September 2, 2004 --- Class 2  --- Hierarchical File System, Special Characters

Activities:

	Introduction to UNIX and hierarchical file system

		UNIX has a hierarical file system.  It is a lot like a
		file cabinet, with subdirectories playing the roles of drawers
		and file playing the role of file folders.  However, in
		UNIX there are more levels in that any subdirectory can have
		a file in it that is itself a subdirectory.

		There is a special directory called the root of the file
		system and denoted by / .  A full path name of a file
		starts with /.  However, when you are typing commands to
		a "shell", or UNIX interpreter, you are considered to be
		"in" a particular directory.  The command pwd stands
		for "print working directory" and will tell you where you
		are.  You can refer to a file with a relative path name
		by starting the name with a subdirectory of your
		current working directory.  You should also know that
		~ stands for your home directory in most shells, and
		that ~username stands for the home directory of user
		whose login name is username.  Thus, ~sg/bin stands for
		my bin subdirectory.

		Commands relevant to directories:
		pwd 	print working directory
		cd	change directory
		mkdir	make a new directory

		The ls command stands for list files.  Explore what it does
		by trying the following commands

		ls
		ls /
		ls -a
		ls -l
		ls -lt

		To find out more about ls, use the manual command
		man ls

		Special directory names

		.	stands for the current directory
		..	stands for one level up in the directory
			hierarchy

	Special or Metacharacters in UNIX
		UNIX has a number of characters that are special to the
		command interpreter, or shell.
		The following characters are used in filename matching:
		* ? [ ] - { } ,

		* matches any number of characters, so 
		ls m* 
		lists all files that start with the letter m.  UNIX is
		case sensitive.  So ls M* is not the same.

		? matches any single character, so
		ls m?
		lists only files whose names are two characters long and
		begin with m.
		ls m??
		would match files whose names are three characters long and 
		begin with m.

		[ and ] are used to make a list of single characters.
		[abf] matches any of the characters a, b or f.
		[a-j] matches the range of characters a, b, ... j.
		[0-9] matches any digit.
		For example,
		ls *.[cf]  
		would match any file that ends in .c or .f, which is usually
		how we name C and Fortran programs, respectively.

		{ and } are used to make a list of multicharacter matches.
		For example,
		{abc,red,green}  will match the three strings "abc",
		"red" or "green".

		It is also possible to get the ls command to list all files
		that do not match a certain pattern.  We used the man page
		to try to do this, but it did not seem to work during class.
		After class, on a Linux machine I found that either of these 
		forms works:

		ls -I "*.c"
		ls --ignore="*.c"

		
		The double quotes keep the expression with the metacharacter
		from being expanded.  I have also verified that they work 
		on the Suns.

		There are other special characters in UNIX that do things
		other than match characters.

		$ is used to denote a variable name.  For example there is
		a path variable.  Try the following examples

		echo path
		echo $path

		In the second line we get the path variable, not just the
		string path.

		; is used to separate two commands on a single line, e.g.
		echo $path; date

		\ at the end of the line is used for line coninuation if
		you have a long command.  The carriage return must immediately
		follow the \.  \ is also used to escape the special meaning
		of a character.  Try this

		echo \$path

		/ we have already seen is used for separating (sub)directory
		names, and for root

		~ is used for home directories

		( ) are used in pairs to combine the output of two UNIX
		commands.  This will be explained more when we need it.

		& at the end of a command line puts the command in the
		background so that you immediately get back a prompt and
		can issue another command.  The command in the background
		continues to run, however.

		| < > are very useful as we will now explain.