CS510B Project Part IV

In project part IV, you are required to implement the pipe “|” support.  For simplicity, only 1 pipe is required.  This part of code should be added to the existing code from project part I, II, and III.

Let us look at command “ls | grep fi”.  Two processes should execute this command.  A pipe connects these 2 processes.  One process executes “ls”, and writes the output to the pipe; the other executes “grep fi”, and reads input from the pipe.  They should have parent/child or sibling relationship.  The output of process “ls” is the input of process “grep fi”. 

Processes connected by pipes form a process group.  That process group could be foreground or background.  So foreground/background (“&”) should be handled before pipe (“|”).  By default, a UNIX program reads input from STDIN_FILENO, and writes output to STDOUT_FILENO.  So the process writing output to the pipe should dup pipe_fd[1] (write end of pipe) to STDOUT_FILENO; the process reading input from the pipe should dup pipe_fd[0] (read end of pipe) to STDIN_FILENO.  Your program should also check I/O redirect with pipe.  When a process writes to a pipe, it should not have output redirect (“>”).  When a process reads from a pipe, it should not have input redirect (“<”). 

I prefer parent/child relationship to execute “ls | grep fi”.  Then which process should be the parent?  This is a foreground command.  If we let “ls” executed by parent process, and “grep fi” executed by the child process, “ls” finishes before “grep fi”.  The shell process, which is the parent of “ls”, waits for “ls”.  The shell process returns from wait() and prints the shell prompt before “grep fi” finishes.  Then the output will be like this:

Shell_prompt > ls | grep fi

Shell prompt >

File1 file2 file3

Instead of standard format of a UNIX shell:

Shell_prompt > ls | grep fi

File1 file2 file3

Shell_prompt >

So process executing “grep fi” should the parent and process executing “ls” should be the child of it.  And process for “grep fi” should be child of the shell process.  So the shell process waits for “grep fi” process.  When “grep fi” process finishes, it prints the output. Then shell process prints the shell prompt.  Did you see the advantage using parent/child relationship over sibling relationship?  If sibling processes are used to execute “ls” and “grep fi”, there is no guarantee that shell process returns from wait() after “grep fi” finishes.  The shell process may return from wait() and print the shell prompt before “grep fi” prints the result.

Reference program:

Program 14.1, 14.2 and 14.5

System calls to use:

fork(), pipe(), dup2(), close()

Challenge:

How to implement multiple pipes?  For example, “ls | grep fi | sort –r | tee file”.  Shell process forks process for “tee file”; process for “tee file” forks process for “sort –r”; process for “sort –r” forks process for “grep fi”; process for “grep fi” forks process for “ls”.

Final Project:  mysh4 -- a simulation of C shell
  
LINKS:
Book: Advanced Programming in the UNIX Environment
Links