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:
- Try invoking the command with the --help option (that's two hyphens).
For example, try
Some commands (like softwareupdate) provide an extensive listing of available options (see
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$For some commands, the --help option is not valid, and for others it looks like a valid option but doesn't provide any help.
- In some cases, using a single hyphen gives something useful:
Even though -help is not a valid option to the ssh command, it still gives you some insight. Most commands give a "usage" message when invoked with an invalid option or without a required argument.
shows the output from ssh when invoked with the -help option.
Example 3.5. What you get when you type ssh -help.
user-vc8f9gd:~ vanilla$ 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$
- 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