Controlling and Managing Processes

On a typical Linux server, there can be over a hundred processes running at any given time. The purposes of these processes range from system services, such as the Network Time Protocol (NTP) service, to processes that serve information to others, such as the Apache web server. As an administrator of Ubuntu servers, you will need to be able to manage these processes, as well as managing the resources available to them. In this chapter, we’ll take a look at process management, including the ps command, managing job control commands, and more.

As we work through these concepts, we will cover the following topics:

• Managing jobs
• Understanding the ps command
• Changing the priority of processes
• Dealing with misbehaving processes
• Managing system processes
• Scheduling tasks with cron

To begin our exploration of managing processes, let’s take a look first at managing jobs. Not only will this...

Managing jobs

Up until now, everything we have been doing on the shell has been right in front of us, from execution to completion. We’ve installed applications, run programs, and walked through various commands. Each time, we’ve had control of our shell taken from us, and we’ve only been able to start a new task when the previous one had finished. For example, if we were to install the vim-nox package with the apt install command, we would watch helplessly while apt takes care of fetching the package and installing it for us.

While this is going on, our cursor goes away and our shell completes the task for us without allowing us to queue up another command. We can always open a new shell to the server and multitask by having two windows open at once, each doing different tasks. But that’s likely not going to be the most efficient method of multitasking when working with the command line.

Instead, we can actually background a process without waiting...

Understanding the ps command

While managing our server, we’ll need to understand what processes are running and how to manage them. Later in this chapter, we’ll work through starting, stopping, and monitoring processes. But before we get to those concepts, we first need to be able to determine what is actually running on our server. The ps command allows us to do this.

Viewing running processes with ps

When executed by itself, the ps command will show a list of processes run by the user who called the command:

Figure 7.2: The output of the ps command, when run as a normal user and with no options

In Figure 7.2, you can see that when I ran the ps command as my own user with no options, it showed me a list of processes that I am running as myself. In this case, I have a vim session open (running in the background), and in the last line, we also see ps itself, which is also included in the output.

On the left side of the output, you’ll see...

Changing the priority of processes

Processes on a Linux system can be run with an altered priority, giving some processes more priority and others less. This gives you, the administrator, full reign when it comes to ensuring that the most important processes on the system are running with an adequate level of prioritization. There are dedicated commands for this purpose: nice and renice. These commands allow you to launch a process with a specific priority, or change the priority of a process that’s already running.

Nowadays, manually editing the priority of a process is something administrators will find themselves doing less often than they used to. A processor with 32 cores (or many more) is not all that uncommon, and neither is hundreds of gigabytes of RAM. Servers nowadays are certainly more powerful than they used to be, and are nowhere near as resource-starved as machines of old. Many servers (such as virtual machines) and containers are dedicated to a single task...

Dealing with misbehaving processes

Regarding the ps command, by this point you know how to display processes running on your server, as well as how to narrow down the output by string or resource usage. But what can you actually do with that knowledge? As much as we hate to admit it, sometimes the processes our server runs fail or misbehave and you need to restart them. If a process refuses to close normally, you may need to kill that process. In this section, we introduce the kill and killall commands to serve that purpose.

The kill command accepts a PID as an argument and attempts to close a process gracefully. In a typical workflow where you need to terminate a process that won’t do so on its own, you will first use the ps command to find the PID of the culprit. Then, knowing the PID, you can attempt to kill the process. For example, if PID 31258 needed to be killed, you could execute the following:

sudo kill 31258 

If all goes well, the process will end. You...

Managing system processes

System processes, also known as daemons, are programs that run in the background on your server and are typically started automatically when it boots. We don’t usually manage these services directly as they run in the background to perform their duty, with or without needing our input. For example, if our server is a DHCP server and runs the isc-dhcp-server process, this process will run in the background, listening for DHCP requests and providing new IP assignments to them as they come in. Most of the time, when we install an application that runs as a service, Ubuntu will configure it to start when we boot our server, so we don’t have to start it ourselves. Assuming the service doesn’t run into an issue, it will happily continue performing its job forever until we tell it to stop. In Linux, services are managed by its init system, also referred to as PID 1 since the init system of a Linux system always receives that PID. In recent years...

Scheduling tasks with cron

Earlier in this chapter, we worked through starting processes and enabling them to run in the background, and ensuring they start as soon as the server boots. In some cases, you may need an application to perform a job at a specific time, rather than to have it always running in the background. This is where cron comes in. With cron, you can set a process, program, or script to run at a specific time, down to the minute. Each user is able to have their own set of cron configurations (known as a crontab), which can perform any function that a user would be able to do normally. The root user has a crontab as well, which allows system-wide administrative tasks to be performed. Each crontab includes a list of cron jobs (one per line), which we’ll get into shortly. To view a crontab for a user, we can use the crontab command:

crontab -l 

With the -l option, the crontab command will show you a list of jobs for the user who executed the command...

Summary

In this chapter, we learned how to manage processes. We began with a look at the ps command, which we can use to view a list of processes that are currently running. We also took a look at managing jobs, as well as killing processes that, for one reason or another, are misbehaving. We also discussed methods of changing the priority of a process, to ensure we have full control over which processes are given more processing time, and we also learned how we can schedule things to run at a later time and date with cron.

In Chapter 8, Monitoring System Resources, we’ll take a look at some ways we can keep an eye on the resources that are available on our server, where we will learn how to check disk usage and understand memory usage and swap space, as well as a looking at some utilities that can make resource management a breeze.

Relevant videos

• Getting started with tmux (LearnLinuxTV): https://linux.video/tmux-guide
• Backgrounding and foregrounding processes in Linux (LearnLinuxTV): https://linux.video/bg-fg

Further reading

• Ham Vocke, A Quick and Easy Guide to tmux: https://learnlinux.link/tmux-article
• Tmux cheat sheet and quick reference: https://learnlinux.link/tcs
• crontab guru: https://learnlinux.link/ctg

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