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Homework 3
Due: 11:59 PM, April 2, 2013 Reminders: Follow the submission instructions. Remember to comment your code! Total Points: 200 1) Your Shell. (200 pts) Shells are programs that provide a command line interface. Writing your own shell is a classic C exercise that serves as a good introduction to the C library, the system call layer, and the underlying operating system's process management. In this exercise, you will write your own shell. Your shell should have the following features: o maintain a history of commands (up to the last 100, then age off old ones). o implement the following built-in commands: 1) pwd --- use the getcwd() function to print the working directory 2) cd --- use chdir() to change directory to the target. If the target is the character '-', change back to the previous working directory. If the target is '.', do not change directory. If the target is '..' change directory to the parent directory (if it exists). 3) history --- print the history of commands. For example, history 1 ls 2 cd tmp 3 cat my.txt 4 vim my.txt ... o fork and execute a child process (a command) 1) If the user enters an absolute path (command name starts with /), then your shell should try to execute it. 2) If user enters a a command name alone (no forward slashes), then your shell should check these directories in the following order and execute the command if found: /bin /usr/bin /sbin /usr/sbin 3) If user enters a command with a relative path (does not start with a /, but contains one inside the path), then your shell should check the path and should try to execute it from the current directory. For example: ./myprog local/bin/path/myprog If you have an executable, myprog, under /home/user/bin, where you also hapenned to be, then giving myprog should fail as not found, whereas ./myprog should work as normal. o implement !-based shell command recall (e.g., typing !20 at the prompt executes the 20th command in the history, if it exists. This feature should handle events that don't exist gracefully) o add the following built-in commands: 1) pushd --- push current directory on a stack 2) popd --- pop this directory from the stack and chdir to it. 3) dirs --- print the directory stack This assignment tests your ability to read the C library API documentation, design and organize a medium-size, complex C program, understand a C program's interaction with the OS, and design and build data structures (i.e., a stack) in C using structs, unions, and typedefs. This exercise also provides ample opportunity to test your program. Note that this assignment invariably results in someone "fork()-bombing" the machine they are developing the shell on (and endless loop where each child process forks() another child process that forks another child, and so on...). Take extreme care to avoid doing this. OS schedulers have gotten better at confining this type of Denial-of-Service attack so that it only affects the user's own slice of CPU time, but on some older OSs, you can still bring the machine to its knees. If you are using a shared infrastructure, keep another terminal window open to kill stray processes (using the Unix 'kill' command) and always 'nice' your shell using 'nice 19 [cmd]'. This is a popular assignment, and it's likely you'll find the code for many shells online. I do not mind if you refer to these pieces of code for general design ideas and program structure. ******If you do so, however, please cite the location of this code and the author of it in your README file.****** Keep in mind that the point of this exercise is to help you understand the interaction between user-level programs and the OS. Copying someone else's shell doesn't really assist you in fullfilling that goal. __________________________________________________________________________ Note: You may want to consider using GNU getopt library for processing command line options.
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