Up until now you have seen a number of commands and their mysterious options and arguments. In this lesson, we will try to remove some of that mystery. This lesson will introduce the following commands.

Commands can be one of 4 different kinds:

It is often useful to know exactly which of the four kinds of commands is being used and Linux provides a couple of ways to find out.

The type command is a shell builtin that displays the kind of command the shell will execute, given a particular command name. It works like this:

 type command

where “command” is the name of the command you want to examine. Here are some examples:

[me@linuxbox me]$ type type
type is a shell builtin
[me@linuxbox me]$ type ls
ls is aliased to `ls --color=tty'
[me@linuxbox me]$ type cp
cp is /bin/cp

Here we see the results for three different commands. Notice that the one for ls (taken from a Fedora system) and how the ls command is actually an alias for the ls command with the “– color=tty” option added. Now we know why the output from ls is displayed in color!

Sometimes there is more than one version of an executable program installed on a system. While this is not very common on desktop systems, it’s not unusual on large servers. To determine the exact location of a given executable, the which command is used:

[me@linuxbox me]$ which ls
/bin/ls

which only works for executable programs, not builtins nor aliases that are substitutes for actual executable programs.

With this knowledge of what a command is, we can now search for the documentation available for each kind of command.

bash has a built-in help facility available for each of the shell builtins. To use it, type “help” followed by the name of the shell builtin. Optionally, you may add the -m option to change the format of the output. For example:

[me@linuxbox me]$ help -m cd

NAME
    cd - Change the shell working directory.

SYNOPSIS
    cd [-L|-P] [dir]

DESCRIPTION
    Change the shell working directory.
    
    Change the current directory to DIR.  The default DIR is the value of the
    HOME shell variable.
    
    The variable CDPATH defines the search path for the directory containing
    DIR.  Alternative directory names in CDPATH are separated by a colon (:).
    A null directory name is the same as the current directory.  If DIR begins
    with a slash (/), then CDPATH is not used.
    
    If the directory is not found, and the shell option `cdable_vars' is set,
    the word is assumed to be  a variable name.  If that variable has a value,
    its value is used for DIR.
    
    Options:
        -L force symbolic links to be followed
        -P use the physical directory structure without following symbolic
     links
    
    The default is to follow symbolic links, as if `-L' were specified.
    
    Exit Status:
    Returns 0 if the directory is changed; non-zero otherwise.

SEE ALSO
    bash(1)

IMPLEMENTATION
    GNU bash, version 4.1.5(1)-release (i486-pc-linux-gnu)
    Copyright (C) 2009 Free Software Foundation, Inc.

A note on notation: When square brackets appear in the description of a command’s syntax, they indicate optional items. A vertical bar character indicates mutually exclusive items. In the case of the cd command above:

 cd [-L|-P] [dir]

This notation says that the command cd may be followed optionally by either a “-L” or a “-P” and further, optionally followed by the argument “dir”.

Many executable programs support a “–help” option that displays a description of the command’s supported syntax and options. For example:

[me@linuxbox me]$ mkdir --help

Usage: mkdir [OPTION] DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.

   -Z, --context=CONTEXT (SELinux) set security context to CONTEXT
Mandatory arguments to long options are mandatory for short options
too.
   -m, --mode=MODE   set file mode (as in chmod), not a=rwx – umask
   -p, --parents     no error if existing, make parent directories as
                     needed
   -v, --verbose     print a message for each created directory
   --help            display this help and exit
   --version         output version information and exit

Some programs don’t support the “–help” option, but try it anyway. Often it results in an error message that will reveal similar usage information.

Most executable programs intended for command line use provide a formal piece of documentation called a manual or man page. A special paging program called man is used to view them. It is used like this:

 man program

where “program” is the name of the command to view. Man pages vary somewhat in format but generally contain a title, a synopsis of the command’s syntax, a description of the command’s purpose, and a listing and description of each of the command’s options. Man pages, however, do not usually include examples, and are intended as a reference, not a tutorial. As an example, let’s try viewing the man pagefor the ls command:

[me@linuxbox me]$ man ls

On most Linux systems, man uses less to display the manual page, so all of the familiar less commands work while displaying the page.

Many software packages installed on your system have documentation files residing in the /usr/share/doc directory. Most of these are stored in plain text format and can be viewed with less. Some of the files are in HTML format and can be viewed with your web browser. You may encounter some files ending with a “.gz” extension. This indicates that they have been compressed with the gzip compression program. The gzip package includes a special version of less called zless that will display the contents of gzip-compressed text files.