CS 332: Operating Systems

A little command shell

Due 11:10 AM Wednesday, 4/28. Hand in via the Courses system.

Write a small command shell. In particular, your shell should:

1. Print a prompt.
2. Get a command line from the user. The command line may include redirections (<, >), pipes (|), and possibly an ampersand (&) at the end of the line.
3. Search the $PATH environment variable for a program whose name is the first word in the command string, and fork a child process to execute the first such program found, passing the command-line arguments (including the program name itself) to the program (or printing an error message if no command is found). (If the command string includes <, >, or |, the execution of the command will be more complex than this, but analogous.)
4. wait for the execution to finish (unless the command was terminated with an &, in which case you should go to step 5 immediately).
5. Go to step 1.

Your shell should also trap Ctrl-C's. Upon receiving a SIGINT, it should tell the user to type Ctrl-] followed by <return> to quit your shell.

You may work with a partner or alone, as you wish.

Here are a few technical points that may interest you:

• Typing Ctrl-] sends a character with ASCII value 29. This character, known as "GS" or "Group Separator," goes along with "FS" (28, Ctrl-\, "File Separator"), "RS" (30, Ctrl-^, "Record Separator"), and "US" (31, Ctrl-_, "Unit Separator"). The ASCII standard says "These information separators may be used with data in optional fashion, except that their hierarchical relationship shall be: FS as the most inclusive, then GS, then RS, and US as least inclusive [is this a political statement?]. The content and length of a file, group, record, or unit are not specified." So in other words, nobody uses these characters anymore, except when they need a character that nobody uses anymore.
• "man execv" will get you a started using exec. The "v" in "execv" stands for "vector," which means that when you call execv(), you give it a pre-allocated argv of command-line argument strings. The "p" in "execvp" stands for "path," which means that "execvp" looks for the file-to-be-executed in all the directories specified in your PATH environment variable. To see what your PATH is, type "echo $PATH" at the Unix prompt. If you want to look all over the place for the file-to-be-executed, use execvp(). If you only want to look in the current directory, use execv().

Important constraints

I would appreciate your assistance in making this program logistically easy to grade. Following these rules will be worth one point.

1. Write your program in a single source file named shell.c or shell.cpp (all lower case).
2. If you need to submit a revised version of your program, call it shell1.c. A revision of the revision should be called shell2.c, etc.
3. Don't submit a folder for this assignment--just shell.c. If you have "readme" information to communicate to me, just put it in the comment at the top of shell.c.