Unix Command Basics

A Unix command is built from a command name and optionally one or more command line arguments. Arguments can be either flags or data.

ls -a -l /etc /var
^^ ^^^^^ ^^^^^^^^^
|   |    |
|   |    Data Arguments
|   Flags
Command name

For many Unix commands, the flags must come before the data arguments. A few commands require that certain flags appear in a specific order. Some commands allow flags and data arguments to appear in any order. Unix systems do not enforce any rules regarding arguments. How they behave is entirely up to the programmer writing the command. However, the vast majority of commands follow conventions, so there is a great deal of consistency in Unix command syntax.

The components of a Unix command are separated by white space (space or tab characters). Hence, if an argument contains any white space, it must be enclosed in quotes (single or double) so that it will not be interpreted as multiple separate arguments.

Example 3.4. White space in an Argument

Suppose you have a directory called My Programs, and you want to see what's in it. You might try the following:

shell-prompt: ls My Programs

The above command fails because "My" and "Programs" are interpreted as two separate arguments. The ls command will look for two separate files or directories called "My" and "Programs". In this case, we must use quotes to bind the parts of the directory name together into a single argument. Either single or double quotes are accepted by all common Unix shells. The difference between single and double quotes is covered in Chapter 4, Unix Shell Scripting.

shell-prompt: ls 'My Programs'
shell-prompt: ls "My Programs"

As an alternative to using quotes, we can escape the space by preceding it with a backslash ('\') character. This will save one keystroke if there is only one character to be escaped in the text.

shell-prompt: ls My\ Programs

Example 3.5. Practice Break

Try the following commands:

shell-prompt: ls
shell-prompt: ls -al
shell-prompt: ls /etc
shell-prompt: ls -al /etc
shell-prompt: mkdir My Programs
shell-prompt: ls
shell-prompt: rmdir My
shell-prompt: rmdir Programs
shell-prompt: mkdir 'My Programs'
shell-prompt: ls
shell-prompt: ls My Programs
shell-prompt: ls "My Programs"



Be sure to thoroughly review the instructions in Section 2, “Practice Problem Instructions” before doing the practice problems below.
  1. What are the three major components of a Unix command?

  2. What are the two sources of the command name?

  3. How do we know whether an argument is a flag or data?

  4. What is the advantage of short flags and the advantage of long flags?

  5. What do data argument represent?

  6. What rules does Unix enforce regarding the order of arguments?

  7. What separates one Unix argument from the next?

  8. Can an argument contain whitespace? If so, how?