Homework 2

Due Date: November 2, 2000	200 Points

1.	(20 points) Chapter 14, exercise 6

2.	(20 points) Chapter 14, exercise 9

3.	(160 points) This exercise asks you to determine how the various shells access environment variables, and test for 
a potential problem.

a.	Write a program called envalter to add environment variables to an environment and then spawn a subpro-
gram. Your program should take the following arguments:

-b env	add the environment variable env to the beginning of the environment. Env may be an environ-
ment variable name or a name and value (var or var=value, respectively).

-d env	delete all occurrences of the environment variable env from the environment. If env is an environ-
ment variable name, delete all environment variables with that name. If env is a name and value, 
delete only those variables with the given name and value.

-e env	like -b, except the environment variable env is added to the end of the environment.

program	execute the program in the new environment

Hint: use execve(2) to execute the program. Do not use system(3)!

b.	Write a second program called shell that determines whether a given shell uses the first or last search path in 
the environment. This program should take the following arguments:

-f dir	put the first xyzzy program in this directory (if not given, use the directory xyzzy1)

-l dir	put the second xyzzy program in this directory (if not given, use the directory xyzzy2)

shell	use the named shell

Hint: Create two programs called xyzzy. One should print "it's the first" and the other "it's the last". Use the 
program you wrote in part a to delete the current search path, and add two new search paths. The first adds a 
search path containing the directory with the first xyzzy to the front of the environment, and the second adds 
a search path containing the directory with the second xyzzy to the end of the environment. Then spawn a 
shell and see which program is executed.

c.	Bundle your programs into a distribution mechanism that works as follows. After un-taring the program, the 
recipient types "make" to compile (set up) both programs. The recipient can then type "./check shell" where 
shell is the name of a shell (either relative or full path name) and the program will print: one of:
shell: uses the first occurrence of the environment variable
shell: uses the last occurrence of the environment variable