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.
   1. Write a program called envalter to add environment variables to an environment and then spawn a subprogram. Your program should take the following arguments:
      • -b env add the environment variable env to the beginning of the environment. Env may be an environment 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 environment 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)!
   2. 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.
   3. 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
      

Matt Bishop Office: 3059 Engineering Unit II Phone: +1 (530) 752-8060 Fax: +1 (530) 752-4767 Email: bishop@cs.ucdavis.edu

Copyright Matt Bishop, 2000. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print.