␡
- Using the Unix Manual
- Printing man Pages
- Using Commands' Built-in Help
- Using the Web to Get Help
- Getting Help from Other People
This chapter is from the book
This chapter is from the book
This chapter is from the book
This chapter is from the book
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.
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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
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$For some commands, the --help option is not valid, and for others it looks like a valid option but doesn't provide any help. - In some cases, using a single hyphen gives something useful:
ssh -help
Even though -help is not a valid option to the ssh command, it still gives you some insight. Most commands give a "usage" message when invoked with an invalid option or without a required argument.
Figure 3.5
shows the output from ssh when invoked with the -help option.
Example 3.5. What you get when you type ssh -help.
user-vc8f9gd:~ vanilla$ 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$ - 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