You're reading from The Linux DevOps Handbook

Product typeBook

Published inNov 2023

PublisherPackt

ISBN-139781803245669

Edition1st Edition

Concepts

DevOps

Authors (2):

Damian Wojsław

Grzegorz Adamowicz

View More author details

Intermediate Linux

In this chapter, we’re going to continue with the introduction to the Linux shell. The topic itself is vast and warrants a book of its own. We will be coming back to the topics from the previous chapter and introducing new ones.

In this chapter, we will cover the following topics:

Globs
Automating repetitive tasks
Software installation
Managing users
Secure Shell (SSH) protocol

Technical requirements

It is highly recommended that you have a Linux system installed and ready for use. We recommend it be a virtual machine or a laptop that you can safely reinstall from scratch in case something goes horribly wrong. This will let you follow the examples in this book and perform any kind of exercise that we give you.

We are not going to cover an installation. Every distribution may use its own installer, be it graphical or text (depending on the distribution and which variant you’ve picked). You’ll need to note down or remember the name of your user (conveniently called username or login) and password. There are ways to get into the system if you have physical access and you don’t know either the login or password or both, but they are way outside the scope of this book.

Our main distribution in this book is Debian. However, you should be okay with any of the major ones we covered in the previous chapter, so long as it isn’t Alpine...

Globs

There is a lot that a shell can do for you to make your life easier. One of them is allowing for a level of uncertainty when typing in arguments on the shell. To that end, the shell defines several special characters that are treated like symbols for something, not like literal input. These are called global patterns, or globs. The characters that are used in globs are sometimes referred to as wildcards.

Do not confuse globs with regular expressions (regexps). While globs are quite a powerful tool on their own, they are no match for regexps. On the other hand, regexps are not evaluated by bash when it performs pattern matching.

The following table describes shell globs and their meaning. We’re going to explain their exact meaning through several examples:

...

Automating repetitive tasks

There are times when you’ll want to make some tasks repetitive. You may write a script that will create a backup of a database, check users’ home directory permissions, or dump current operating system preformance metrics into a file. Modern Linux distributions provide you with two ways of setting these up. There is a third method that allows you to run a task once, at a delayed time (the at command), but here, we’re interested in repetitive tasks.

Cron jobs

Cron is a traditional way of running tasks that need to be executed regularly at specified intervals. Usually, they should be obsolete by systemd timers, but a lot of software provides repeatability through the use of cron jobs and Alpine Linux won’t have this in the name of the minimal-sized distribution.

Cron jobs are essentially commands that are run at predefined intervals. The command and their trigger timers are defined in configuration files that live in the...

Software installation

Depending on which distribution you’ve chosen and the type of installation you’ve decided on, your system may lack software that’s essential for your everyday work. It may also be that you one day require a piece of software that isn’t installed by default.

Linux distributions pioneered something that other operating systems mirrored later on. The common way of installing software on the Linux operating system is by running an appropriate command that will fetch a binary, put it properly on the system, add some configuration if required, and make it available to users. Today, this may not sound revolutionary at all. After all, we’re living in a world of Apple App Store, Google Play, and Microsoft Apps. But back in the day when Windows and macOS users had to navigate the internet to find a suitable installer for their software, Linux users could install most of it with a single command.

This is important in the automated...

Managing users

The user in a Linux system is defined by a set of three files:

/etc/passwd: This file contains information about the user – that is, the user’s name, unique numerical ID (UID) in the system, the primary group the user belongs to GID, the path to the home directory, and the shell that is loaded when the user logs in. A typical entry looks like this:

   admin:x:1000:1000:Debian:/home/admin:/bin/bash

Each line describes exactly one user. Fields are separated by a colon. The second field will only contain anything other than x in very exotic cases. Here, x means that the password is stored separately in the /etc/shadow file. The reason is that permissions for the /etc/passwd file have to be a bit more relaxed so that the login process can work. /etc/shadow can only be read by root and root group and written to only by root:

    root@myhome:~# ls -ahl /etc/passwd
    -rw-r--r-- 1 root...

Secure Shell (SSH) protocol

In the DevOps world, almost nothing runs locally on your laptop or PC. There is one golden standard among ways to reach remote systems and it’s the SSH protocol. SSH was developed in 1995 as a secure, encrypted remote shell access tool that would replace plaintext utilities such as telnet or rsh. The main reason for this is that in distributed networks, it is too easy to eavesdrop on communication and anything that is being transmitted in open text can easily be intercepted. This includes important data such as login details.

The most commonly used SSH server (and the client) in the Linux world is OpenSSH (https://www.openssh.com/). Other open source servers that are still maintained at the time of writing are lsh (http://www.lysator.liu.se/~nisse/lsh/), wolfSSH (https://www.wolfssl.com/products/wolfssh/), and Dropbear (https://matt.ucc.asn.au/dropbear/dropbear.html).

SSH is mainly used to log into a remote machine to execute commands. But it...

Summary

This chapter concludes our introduction to the basic Linux operations you will need in your daily work. It has by no means been comprehensive enough to explain everything that you need to know to manage your Linux system, but it is enough to get you started and should be enough to help you manage your system. In the next chapter, we are going to cover writing shell scripts from the very beginning and guide you through basic and more advanced topics.

Exercises

Try out the following exercises to test your knowledge of this chapter:

In Debian/Ubuntu, install the vim package.
Create a cron job that will create a file called /tmp/cronfile every Saturday at 10:00 A.M.
Create a group called admins and add an existing user to it.

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Authors (2)

Damian Wojsław

Damian Wojsław has been working in the IT industry since 2001. He specializes in administration and troubleshooting of Linux servers. Being a system operator and support engineer he has found DevOps philosophy a natural evolution of the way sysops work with developers and other members of the software team.
Read more about Damian Wojsław

Grzegorz Adamowicz

Grzegorz Adamowicz has been working in the IT industry since 2006 in a number of positions, including Systems Administrator, Backend Developer (PHP, Python), Systems Architect and Site Reliability Engineer. Professionally was focused on building tools and automations inside projects he is involved in. He's also engaged with the professional community by organizing events like conferences and workshops. Grzegorz worked in many industries including Oil & Gas, Hotel, Fintech, DeFI, Automotive, Space and many more.
Read more about Grzegorz Adamowicz

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