CS 497C - Introduction to UNIX
Lecture 4: Understanding the UNIX Command
Chin-Chih Chang chang@cs.twsu.edu

The PATH: Locating Commands
* UNIX obtains the list of directories that has to be searched from of an 
  environment variable – PATH.
* If you evaluate the value of PATH, you'll find a directory list separated 
  by colons:
$ echo $PATH
/bin:/usr/bin:/usr/X11R6/bin:/usr/local/bin:.

The PATH: Locating Commands
* There are five directories in this list, and when you issue a command, the 
  system will search this list in the sequence specified to locate and 
  execute it.
* . indicates the current directory.
* To know in which directory the command is located, you can use the type 
  command.
$ type ls
ls is /bin/ls

Internal and External Commands
* The agency that actually does all this work is known as shell - a command 
  interpreter.
* Since ls is a file having an independent existence in the /bin directory 
  (or /usr/bin), it is called an external command. Most commands are external 
  in nature.
* The commands built in shell are known as internal commands.

Internal and External Commands
* The type command itself is a shell built-in. Whether or not you are able to 
  execute it depends on the shell you use.
* In some versions of the C shell, the type command won’t work. In this case, 
  which or whereis can be used.
* The shell uses its internal command even though there is an external command 
  of the same name.

Command Structure
* An UNIX command is composed of two parts: command and argument.
* Commands and arguments are separated by any number of spaces or tabs know as 
  whitespace.
$ ls-l
ls-l: Command not found.
* The argument that begins with a - symbol is  called an option.

Options and Filenames
* Every command has a fixed set of options.
* The command with its arguments and options is entered in one line that is 
  referred to as the command line.
* Some commands accept a single filename, some accept more than one filenames.
* Wrong options will be indicated.
* An option can have its own arguments.
* pine -f mail-may

Combining Options
* Options that begin with a – sign can normally be combined with only one - 
  sign.
* For instance, this command has three options:
ls -l -a -t
* The -l option provides most details of a file's attributes. The -t option 
  sorts files according to the time of modification. The -a option lists 
  the hidden files.

Combining Options
* You can combine these options in this way:
ls -lat
ls -atl
* This facility reduces your typing load. The shell parses (break up) the 
  option combination into separate options.
* Some commands won't let you combine options in the way you did just now.
tar -cv -f /dev/fd0 -b 18

Combining Options
* There are four options here, but two of them (-f and -b) has their own 
  arguments. These arguments are called option parameters.
* We combined the -c and -v options here. We can combine the -f and -b 
  options, provided that their own parameters are placed in the same sequence.
tar -cvfb /dev/fd0 18
* The following one won't work:
tar -cvfb 18 /dev/fd0

Exceptions and Variations
* All commands don’t compulsorily use options and arguments.
* Commands like clear don't accept any argument.
* The who and date commands may or may not be specified with arguments.

Exceptions and Variations
* The ls command permits more variations:
 - Without any argument (ls)
 - With only one option (ls -l)
 - With only filenames (ls chap01 chap02)
 - Using a combination of both (ls -la chap01 chap02)
 - This text discusses commands that use options. The vast majority will 
   conform to the option rules that have been discussed but there are some 
   exceptions.

Flexibility of Command Usage
* UNIX allows you to specify more than one command in the same command line. 
  Each command is separated from the other by a ; (semicolon):
who; ls -l
* The ; is one of special characters that are understood by the shell. These 
  special characters are known as metacharacters.

A Command Line Can Overflow
* When a command has more characters than the terminal width, it simply 
  overflows to the next line.
* You may want to spread the command into multiple lines. The shell issues a 
  secondary prompt, usually > (or ?).
$ echo "This is a two-line
> text message"
In C shell you have to enter a \ (backslash).

Flexibility of Command Usage
* Subsequent commands can be entered at the keyboard without waiting for the 
  prompt. There is a type-ahead buffer (a temporary storage area) which stores 
  all these commands and passes them on for execution.
* A command may not behave in the way you want because UNIX comes in many 
  flavors.

Flexibility of Command Usage
* The uname can print system information.
$ uname
Linux
The uname -r command shows the version number of the kernel. 
$ uname -r
2.2.19