Operating Systems Spring 2024
Please review the general instructions for assignments.
The goals of this project are:
You will write a program called myshell which is capable of executing, managing, and monitoring user level programs. This program will be similar in purpose and design to everyday shells like bash or tcsh, although the syntax will be slightly different. myshell will be invoked without any arguments, and will support several different commands.
Your shell should print out a suitable prompt like myshell> when it is ready to accept input.
It must read a line of input, accepting several possible commands:
The first few command in this list will be “built in” commands in which the shell will do the actual work. The list command should cause the shell to list the contents of the current directory, in the same way as the dirlist program from the previous assignment. Go right ahead and re-use the code that you already wrote, being sure to fix any bugs or problems. For example:
myshell> list
NAME SIZE TYPE MODE OWNER
-----------------------------------------
program.c 3264 B file 0644 dthain
test 32 B dir 0755 dthain
homework.doc 7585 B file 0644 dthain
courses 16 B link 0600 dthain
-----------------------------------------
The chdir command should cause the shell to change its working directory to the named directory:
myshell> chdir /tmp
The pwd command should cause the shell to print the current working directory:
myshell> pwd
/escnfs/home/dthain
The next few commands will focus on creating and managing sub-processes that execute external programs.
The start command should start another program with command line arguments, print out the process number of the running program, and then go back to accepting input. For example:
myshell> start cp data.txt copy.txt
myshell: process 346 started
myshell>
The wait command causes the shell to wait for any child process to exit. When this happens, indicate whether the exit was normal or abnormal, along with the exit code or signal number and name, respectively. Display any errors encountered. For example:
myshell> wait
myshell: process 502 exited normally with status 5
myshell> wait
myshell: process 347 exited abnormally with signal 11: Segmentation fault.
myshell> wait
myshell: No children.
The waitfor command should do the same thing, but waits for a specific child process to exit:
myshell> waitfor 346
myshell: process 346 exited normally with status 0
myshell> waitfor 346
myshell: No such process.
The run command should combine the behavior of start and waitfor. run should start a program, wait for that particular process to finish, and print the exit status. For example:
myshell> run date
Mon Jan 19 11:51:57 EST 2009
myshell: process 348 exited normally with status 0
The kill command should kill a process by taking the pid of a specific child process.
myshell> kill 346
myshell: process 346 has been killed
myshell> kill 346
myshell: unable to kill process 346
After each command completes, your program must continue to print a prompt and accept another line of input. The shell should exit with status zero if the command is quit or exit or the input reaches end-of-file. If the user types a blank line, simply print another prompt and accept a new line of input. If the user types any other command, the shell should print a reasonable error message:
myshell> bargle ls -la
myshell: unknown command: bargle
To keep things simple, your shell doesn’t need to deal with arbitrarily long commands. It must accept input lines of up to 1024 characters, and must handle up to 128 distinct words on a line.
You will need to read the manual pages for the following system and library calls, and possibly others:
fgets, printf, strtok, strsignal, opendir, readdir, closedir, stat, chdir, getwd, fork, execvp, wait, waitpid, kill, exit, signal
Use fgets to read one line of text after printing the prompt. Note that if you printf a prompt that has no newline on the end, it will not immediately display. Call fflush(stdout) to force the output.
Breaking the input line into separate word is a little tricky, but is only a few lines of code once you get it right. Call strtok(line," \t\n") once to obtain the first word, and then strtok(0," \t\n") repeatedly to get the rest, until it returns null. Declare an array of pointers char *words[100], then, for each word found by strtok, save a pointer to the word in words[i]. Keep track of the number of words as nwords, then set words[nwords] = 0; when you have found the last one.
Once you have broken the input line into words, you can check words[0] for the command name, use strcmp to check for string equality and atoi to convert a string to an integer.
Make sure to stop if fgets returns null, indicating end-of-file. This allows you to run myshell and read commands from a file. For example, if you create myscript with the following contents:
start ls
wait
start date
wait
Then, you can run the shell on that input like this:
./myshell < myscript
Make sure to test your program on a wide variety of conditions. Try running multiple programs simultaneously. Create some simple programs that crash or exit with values other than zero, to make sure that wait and run report the correct exit status.
Make sure to carefully handle all possible error conditions. Every system call can fail in a number of ways. You must cleanly handle all possible errors with a reasonable error message, as discussed in Project 1. It is up to you to read the man pages carefully and learn what errors are possible.
Your grade will be based on:
This assignment is due on Friday, January 9th at 11:59PM.
Review the general instructions for assignments.
Please turn in only the source code files myshell.c and a Makefile that builds the executable. Do not turn in executables or other files, since those just take up space, and we will build your code from source anyway.