2.2 Lesson 1

When started with the --help parameter, most commands display some brief instructions about their usage. Although not all commands provide this switch, it is still a good first try to learn more about the parameters of a command. Be aware that the instructions from --help often are rather brief compared to the other sources of documentation which we will discuss in the rest of this lesson.

Most commands provide a manual page or “man” page. This documentation is usually installed with the software and can be accessed with the man command. The command whose man page should be displayed is added to man as an argument:

This command opens the man page for mkdir. You can use the up and down arrow keys or the space bar to navigate through the man page. To exit the man page, press Q.

Each man page is divided in maximum of 11 sections, though many of these sections are optional:

In practice, most man pages don’t contain all of these parts.

Man pages are organized in eight categories, numbered from 1 to 8:

Each man page belongs to exactly one category. However, multiple categories can contain man pages with the same name. Let’s take the passwd command as an example. This command can be used to change a user’s password. Since passwd is a user command, its man page resides in category 1. In addition to the passwd command, the password database file /etc/passwd also has a man page which is called passwd, too. As this file is a configuration file, it belongs to category 5. When referring to a man page, the category is often added to the name of the man page, as in passwd(1) or passwd(5) to identify the respective man page.

By default, man passwd displays the first available man page, in this case passwd(1). The category of the desired man page can be specified in a command such as man 1 passwd or man 5 passwd.

We have already discussed how to navigate through a man page and how to return to the command line. Internally, man uses the less command to display the man page’s content. less lets you search for text within a man page. To search for the word linux you can just use /linux for forward searching from the point that you are on the page, or ?linux to start a backward search. This action highlights the all the matching results and moves the page to the first highlighted match. In both cases you can type N to jump to the next match. In order to find more information about these additional features, press H and a menu with all the information will be displayed.

Another tool that will help you while working with the Linux system are the info pages. The info pages are usually more detailed than the man pages and are formatted in hypertext, similar to web pages on the Internet.

The info pages can be displayed like so:

For each info page, info reads an info file that is structured into individual nodes within a tree. Each node contains a simple topic and the info command contains hyperlinks that can help you move from one to the other. You can access the link by pressing enter while placing the cursor on one of the leading asterisks.

Similar to man, the info tool also has page navigation commands. You can find out more about these command by pressing ? while being on the info page. These tools will help you navigate the page easier as well as understand how to access the nodes and move within the node tree.

As mentioned before, the /usr/share/doc/ directory stores most documentation of the commands that the system is using. This directory contains a directory for most packages installed on the system. The name of the directory is usually the name of the package and occasionally its version. These directories include a README or readme.txt file that contains the package’s basic documentation. Alongside the README file, the folder can also contain other documentation files, such as the changelog which includes the history of the program in detail, or examples of configuration files for the specific package.

The information within the README file varies from one package to another. All files are written in plain text, therefore they can be read with any preferred text editor. The exact number and kinds of files depend on the package. Check some of the directories to get an overview of their contents.

A Linux system is built from numerous directories and files. Linux has many tools to locate a particular file within a system. The quickest one is the command locate.

locate searches within a database and then outputs every name that matches the given string:

The locate command supports the usage of wildcards and regular expressions as well, therefore the search string does not have to match the entire name of the desired file. You will learn more about regular expressions in a later chapter.

By default, locate behaves as if the pattern would be surrounded by asterisks, so locate PATTERN is the same as locate *PATTERN*. This allows you just provide substrings instead of the exact filename. You can modify this behavior with the different options that you can find explained in the locate man page.

Because locate is reading from a database, you may not find a file that you recently created. The database is managed by a program named updatedb. Usually it is run periodically, but if you have root privileges and you need the database to be updated immediately, you can run the updatedb command yourself at any time.

find is another very popular tool that is used to search for files. This command has a different approach, compared to the locate command. find command searches a directory tree recursively, including its subdirectories. find does such a search at each invocation, it does not maintain a database like locate. Similar to locate, find also supports wildcards and regular expressions.

find requires at least the path it should search. Furthermore, so-called expressions can be added to provide filter criteria for which files to display. An example is the -name expression, which looks for files with a specific name:

The first find command searches for the file within the current Downloads directory, whereas the second one searches for the file in the user’s home directory.

The find command is very complex, therefore it will not be covered in the Linux Essentials exam. However, it is a powerful tool which is particularly handy in practice.