Program A (xssh2)

o Features ( Basic capabilities )

  - Evaluate (execute) commands located in standard places (execvp)
  - Evaluate scripts
    + Environment
    + Variables
  - Concurrent command execution
  - Redirect stdin and stdout

o Usage

	xssh2 [-x] [-d DebugLevel] [Filename]

	-x		The command line after variable substitution should
			  be displayed before the command is evaluated.
	-d DebugLevel	Output debug messages
			  0: don't output (default)
			  1: ...
	Filename	Input is from the file instead of stdin

o Informal Semantics of Simple Cases

    Non-builtin commands
	A Unix executable that can be found in a directory listed in the PATH
	  environment variable or is an absolute pathname
    Built-in Commands (W1, W2, ... stand for Word1, Word2, ...)
	chdir		Change the current directory to $HOME
	chdir W1	Change the current directory to word W1
	echo W1 W2 ...	Display the words W1, W2, ... followed by a newline.
			  Multiple spaces/tabs should be reduced to a single
			  space.
	export		Display the environment variables
	export W1 W2 ...  Export the variables W1, W2, ... to the environment
	quit		Quit the shell with the exit status set to that of
			  the previous command executed
	quit W		Quit the shell with an exit status of W
	set		Display the contents of the variable table
	set W1		Set the variable W1 to the empty string
	set W1 W2 ...	Set the variable W1 to the concatenation of words
	  		  W2, W3, ...
	sleep W1	Pause for W1 seconds
	wait		Wait for ANY child process to terminate
	wait W1		Wait for process W1 to terminate

o Complex Constructs (C stands for Command)

	C ;		Sequential execution; C terminates before proceding
	C &		Parallel execution; exec C in the background and
			  continue processing commands
	C ,		Sequential background execution; exec C in the
			  background, but any background command following
			  C can not start until C terminates

o Redirection (C stands for Command and F stands for File)

	C < F1 > F2	Redirect stdin to the file F1 and stdout to the file
			  F2.  Both redirections are optional; e.g.,
			  "C > F" means redirect stdout to F but do not
			  redirect stdin.


<< Other Features >>

o A newline character is replaced with ';'.
o A sequence of complex commands on one line has equal precedence; e.g.,
    C1 ; C2 ; C3 ;	Run C1, C2, and C3 sequentially in the foreground
    C1 & C2 & C3 &	Run C1, C2, and C3 concurrently in the background
    C1 , C2 , C3 ,	Run C1, C2, and C3 sequentially in the background
    C1 ; C2 & C3 ;	Run C1 in the foreground, C2 in the background and
    			  C3 in the foreground concurrent with C2; i.e.,
			  C3 doesn't need to wait for C2 to terminate.
    C1 & C2 , C3 , C4 ;	Run C1, C2 and C3 in the background, but C3 can not
    			  start running until C2 has terminated.  Run C4
			  in the foreground after starting C1 in the
			  background.
    In each case, the command can contain stdin/stdout redirection.
o Multiple spaces/tabs are reduced to a single space during the substitution
    and line scanning phase.
o The command line prompt should be the three character sequence '>> ' (i.e.,
    >, >, space).
o A child process inherits all of its parent's environment variables.
o '$' as the first character of a word signifies variable substitution; e.g.,
    $XY$Z means "the value of the variable XY$Z".
o All undefined variables have a value of the null string.
o The '#' character signifies the beginning of a comment.  All other
    characters following and including the '#' should be ignored during
    interpretation.
o Blank lines should be ignored.

<< Special Variables >>

  $$	PID of this shell
  $?	Decimal value returned by last command executed
  $!	PID of last background command

<< Implementation Notes >>

o You are not allowed to use the library function system(3).


<< Implementation Sketch >>

o -x command line flag

	Already recognized in xsshA.c
	Need to implement do_subst() to populate xword[]

o -d DebugLevel

	Modify DEBUG macro to handle DebugLevel

o "xssh2 Filename"

	Read input from file until EOF
	Temporarily redirect stdin to read from Filename	[dup2]

o Read-Eval Loop

	Read line from stdin until EOF or quit command {
	  word[] = Parse into words;
	  xword[] = Do variable substitution;
	  if (builtin command)	Eval builtin command;
	  else			Eval non-builtin command;
	}

o Non-builtin commands

	Lift code from HW3
	Need exit status of commands				[waitpid]

o chdir

	Remember current working directory is $HOME
	Need $HOME from environment				[getenv,putenv]
	Change kernel's understanding of current working
	  directory is $HOME					[chdir]

o chdir W1

	Same as "chdir", but need to know current directory.	[getcwd]

o echo W1 W2 ...

	Lift code from HW1

o export

	Walk through environ(5) and display (see lecture notes)	[environ]

o export W1 W2 ...

	Update environment					[setenv]

o quit

	Keep track of exit status of each command
	Lift code from HW3

o quit W

	Set exit status
	Lift code from HW3

o set

	Walk through symbol table displaying contents		[map]

o set W1

	Set variable W1 to ""					[map]

o set W1 W2 ...

	Set variable W1 to concatenation of W2 ...		[map,strcat]

o sleep W1

	Release CPU for W1 seconds				[sleep]

o wait

	Wait for any child to terminate				[waitpid]
	[ waitpid(-1, ...) ]
	How to set exit status					???

o wait W1

	Same as "wait" except wait for specific PID

o C ;

	Sequential execution					[fork,execvp]
	Wait on child to terminate				[waitpid]

o C &

	Run C in background but don't wait on child		[fork,execvp]

o C ,

	Run C in background but if next command is another
	  sequential background command, wait on C to
	  terminate before execing
	Scan ahead and continue to process commands that can
	  be run

o C < F1 > F2

	Redirect stdin/stdout before execing			[dup2]

o List of Commands

	Parse xword[] into commands (yword[]???)