Part 1: man Pages
As previously mentioned, UNIX was the operating system from which the Linux foundation was built. The developers of UNIX created help documents called man pages (man stands for manual).
Man pages are used to describe the features of commands. They will provide you with a basic description of the purpose of the command, as well as provide details regarding the options of the command.
1. Viewing man pages
To view a man page for a command, execute man command in a terminal window. For example, the command man cal will display the man page for thecal command:
2. Controlling the man Page Display
The man command uses a "pager" to display documents. Normally this pager is the less command, but on some distributions it may be the more command.
If you want to view the various movement commands that are available, you can type the letter h while viewing a man page. This will display a help page (note: If you are working on a Linux distribution that uses the more command as a pager, your output will be different than the example shown here):
If your distribution uses the less command, you might be a bit overwhelmed with the large number of "commands" that are available. The following table provides a summary of the more useful commands:
| Command | Function |
|---|---|
| Return (or Enter) | Go down one line |
| Space | Go down one page |
| /term | Search for term |
| n | Find next search item |
| 1G | Go to beginning |
| G | Go to end |
| h | Display help |
| q | Quit man page |
3. Sections of the man Page
Man pages are broken into sections. Each section is designed to provide specific information about a command. While there are common sections that you will see in most man pages, some developers also create sections that you will only see in a specific man page.
The following table describes some of the more common sections that you will find in man pages:
| Section name | Purpose |
|---|---|
| NAME | Provides the name of the command and a very brief description. |
| SYNOPSIS | Provides examples of how the command is executed. See below for more information. |
| DESCRIPTION | Provides a more detailed description of the command. |
| OPTIONS | Lists the options for the command as well as a description of how they are used. Often this information will be found in the DESCRIPTION section and not in a separate OPTIONS section. |
| FILES | Lists the files that are associated with the command as well as a description of how they are used. These files may be used to configure the command‘s more advanced features. Often this information will be found in the DESCRIPTION section and not in a separate OPTIONS section. |
| AUTHOR | The name of the person who created the man page and (sometimes) how to contact the person. |
| REPORTING BUGS | Provides details on how to report problems with the command. |
| COPYRIGHT | Provides basic copyright information. |
| SEE ALSO | Provides you with an idea of where you can find additional information. This also will often include other commands that are related to this command. |
Part 2: man Pages Categrized by Sections
Until now, we have been displaying man pages for commands. However, sometimes configuration files also have man pages. Configuration files (sometimes called system files) contain information that is used to store information about the Operating System or services.
Additionally, there are several different types of commands (user commands, system commands, and administration commands) as well as other features that require documentation, such as libraries and Kernel components.
As a result, there are thousands of man pages on a typical Linux distribution. To organize all of these man pages, the pages are categorized by sections, much like each individual man page is broken into sections.
By default there are nine default sections of man pages:
- Executable programs or shell commands
- System calls (functions provided by the kernel)
- Library calls (functions within program libraries)
- Special files (usually found in
/dev) - File formats and conventions, e.g.
/etc/passwd - Games
- Miscellaneous (including macro packages and conventions), e.g.
man(7),groff(7) - System administration commands (usually only for root)
- Kernel routines [Non standard]
When you use the man command, it searches each of these sections in order until it finds the first "match". For example, if you execute the command man cal, the first section (Executable programs or shell commands) is searched for a man page called cal. If not found, then the second section is searched. If no man page is found after searching all sections, you will receive an error message:
1. Determining Which Section
To determine which section a specific man page belongs to, look at the numeric value on the first line of the output of the man page. For example, if you execute the command man cal, you will see that the cal command belongs to the first section of man pages:
2. Sepcifying a Section
In some cases you will need to specify the section in order to display the correct man page. This is necessary because sometimes there will be man pages with the same name in different sections.
For example, there is a command called passwd that allows you to change your password. There is also a file called passwd that stores account information. Both the command and the file have a man page.
The passwd command is a "user" command, so the command man passwd will display the man page for the passwd command by default:
To specify a different section, provide the number of the section as the first argument of the man command. For example, the command man 5 passwd will look for the passwd man page just in section 5:
3. Searching Sections
Sometimes it isn‘t clear what section a man page is stored in. In cases like this, you can search for a man page by name.
The -f option to the man command will display man pages that match, or partially match, a specific name and provide a brief description of each man page:
Note that on most Linux distributions, the whatis command does the same thing as man -f. On those distributions, both will produce the same output.
4. Searching man Pages by Keyword
Unfortunately, you won‘t always remember the exact name of the man page that you want to view. In these cases you can search for man pages that match a keyword by using the -k option to the man command.
For example, what if you knew you wanted a man page that displays how to change your password, but you didn‘t remember the exact name? You could run the command man -k password:
When you use this option, you may end up with a large amount of output. The preceding command, for example, provided over 60 results.
Recall that there are thousands of man pages, so when you search for a keyword, be as specific as possible. Using a generic word, such as "the" could result in hundreds or even thousands of results.
Note that on most Linux distributions, the apropos command does the same thing as man -k. On those distributions, both will produce the same output.
Part 2: info Command
Man pages are great sources of information, but they do tend to have a few disadvantages. One example of a disadvantage is that each man page is a separate document, not related to any other man page. While some man pages have a SEE ALSO section that may refer to other man pages, they really tend to be unrelated sources of documentation.
The info command also provides documentation on operating system commands and features. The goal of this command is slightly different from man pages: to provide a documentation resource that provides a logical organizational structure, making reading documentation easier.
Within info documents, information is broken down into categories that work much like a table of contents that you would find in a book. Hyperlinks are provided to pages with information on individual topics for a specific command or feature. In fact, all of the documentation is merged into a single "book" in which you can go to the top level of documentation and view the table of contents representing all of the documentation available.
Another advantage of info over man pages is that the writing style of info documents is typically more conducive to learning a topic. Consider man pages to be more of a reference resource and info documents to be more of a learning guide.