In this section, you'll learn how to run Linux command-line programs and what the basic structure of the command line is. You will also learn what program options and arguments are and why they are important for customizing your commands.

When we say the Linux command line, what we really mean is the shell. It's important to know that the shell is not the same as a terminal emulator. A Terminal is a screen or window that lets you access a Linux server's input and output. A shell is just a program that runs on the server as does any other command and which awaits, interprets, processes, executes, and responds to commands typed in by the user.

First, open up a new terminal emulator and log in to your CentOS 7 server by using SSH, as we learned in the Chapter 1, Introduction to Linux. Log in using your normal user account, which you set...

In this section, you will learn how shell expansion works and how we can use file globbing to make our lives easier when using commands that deal with a lot of input files. We will discuss all existing and available shell globbing character classes and show you important use cases and examples for each of them. When working with commands that use file or directory names as arguments, such as the ls command, it is very helpful to learn about file and directory globbing. These are special characters typed in the shell that behave differently than regular characters. All globbing characters are going to be replaced by the shell with a list of files matching the characters' pattern right before any command can use them as parameters. It's a notation to simplify working with files, especially when dealing with a large number of files that you need to type and...

As we learned in the previous section, the shell has a list of special characters that have a special meaning in the shell and trigger some functionality, such as using the wildcard character as filenames. But there are even more special characters than the ones we showed you before. If you want to work with such special characters, for example, using filenames that contain question mark symbols, which are valid filenames, you have a problem, as the shell always first tries to apply special actions to special characters, so they will not work as normal filename characters. The solution here is to disable all special meanings of such characters using various approaches, such as quoting, so that we can treat them as any other normal literal character. As you now know, in the Linux Bash shell, there are some special characters, such as * # [ ] . ~ ! $ { } < &gt...

Before we can start teaching you how to get help using the various forms of documentation available for Linux commands, we first have to learn how to read the default command syntax documentation. Most of the provided standard shell commands in Linux follow a uniform format describing their usage. Afterward, we will show you how to get help.

When working with the Linux command line, getting help and looking up information and documentation is very important because the command line can be very complex and nobody knows and can remember everything. On every Linux system, there are several ways available to get help, depending on the kind of level of information you need to know. In this section, we will tap into the different sources of documentation.

In a previous section, you already learned the general structure of Bash shell commands and everything you need to know...

In this section, we will learn how to work in the shell efficiently. We will introduce some important practices and techniques that will improve your productivity and make you a faster shell command hacker. This can make you a happier person because, eventually, you will be able to advance to feeling very comfortable working in the shell. Please note, in this section, we will show you a lot of keyboard shortcuts. Learning keyboard shortcuts is like learning any other craft, you begin slowly and gradually, because learning too many new skills at once can leave you overwhelmed and make you forget more quickly than learning in smaller chunks. My tip is to start by learning the first three to four command editing shortcuts and then incorporate more from day to day or week to week. We will start with the command editing shortcuts. Now, if you don't...

In this section, you will learn why every command can use three standard streams for accessing its input and output. Also, you will learn how to work with those input and output streams and how to use redirection. Finally, we will learn how to use pipes and why they are so important. One philosophy of the Linux operating system is that every command has exactly one functionality in the system, nothing more, and nothing less. For example, there's one command to list files, another to sort text, and one to print the file's content, and so on.

Now, one of the most important features of the shell is to connect different commands to create custom tailored solutions and tools for all kinds of problems and workflows. But before we can show you how to concatenate different commands together to build something powerful, we first need to know how...

In this section, we will introduce the wonderful art of regular expressions. You will learn what they are and why they are so powerful. There are a lot of different regular expression characters available, and here we will introduce the most important ones. Afterward, you will learn how to apply regular expressions with the grep command to find, extract, and filter useful information out of text files. Regular expressions, or regexps for short, are a very powerful concept used to search through text using special patterns, describing the structure of the search term instead of a constant string of characters, which is also called literal text search in this context. Using regular expressions can save you a lot of time, by not doing repetitive work, and Linux system administrators use them quite heavily in their everyday work.

In the File globbing...

In this section, we will learn about the sed command, the powerful stream editor. We will give you a brief introduction on how sed works and we'll be showing you the substitution mode for automatically replacing text and files, which is one of the most important modes available. Next, we will learn about the sed command. Let's first examine its syntax:

sed [OPTION] 'pattern rule' FILE  

sed stands for stream editor and this command can edit files automatically without any user interaction. It processes input files on a line-by-line basis. Oftentimes, sed is used in shell scripts to transform any command's output to a desired form for further processing. Most everyday use cases for sed follow a similar pattern, which, in its most simple form, is first used with a regular expression or other pattern to define which lines to change in an...

In this section, we will show you what the command awk is all about and why it can be important for us. We will also show you how to use it for text file manipulation and processing. awk is another very important tool for text processing and manipulation. It can be used as a complete scripting language to work on text files or streams. It contains some very powerful programming constructs, including variables: if...else, while, do while and for loops; arrays; functions; and mathematical operations. awk also works on a line-by-line basis, as sed does. One of the key features of awk and the main difference to sed is that it splits input lines into fields automatically. But how does it work and why is it so helpful?

awk enables you to create rule and action pairs, and, for each record that matches this rule or condition, the action will fire. The rules are also called...

In this section, you will learn how to navigate the Linux filesystem. You will also learn how the Linux filesystem is structured. If we print out the folder structure of the top-level directories beneath the root directory by executing the tree -d -L 1 / command, you will see a list of strange-sounding directory names. These directory names are the same on any Linux distribution and they follow a standard called the filesystem hierarchy standard (FHS). Each of these standard directories in the Linux filesystem has a specific purpose, and the user can expect certain files in certain locations, and it also means that a program can predict where the files are located, and it also means that any program working with those system directories can predict where the files are located. The following are the directories:

• The / slash is the primary hierarchy...

In this chapter, we started off with an introduction to the command line, file globbing, and quoting commands. We progressed towards practical execution by working with the shell, standard streams, and regular expressions. We also covered functionalities of sed, awk, and the Linux filesystem.

In the next chapter, we'll cover concepts pertaining to files.