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Using Commands' Built-in Help

The most common way to learn about a command is to read its built-in help. Many commands support an option that displays information about the command, and almost all commands display a usage message (saying how they are best used) if they are invoked with improper arguments or options.

Built-in help is terse, often consisting only of a usage message using the format shown in Figure 3.2. Still, it is easily available and often reminds you of the available options and required arguments.

There isn't a consistent way to get built-in help from commands, but there are several ways that work.

To see the built-in help from a command:

  1. Try invoking the command with the --help option (that's two hyphens). For example, try softwareupdate --help Some commands (like softwareupdate) provide an extensive listing of available options (see Figure 3.4 for a partial listing from the ssh command).

    Example 3.4. Some commands (like softwareupdate) give an extensive listing of available options; this is the result when you type softwareupdate --help.

    user-vc8f9gd:~ vanilla$ softwareupdate --help
    usage: softwareupdate <mode> [<args> ...]
           -l | --list             List all appropriate updates
           -d | --download         Download Only
           -i | --install          Install
                     <label> ...   specific updates
                     -a | --all              all appropriate updates
                     -r | --recommended      only recommended updates
                     -u | --url <url> ...    from signed package URLs
           Per-user preferences:
           --ignore <label> ...         Ignore specific updates
           --reset-ignored               Clear all ignored updates
           --schedule (on | off)         Set automatic checking
    
           -h | --help         Print this help
    user-vc8f9gd:~ vanilla$
    For some commands, the --help option is not valid, and for others it looks like a valid option but doesn't provide any help.
  2. In some cases, using a single hyphen gives something useful: ssh -help Even though -help is not a valid option to the ssh command, it still gives you some insight. Most commands give a "usage" message when invoked with an invalid option or without a required argument. Figure 3.5 shows the output from ssh when invoked with the -help option.

    Example 3.5. What you get when you type ssh -help.

    user-vc8f9gd:~ vanilla$ ssh -help
    ssh: illegal option — h
    usage: ssh  [-1246AaCfghkNnqsTtVvXxY] [-b bind_address] [-c cipher_spec]
                [-D port] [-e escape_char] [-F configfile] [-i identity_file]
                [-L port:host:hostport] [-l login_name] [-m mac_spec] [-o option]
                [-p port] [-R port:host:hostport] [user@]hostname [command]
    user-vc8f9gd:~ vanilla$
  3. In some cases the -h option will produce a help message. The man command is an example of a command that will give you a help message if you invoke it with the -h option: man -h
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