Project 1: Implementing a Simple Shell

(c) 1997, Howard E. Motteler; Modified by Gary L. Burt, 1998, 1999, 2000, 2001

Assigned: 3 April 2001
Due:8 May 2001

Project Goals

The Project

Shell

You are to design and implement a simple shell, "mysh". Some of the commands of this shell are to be internal and some are to external. The internal commands for project will be the command to list the directory (the UNIX "ls" command), the change directory command (cd) and the "exit" command).

The following is a prototype of what your main might look like:

while ( TRUE )					/* repeat forever           */
{
    read_command( command, parameters);	/* read input from terminal */

    if ( fork( ) != 0 )				/* fork off child process   */
    {
	  /* Parent Code */
	  waitpid( -1, &status, 0 )		/* wait for child to exit   */
    }
    else
    {
	  /* Child code */
        execve( command, parameters, 0 ); /* execute the command	    */
    }
}

Pipe and Sockets

On the client side, mysh will create a pipe and execute the program client-server. The client-server will wait until it receives a command on the pipe. Commands that it will be expected to handle are:

1. remoteput
2. remoteget
3. remotels

Sample of the socket code:

Specifications

cmd
cmd > file
cmd < file
cmd < file > file
cmd | cmd
cmd ; cmd
!!
!nr

where:

• "file" is any valid UNIX filename.
• "cmd" is a program name followed by zero or more arguments.
• There will no wildcards, no in-line command evaluation, no macro substitutions, and commands and filenames which does not include the symbols ">", "<", ";" or "|".
• There can be any of commands separated by the "|" or ";" tokens.
• There are internal and external commands. The external commands will by the ones you use fork and exec and get existing programs to execute. Internal commands will be handled by the program mysh by code that you must write:
  1. When "cmd" is exit, your shell should terminate.
  2. For the special case when "cmd" is "ls", ls -i, ls -l, or li -li your shell will list the files for the directory given in the "file" argument. When there is no "file" specified, you are to use all of the current working directory. For the -i option, you are to list the filename(s) and the inode number from the directory:
     1267 foo
     For the -l option, you are to display the filename(s) and the permissions of the file(s) in the format that is used on UNIX: -rwxrw-r--
     -rwxrw-r-- foo
     The combined version would be:
     -rwxrw-r-- 1267 foo
     You will find this information in the directory entry and the assocated inode struction.
  3. When the "cmd" is cd, your mysh must make the necessary system call in order for the cd to work correctly.
  4. When the "cmd" is remoteput, it will be followed with a filename (using either an absolute or relative pathname). mysh will send the file's contents to the socket-client via a pipe. Then the socket-client will transfer the contents to the socket-server. The socket-server will will save the contents to a file with the same name.
  5. When the "cmd" is remoteget and a filename, mysh will send the name to the socket-client, which will request the file from the socket-server. The socket-client will save the file's contents in the current working directory.
  6. When the "cmd" is remotels, mysh will request the a listing of the directory on the other side of the socket. It will be assumed that all work will be done in only one directory on the socket-server.
  7. When the "cmd" is "restartclient", mysh will execute the socket-client software.

  You must provide error-handling for all internal commands, similiar to how Linux does it. (You can let Linux message print out, so you don't have to do things!) Transfer to data must be coordinated using a semaphore!

• Your shell should also be able to parse and execute command lines above ending with an "&" that puts the specified process or processes into the background. (With multiple commands, only put put the last command into the backgroud.)
• You can assume that all command lines are of the form described here, and you do not have to check for any other sorts of input. In particular, you do not have to expand filename wildcards.
• Use fork() and either execl(), execve() or execvp() to execute the commands. The man difference is that execvp() will search your PATH for the specified executable, and doesn't have an explicit environment argument. Additional information about these calls is available from the man pages.
• Your shell should prompt the user for each command line with the string "mysh[n>". The number n is the number of the command )(for the history version).
• Your shell will allow two forms of the history command. The first is the double explanation points ("bang-bang") which will repeat the last line of input that was typed in. The second version is the explanation point with a number ("bang-n") . The number is a command from the history buffer. You will be required to keep the last fifty commands in the buffer, and know the history number of each command.
• The final version you submit should not print any test messages or other extraneous jabber. You should print errors returned by execvp() or execve(); you can use strerror(errno) to make your error reporting more informative; errno is an external variable set to the most recent error, and strerror() translates this to a human-readable string. You may find the UNIX string manipulation library routines (such as strtok) convenient for parsing the command line; do a "man string" to learn more about these functions.
• To redirect input from a file to a program, so the file appears as "standard input" to the program (e.g., as in the command " wc < test.dat"), you will have:
  1. to manipulate file descriptors. To connect a file to the UNIX "standard input" (by convention file descriptor 0), you can use the dup2()system call, as in the following example.

     
     
     if (( fd = open ( fname, O_RDONLY )) == -1 )
     {
         fprintf( stderr, "mysh error: can't open %s\n", fname);
         exit(1);
     }
     
     dup2(fd, 0);
     close(fd);
     
     if(execvp(cmd, args == -1 )
     {
         fprintf(stderr, "mysh error: %s\n", strerror(errno) );
     }
     

  2. OR you can use a real pipe (see system call for pipe).

• If we wanted to execute the command "wc -l", "cmd" in the above example would be the string "wc", and the string array "args" would have its first element pointing to "wc", its second element pointing to "-l", and its third element the null pointer 0, indicating the end of the arg array.
• The "&" function is easy to implement--without the "&", the shell waits for the child process; with the "&" it does not wait.
• The pipe function "|" is a little more difficult, and can be implemented with either a temporary file, or preferably with a real UNIX pipe.
• The ";" is used to separate commands on the command line. Process the first part independently and when it is finished, then process the second part.

Grading

Do not use the system() system call or invoke the UNIX shell to implement your shell!

You must do the project described here. Doing some other similar or dissimilar project, matter how difficult or clever, may be worth 0 points. This is not a group project; please do your own work, and be careful about sharing your code. It is OK to discuss design issues, but in your documentation, you should give credit to your sources.

You will be provided with a copy of the test plan that the TAs will use when the grade this project, so that you will know exactly how they will test this project.

You must have a makefile and a README file. You are to provide all necessary instructions on how to build and execute your programs in the README file, so that the TAs can do the best job of building and testing your system. Anything that you feel will help the TA should be included.

What to turn in

If your code is in more than one file, you will have to a makefile. For help with a makefile, see the TA.

Hints and Tips

• To get started, you should read the man pages for fork, execl, execve, execvp, dup, wait, pip, getpid and any string functions you might want to use. ("man string" will bring up a summary page on the basic string functions.)
• The functions execve and execvp take an array of strings as their arguments. This array is similar to the argv[] array passed to C programs. When you create this array, it is important that the end of the array is marked with a null pointer (Not a null string!); for example if there are k string pointers in your array args, you would set args[k]="\0".
• You will probably need the following "include" files.
  
  

          #include <stdio.h>
          #include <unistd.h>
          #include <sys/types.h>
          #include <sys/stat.h>
          #include <fcntl.h>
          #include <string.h>
          #include <stdlib.h>
          #include <errno.h>
          #include <netinet/in.h>
          #include <sys/socket.h>
          #include <sys/wait.h>
          #include <netdb.h>
          #include <sys/ipc.h>
          #include <sys/sem.h>
          #include <sys/shm.h>
        

• You should develop your code in a series of stages:
1. write and test a parser to read the command lines;
2. get the simple commands to work;
3. get the history portion to work;
4. get "&" to work;
5. get I/O redirection to work;
6. get the cd command to work;
7. get pipes to work.
• When working with a file version of a pipe, you will have to capture the output from the first command in a temporary file. You can store the file in the directory /tmp, which is for that purpose. However, you will have to make sure that your filename is unique. Since you might have a file in that directory left over from an earlier session, so you need a way to guarantee a unique filename. You can use your login ID plus the PID of the current process. Also remember to remove the file when it is no longer needed!

  When working with the system pipe, remember to remove the pipe when terminates (whether it was intended or not).

• This project is to be Linux-only. It should run on all versions of UNIX, but there is not enough time to worry about it.
• This is a non-trivial program. Do not wait to start implementation because you will not have enough time if you do.

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