29 in the Minutes column would specify the first thirty minutes of an hour, while 1,5 in the Day of Week column would specify Monday and Friday.

Let's take a look at another example of a cron table that uses hyphens and ranges so you can get a sense of how each character works.

# m h  dom mon dow   command
00-04 * * * * bash /path/to/script.sh       # Runs every minute for the first five minutes of every hour
00 00 * * 0,6 bash /path/to/script.sh       # Runs at midnight every Saturday and Sunday
00 03 01-15 * * bash /path/to/script.sh     # Runs at 3am for the first fifteen days of every month 
00,30 * * * * bash /path/to/script.sh       # Runs at the start and middle of every hour
00 00,12 * * * bash /path/to/script.sh      # Runs every day at midnight and noon
00 * 01-07 01,06 * bash /path/to/script.sh  # Runs at the start of every hour for the first seven days of the months of January and June

A program that is being run by cron is only as powerful as your imagination can stretch! If you're familiar with the social network Twitter then you might have come across some Twitter accounts which create posts automatically like Emoji Aquarium, Old Fruit Pictures, or Endless Screaming. Many of these "bot" accounts are powered by cron, which uses Twitter's HTTP API to post tweets regularly.