Homework 7 Starting the DASH

Begin constructing a shell.

This homework is worth 20 points.

Goals

When you finish this homework you should:

Established a framework for the next several projects.
Used many of the information gathering techniques discussed in class.

Assignment

This project is the beginning of a series of assignments aimed at constructing DAn's SHell or dash. The purpose of this shell is to experiment with the concepts we have discussed. We will be adding to this so you need to keep your code open and flex able.

This shell will be line oriented. The basic loop will be

while not done
      read a line (command)
      process the line

Parsing the command line is your first challenge. There are many ways you could accomplish this task. In some sense it depends on what you know and how much time you wish to invest in this portion of the project..

A simple solution would include c++ style strings, getline and calls to find. This will require you to write code to mess with skipping white space. This should be a solution that anyone can use however. String streams might be really useful in this case.
A more tools based solution would involve the library call strtok and c style strings. This solution would handle the white space problem, but requires c style strings.
The right way to do it would involve flex and bison, but that is way beyond the scope of this class. I would be happy to help anyone attempting this solution.

We will be parsing input in the following form:

     command: cmd  | cmd redirectl | cmd pipe cmd |  builtin
     cmd: STRING args 
     args : STRING | STRING argl | ε
     pipe: "|"
     redirectl: redirect | redirect redirectl
     redirect: < STRING | > STRING  | ε
     builtin: buitin_s | builtin_a 
     builtin_s: "UID" | "PID" | "PPID" | "NAME" | "WD" | "INFO" | "EXIT" | "UP" | "HOME" | "HD" 
     builtin_a: prompt | run | down | go | set
     prompt: "PROMPT" INTEGER
     run: "RUN" STRING
     down: "DOWN" STRING
     go: "GO" STRING 
     set: "SET" STRING STRING

Note: the grammar above may contain errors and is most certainly incomplete.

For now, you need to deal with the following built in commands:

UID: print the user's uid.
PID: print the process id.
PPID: print the parent process id.
NAME: print the user name from the password file.
WD: print the current working directory
HD: print the user's home directory
INFO: print all of the above.
EXIT: exit the program
HOME: change the current working directory to the users home directory
UP: change the current working directory up one level if possible.
PROMPT n: Change the prompt setting to the integer n.
RUN filename: execute the commands in the named file.
DOWN path: change the current working directory to the sub-directory listed. (Append path to cwd)
GO path: change the current working directory to be path. Path should start with a /.
SET name value: set the value of the environment variable name to be value.

The integer associated with prompt determines how the prompt is displayed. Treat this integer as a set of bit flags, and display the following

Value Value to Display

0 ">"

1 Environment variable PROMPT

2 Current directory basename

4 The first part of the host name (up to the first .)

8 username

These are displayed in descending order. The delimiter between portions is :.Assume that the machine is Mirkwood, the current directory is foo and the user name is bennett, and the environment variable PROMPT is "%". The following prompts would be :

n Prompt

0 >

1 %

2 foo

3 foo:%

5 Mirkwood:%

15 bennett:Mirkwood:foo:%
I will not be adding any features to the prompt, so you may embellish on this if you wish.

Value	Value to Display
0	">"
1	Environment variable PROMPT
2	Current directory basename
4	The first part of the host name (up to the first .)
8	username

n	Prompt
0	>
1	%
2	foo
3	foo:%
5	Mirkwood:%
15	bennett:Mirkwood:foo:%

Your program should recognize the following command line arguments when it is started.

-s or --start starts an instance of dash which attempts to read and execute the commands in ~/.dash/rc
-l or --log starts a log file, with a temporary file filename in ~/.dash/logs. Using an appropriate name, this log file should contain a history of commands executed, including builtins.
-d or --debug runs the shell in a diagnostic mode, printing out which actions are about to occur to the standard output.

Discussion

This should behave like a shell, running silently unless the debug command is given. The following would be an example of running the shell

$ dash
> SET PROMPT " "
> PROMPT 9
bennett: ID
   The process id is: 14234
bennett: NAME
   Your username is: bennett
bennett: WD 
   The working directory is: /home/bennett
bennett: UP
bennett: WD
   The working directory is: /home
bennett: UP
bennett: WD
   The working directory is: /
bennett: HOME
bennett: WD
   The working directory is: /home/bennett
bennett: DOWN foo
bennett: WD
   The working directory is: /home/bennett/foo
bennett: DOWN foo
   Error the directory: /home/bennett/foo/foo does not exist.
bennett: EXIT
$

We will be adding to this program, however, I will not modify any of the above specification unless it contains an error.
You may embellish, but before you make any major changes please consult with me.

Submission

Email a copy of the C code to complete the final task to your instructor by class time. If your code consists of a single file, the source code will be sufficient, if the code is in multiple files, you should submit a tar file containing the source code and a make configuration file required to build the project.