help

What is the help command?

The help command is a built-in shell utility that provides concise information about other shell built-in commands. It’s your quick reference guide for understanding the syntax and usage of these internal commands. Crucially, help only works for shell built-ins; it won’t provide information on external commands (like ls, grep, etc.).

How to Use help

The syntax is straightforward:

help <command>

Replace <command> with the name of the shell built-in command you want information on.

Example 1: Getting help on the cd command

The cd command (change directory) is a fundamental shell built-in. Let’s see what help tells us:

help cd

This will display output similar to:

cd: cd [-L|[-P [-e]] [-H]] [dir]
    Change the shell working directory.

    Change the current working directory to DIR.  If DIR is not supplied,
    the value of HOME is used.

    Options:
      -L  If the specified directory is a symbolic link, follow it.  This is
          the default behavior.
      -P  If the specified directory is a symbolic link, do not follow it.
      -e  If DIR does not exist, exit with an error.
      -H  If the specified directory is a symbolic link, follow it if the
          link refers to a directory.  Do not follow it if the link refers
          to a file.  This option is only effective if a directory is
          specified.

This output clearly describes the cd command’s syntax, options, and functionality.

Example 2: Exploring the alias command

The alias command allows you to create shortcuts for longer commands. Let’s use help to understand it better:

help alias

You’ll see information explaining how to create, list, and remove aliases within your shell.

Example 3: Understanding export

The export command is vital for setting environment variables. Using help:

help export

This will show you the correct usage of export to manage environment variables, explaining how they’re inherited by child processes.

Example 4: Handling errors with help

If you try to use help on a command that isn’t a shell built-in, you’ll typically get an error message indicating that the command isn’t found. For example:

help ls 

This will likely return an error similar to help: ls: no such builtin command.

Example 5: Combining help with other commands

You can creatively combine help with other shell features. For instance, to see help for all commands containing “echo” in their description, one could pipe the output of help to grep:

help | grep echo

Using help effectively allows for a quick and easy way to understand the functionality of numerous shell built-ins, making it a tool in any Linux user’s arsenal.