Chapter 10: Daemons and Cron

So far, we've explored how to run our own processes. However, computer operating systems also have many of their own processes in the background, silently running and making sure everything works "as you'd expect it to". Lets learn a little bit more about these processes, called daemons (pronounced day-mon, or alternatively, to rhyme with diamond)

What is a Daemon?

In Linux, there are three types of processes; interactive, batch, and daemon:

• Interactive processes are the ones that you're most familiar with. They are associated with a terminal, and you can interact with them (such as sending them signals)
• Batch processes are less common. They are not associated with a terminal, but are groups of tasks from a list submitted to a process queue. They are best suited for performing recurring operations when system usage is low - think monthly system backups

This leaves daemons. Daemon processes run in the background, and are spawned (term for creating a process) right after a system boots. Specifically, they are spawned from the init process by forking it, something we explored in Chapter 9. In this way, orphans and daemons are somewhat similar in that their parent is the init process; creating an orphan is one way of making your own pseudo-daemon.

Examples of Daemons

In Linux, it is specified that the names of daemon processes end with "d". Using ps and/or htop(especially when listing processes as a tree with htop -t or htop --tree), you might be able to catch some daemons yourself. Here are some common examples:

• systemd: A system and service manager. On modern systems, this will be the init process and have a PID of 1 (including on our server)
• logind: Manages user login and seats
• sshd: Manages all SSH connections. This is what handles your request to connect to the C2S2 and ECELinux servers, as well as the connection while you're active
• httpd: The HTTP service manager, usually run on web servers
• ftpd: The FTP (File Transfer Protocol) service manager. FTP is used to transfer files between one computer and another using the TCP protocol
• crond: A scheduling daemon, used for scheduling tasks. This is commonly used by users to schedule their own tasks - we'll explore this in a bit!

If you're curious about what daemons are currently running on a system, one way to view them is to use the service command. service is used to manage daemons, and running service --status-all will show you the status of all currently running daemons.

Case Study: cron 

Background

One of the more popular daemons to use is cron. cron is lightweight daemon used to schedule processes, and can be used by users to run processes regularly, even when they're not logged in. While many newer users have opted to use systemd itself to schedule their processes, cron is what started it all.

To use cron, we must first use a crontab . We can create one with the command

crontab -e

This command edits your existing crontab if it exists, and creates one if not.

Crontab Formatting - Time

Crontabs require a specific syntax for specifying a command and when it should be run. Specifically, they follow the following syntax:

* * * * * <command to be executed>
– – – – –
| | | | |
| | | | `—– day of week (0 – 6) (Sunday=0)
| | | `——- month (1 – 12)
| | `——— day of month (1 – 31)
| `———– hour (0 – 23)
`————- min (0 – 59)

Here, we can see how we can specify when a command should be run simply by specifying the minutes, hours, days, months, and days of week (if necessary) when it should be run. For example, 0 0 1 1 * <command> will run the command every January 1st at midnight.

In addition to this, there are a few more characters that Crontabs use:

• * is a wildcard, and can stand for anything. For example, * * 1 * * <command> will run the command on the first of every month
• - specify a range. For example, 0 0 * * 1-5 <command> will run the command at midnight on every weekday
• / is used for time intervals. For example, */10 * * * * <command> will run the command every 10 minutes.

These can all be combined as well. For example, 0-30/5 12 * * * <command> will run the command during the first half of the 12 p.m. hour every 5 minutes.

Finally, some patterns are so common that Crontabs have specific keywords for them. These are:

For example, @hourly <command> will run the command once every hour.

Crontab Formatting - Commands

The other element of a Crontab entry is the command. This will be the same as any normal command you might run from a terminal (under the hood, cron uses the default shell located at /bin/sh to run the commands. For C2S2, this is Bash). HOWEVER, you must remember that cron isn't you! It may not have the same notion of which version of Python to use, and isn't guaranteed to start in any particular directory.

For example, suppose I have a Python file titled test.py in my home folder. To run it from my terminal, one might simply do

python test.py

However, cron may not know about the specific version of Python you want to use, and isn't guaranteed to start in the same directory. Therefore, we must specify absolute paths to everything we use. Modifying this command for a Crontab entry, it might look like:

/usr/bin/python /home/<netid>/test.py

Full cron Example

Alright - let's put it all together! For this, in your home directory, you'll need a simply Python script that appends a line to a file. Save the following script as test.py in your home/NetID folder:

file = open( "test.txt", "a" )
file.write( "Hi! This is cron saying hello :)\n" )
file.close()

This script should write a new line to a test.txt file every time it's run.

Next, use crontab -e to edit your Crontab. Insert the following entry to run this script every minute (replacing <netid> with your NetID, such that the path points to the script we just made):

* * * * * /usr/bin/python /home/<netid>/test.py

Save the Crontab and...that's it! cron should take care of running the script for you every minute. Wait a few minutes, and come back - you should have a test.txt file waiting for you with a few entries! When you're satisfied, you can delete this entry from the Crontab to avoid cron scheduling it (as well as delete your Crontab with crontab -r, if you wish)

For more information on cron, this cheatsheet is very helpful

Addendum: Why "daemon"?

Many have gotten confused over the name "daemon" - what does it actually mean? I certainly don't know enough about it , so I figured I'd rather quote this good article on them:

"The earliest form of the word, daemon, was spelled as daimon, a form of guardian angel – attendant spirits that helped form the character of people they assisted. Socrates claimed to have one that served him in a limited way, but correctly. Socrates’ daimon only told him when to keep his mouth shut. Socrates described his daimon during his trial in 399 BC, so the belief in daimons has been around for quite some time. Sometimes, the spelling of daimon is shown as daemon. Daimon and daemon, here, mean the same thing.

While a daemon is an attendant, a demon is an evil character from the Bible. The differences in spelling is intentional and was apparently decided upon in the 16th century. Daemons are the good guys, and demons are the bad ones.

The use of the word, daemon, in computing came about in 1963. Project MAC is shorthand for Project on Mathematics and Computation, and was created at the Massachusetts Institute of Technology. It was here that the word, daemon, came into common use to mean any system process that monitors other tasks and performs predetermined actions depending on their behavior, The word, daemon was named for Maxwell’s daemon.

Maxwell’s daemon is the result of a thought experiment. In 1871, James Clerk Maxwell imagined an intelligent and resourceful being that was able to observe and direct the travel of individual molecules in a specific direction. The purpose of the thought exercise was to show the possibility of contradicting the second law of thermodynamics"