2. Introduction to Linux

What exactly is Linux? The answer can be a complex one. Technically, Linux is a piece of software called the kernel. The kernel handles tasks such as booting the system and interacting with hardware devices. By itself, the kernel doesn’t really provide users with any functionality. The rest of the operating system (OS), consisting of the filesystem and a large number of commands, is what provides users with useful features.

Although Linux is technically just the kernel, many people refer to the entire OS as Linux. In reality, the collection of software that makes up the OS is known as a Linux distribution (also called a distro). Many distros are available to choose from, which often results in some confusion for novice Linux users.

First consider which distro you want to use. This topic is explored in more detail later in this chapter.

Consider installing the distro on a Virtual Machine (VM). By using a VM, you can install multiple distros and still use your host operating system. Several VM software choices are available, including VirtualBox, VMware, and Parallels Desktop (for Mac users). Some of these products have free versions, whereas others might charge a license fee.

During the installation, consider accepting the defaults. Typically, the defaults provide you with the software that you need as a developer. You can always reinstall the distro in the future or add additional software using the tools described in Chapter 6, “System Administration.”

Cost—Some distros are entirely free, whereas some charge for support and access to updates.

Features—Some distros provide limited access to software based on the objective of the distro. For example, a security-hardened distro only provides software that has met rigorous security benchmarks.

Function—Some distros are specifically tailored to meet a specific function. For example, a distro might be designed to host database applications.

Support—The organization that creates the distro might provide support, or the support might be completely community sponsored. For most distros used in a corporate environment, the organization that creates and maintains the distro provides support.

Although these might not be all the features you or your organization consider when choosing a distro, the preceding list provides you with an idea of what features are normally considered. Because literally hundreds of distros exist, I recommend exploring http://distrowatch.com, a website devoted to monitoring these distros, their features, and their popularity.

Linux distributions are generally split into three primary families:²

Debian—Known to be a favorite of the hard-core Linux geeks, Debian was one of the first Linux distributions. Popular variations of Debian include Ubuntu and Mint.

Red Hat:—Designed to be a commercial distro, Red Hat Linux was introduced shortly after Debian. It is now called Red Hat Enterprise Linux (RHEL). Because it is a commercial distro, payment of a license fee (for support and updates) is required to use RHEL. However, several free Red Hat–based distros exist, including Fedora and CENTOS.

Slackware/SUSE—Slackware was the original “founding father” of this family of distros, but SUSE is the most popular now. SUSE is like RHEL in that it is designed to be a commercial distro. OpenSUSE is a free variation.

Regarding the information provided in this book, in most cases which distro you use does not matter. However, for some topics the distro does matter. When these situations arise, I provide detailed information regarding the differences between the distros. Otherwise, the examples make use of different distros to highlight that core functionality is largely the same across Linux distributions.³

Via the GUI—On laptops, desktops, and some servers, a GUI-based login appears by default.

Via the command line—Administrators commonly do not install the GUI software on servers because that software is a huge hardware hog (CPU, RAM, and so on). On these servers, you typically see a command-line login.

Via the network—A network-based login could either be via command line (very common) or GUI (not as common). The machine that you log into must have special software installed and enabled to allow this sort of access.

The good news is that regardless of which display manager you use, the basic operation shouldn’t change. You either select your user name from a list or type your user name in the dialog box provided. Then you are prompted for your password. As you become more experienced, try exploring other options provided by your display manager, such as shutting down your system.

To log out of a command-line environment, type exit and then press the Enter key. To return to the GUI screen, press either Ctrl+Alt+F1 or Ctrl+Alt+F2.

GNOME

KDE

Unity+

Cinnamon

Xfce

MATE

With so many desktops available, you might ask yourself “Which one should I use?” To some extent, that question might be answered for you when you choose your Linux distribution. Most distros have a “favored default” desktop which will be installed and used automatically. Although other desktops might be available for the distro, you typically need to install them separately.

In some cases, the developers of the distro might provide you the option of choosing your desktop before you install the distro. For example, consider Figure 2.3, which displays the download links for the Mint distribution.

Figure 2.3 shows the different install options provided, primarily divided by the type of desktop. Additionally, a user with administrative access can install multiple desktops on a single system. If there are multiple desktops, you could choose which desktop you want to use before you log in, as demonstrated in Figure 2.4.

After you successfully log in, a desktop appears. The overall look and feel of the desktop varies. However, figuring out where things are on a particular desktop shouldn’t take too long. For example, look at Figure 2.5, which displays the default desktops for MintOS.

As you can see, the Cinnamon and MATE desktops look very similar. Both provide a menu (bottom left) that enables you to access additional programs and features. Both have quick launch icons and both provide system information (bottom right) such as date/time. The choice comes down to exploring the desktop that you are using rather than memorizing where features reside for a specific desktop.

The most common shell program in Linux is the BASH⁷ shell. This book shows BASH shell–based examples.

Command name—This is just the name of the command.

Option(s)—An option (also referred to as a flag) is a predefined value that changes the behavior of the command. The format of options can vary. In some cases, the option begins with a single hyphen followed by a single character; for example: ls -a. In other cases, the option begins with two hyphens followed by a word; for example: ls --all. In some rare cases, the option is just a single character with no hyphen. The format depends on the command because some commands support the single-hyphen method whereas others support the double-hyphen method (and some support both methods).

Arguments—Arguments are used to provide additional information to the command. This information could be things like a filename, user name, or host name.

See Figure 2.7 for a visual example of the command-line components.

bo@mintos:~ > help alias
alias: alias [-p] [name[=value] ... ]
    Define or display aliases.

    Without arguments, 'alias' prints the list of aliases in the reusable
    form 'alias NAME=VALUE' on standard output.

    Otherwise, an alias is defined for each NAME whose VALUE is given.
    A trailing space in VALUE causes the next word to be checked for
    alias substitution when the alias is expanded.

    Options:
      -p    Print all defined aliases in a reusable format

    Exit Status:
    alias returns true unless a NAME is supplied for which no alias has
    been defined.

As you can see from the output of the help alias command, the command accepts the -p option. Because the option is enclosed within square brackets, the option is not required for the command to run. Additionally, you can include a name or name=value argument with the command. Again, the square brackets indicate that these arguments are not required and that name does not require an =value argument.

One drawback of the help command is that is only works on built-in shell commands. These commands are part of the BASH shell, not a separate executable.⁸ The majority of commands that you will execute are not built-in shell commands, so the help command won’t be that helpful.

However, almost all commands have documentation available via a command called the man command (man is short for manual). To view a man page for a command, execute man cmd, replacing cmd with the command name. For example, to view the man page for the cal command, execute man cal.

The man page for a given command⁹ can be quite large. To facilitate the process of reading the man page, you can use several keys to move through the document:

<spacebar>—Move down one screen.

<ENTER>—Move down one line.

b—Move back one screen.

/term—Search for term.

h—Display help screen.

q—Quit displaying man page.

Note that these are just a few of the commands that you can use to display a man page. The h key provides a full list of commands.

A typical man page has several different sections. The output begins with a line like the following:

CAL(1)               BSD General Commands Manual                   CAL(1)

This indicates the command and the category. This chapter covers man page categories a bit later.

The next section provides a brief description of the command like the following:

NAME
     cal, ncal — displays a calendar and the date of Easter

After this description, you see a summary of how you can execute the command:

SYNOPSIS
     cal [-3hjy] [-A number] [-B number] [[month] year]
     cal [-3hj] [-A number] [-B number] -m month [year]
     ncal [-3bhjJpwySM] [-A number] [-B number] [-s country_code] [[month]
         year]
     ncal [-3bhJeoSM] [-A number] [-B number] [year]
     ncal [-CN] [-H yyyy-mm-dd] [-d yyyy-mm]

Recall that square brackets indicate something that isn’t required but which is a valid option. For example, you could just run the cal command with no options or arguments:

bo@mintos:~ > cal
     July 2016
Su Mo Tu We Th Fr Sa
                1  2
3  4  5  6  7  8   9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31

You could also use an option, such as -3:

bo@mintos:~ > cal -3
     June 2016             July 2016            August 2016
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa
          1  2  3  4                  1  2      1  2  3  4  5  6
 5  6  7  8  9 10 11   3  4  5  6  7  8  9   7  8  9 10 11 12 13
12 13 14 15 16 17 18  10 11 12 13 14 15 16  14 15 16 17 18 19 20
19 20 21 22 23 24 25  17 18 19 20 21 22 23  21 22 23 24 25 26 27
26 27 28 29 30        24 25 26 27 28 29 30  28 29 30 31
                      31

However, only options specified in the Synopsis are allowed:

bo@mintos:~ > cal -2
cal: invalid option -- '2'
Usage: cal [general options] [-hjy] [[month] year]
       cal [general options] [-hj] [-m month] [year]
       ncal [general options] [-bhJjpwySM] [-s country_code] [[month] year]
       ncal [general options] [-bhJeoSM] [year]
General options: [-NC31] [-A months] [-B months]
For debug the highlighting: [-H yyyy-mm-dd] [-d yyyy-mm]

Additionally, you can specify a month and year:

bo@mintos:~ > cal 3 1968
     March 1968
Su Mo Tu We Th Fr Sa
                1  2
 3  4  5  6  7  8  9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31

The next section provides a greater level of detail regarding how the command and its options work:

DESCRIPTION
     The cal utility displays a simple calendar in traditional format and
     ncal offers an alternative layout, more options and the date of Easter.
     The new format is a little cramped but it makes a year fit on a 25x80 terminal.
     If arguments are not specified, the current month is displayed.
     The options are as follows:
     -h      Turns off highlighting of today.

Some commands also provide a See Also section that includes a list of related commands:

SEE ALSO
     calendar(3), strftime(3)

Depending on the man page that you are viewing, you might see additional sections. However, the critical ones have been described here.

bo@mintos:~ > man man
{output omitted}
       The table below shows the section numbers of the manual followed by
       the types of pages they contain.
       1   Executable programs or shell commands
       2   System calls (functions provided by the kernel)
       3   Library calls (functions within program libraries)
       4   Special files (usually found in /dev)
       5   File formats and conventions eg /etc/passwd
       6   Games
       7   Miscellaneous  (including  macro  packages  and  conventions),
           e.g. man(7), groff(7)
       8   System administration commands (usually only for root)
       9   Kernel routines [Non standard]

In most cases, you don’t have to worry much about these categories. When you execute the man command, it first searches category 1 for the man page. If it doesn’t find it, it then searches the next category.¹¹ Eventually it finds and displays the man page or displays an error if it can’t find the man page in any category:

bo@mintos:~ > man nope
No manual entry for nope

In some cases you must specify the category. For example, there is a user command named passwd (category 1) and a file format named passwd (category 5). If you execute the man passwd command, the passwd command man page displays. To view the man page for the passwd file, you must execute the man 5 passwd command.

The documentation found in info pages tends to be more verbose than in man pages. The sections of info pages are also organized differently than man pages. Instead of one long document of text, info pages appear in hyperlinked sections. For example, if you scroll down the document for the ls command (use the down-arrow key on your keyboard), you see this:

* Menu:
* Which files are listed::
* What information is listed::
* Sorting the output::
* Details about version sort::
* General output formatting::
* Formatting file timestamps::
* Formatting the file names::

This is a menu of subcategories you can jump to by placing your cursor on a menu item and pressing the Enter key. After you’re in a subcategory, type the letter u to go back up one level. To see additional commands, press the H or ? key.