Homework 5
Assigned 29 Nov
Due 13 Dec (the last day of class)

Expand the shell you wrote in HW4 with additional features.  I insist on
the first three.  You may choose which of the others to try for extra
credit.  I don't expect anybody to finish more than a few of them.  In
any case, tell me clearly in writing which of them you've done and want
me to grade.

(1) If your program for HW4 doesn't do everything required for HW4, fix it.
(2) In the assignment file for HW4, I gave the example
	--> setenv PROMPT="yes? "
	yes? printenv PROMPT
	yes? 
This requires that the user be able to use quotation marks to override
the normal handling of spaces (and perhaps other special characters
too).  If you didn't do this for HW4, do it now.
(3) Allow the user to run programs with input and/or output redirected,
using "<" and ">" as in csh.

(4) Allow the user to execute multiple commands on one line, using ";"
as in csh.
(5) Allow the user to pipe one command into another, using "|" as in
csh.  (Note that if you do both this and (4), you'll need to decide
what mixes like "cmd1 ; cmd2 | cmd3 ; cmd4 | cmd5 " should mean.)
(6) Allow the user to run programs without waiting for them to finish,
using "&" as in csh.  (Again, you'll need to decide precedence issues.)
(7) Allow the user to specify his/her own home directory easily, using
"~" as in csh.
(8) Allow the user to specify other users' home directories easily,
using "~username" as in csh.
(9) Allow the user to expand environment variables in the middle of a
command line, using "$variablename" as in csh.
(10) Allow the user to repeat the previous command, using "!!" as in csh.
(11) Allow the user to repeat a previous command that started with a
particular string, using "!string" as in csh.
(12) Allow the user to replace part of a previous command with something
else, using "^old^new" as in csh.
(13) Make your shell capable of running shell scripts nicely (i.e. taking
"#" to indicate a comment, not printing prompts during a script, etc.).
You can run a shell script either by typing "myshell <script" or by
making the first line of the script "#!" followed by the complete
pathname of your shell, e.g. "#!/users/staff/math/sbloch/bin/myshell".
(14) Implement "if...then...else".  You may use one or both of the csh
syntaxes, or the sh syntax, or a reasonable syntax of your own; provide
documentation explaining clearly what the syntax is.
(15) Implement "while...do...".  See (14).
(16) Implement globbing, using "*" and "?" as in csh.  For example, if
the user types "ls *.c", and the current directory includes the files
"that.c", "this.c", and "this.h", your shell should replace the command
with "ls that.c this.c" before executing it.  
(17) Suggest and implement other neat things that ought to be in a shell
(perhaps things that AREN'T in any of the existing shells!)
(18) Implement a "help" command, or write an "info" file, explaining to
a user what your shell does.  I'll grade this primarily on the clarity of
your writing.

Note: If you try to implement semicolons AND redirection AND pipes AND
detached processes, Lex may not be powerful enough to keep straight
which one has precedence over the others, and you may need to use Yacc
(or its GNU version, bison) as well.