Learning the UNIX Operating System, Fourth Edition

Syntax of UNIX Command Lines

UNIX command lines can be simple, one-word entries like the date command. They can also be more complex: you may need to type more than the command name.

A UNIX command may or may not have arguments. An argument can be an option or a filename. The general format for UNIX commands is:

command option(s) filename(s)

There isn’t a single set of rules for writing UNIX commands and arguments, but you can use these general rules in most cases:

• Enter commands in lowercase.

• Options modify the way in which a command works. Options are often single letters prefixed with a dash (-) and set off by any number of spaces or tabs. Multiple options in one command line can be set off individually (like -a -b), or, in some cases, you can combine them after a single dash (like -ab).

  Some commands, including those on Linux systems, also have options made from complete words or phrases, like --delete or --confirm-delete. When you enter a command, you can use this option style, the single-letter options (which all start with a single dash), or both.

• The argument filename is the name of a file that you want to use. If you don’t enter a filename correctly, you may get the response "filename: no such file or directory” or "filename: cannot open.”

  Some commands, like telnet and who (shown earlier in this chapter), have arguments that aren’t filenames.

• You must type spaces between commands, options, and filenames.

• Options come before filenames.

• Two or more commands can be written ...