msdx321 / lab5_template

Lab: Shell Design

Intro

The current homework is to implement a shell, msh. Lets play with the shell a little bit, and figure out what's going on.

$ ps aux | grep gparmer | wc -l

Replace your own user-name for gparmer.

Pipeline

Before we jump into the actual exercies, how do you think the previous command got implemented?

• What forks are being done?
• What execs are being done?
• How is the | implemented?
• Where are waits happening?

Coding Exercise

Write code to do the following:

• Parent forks a child process
• Use a pipe to redirect child's stdout to parent's stdin (note, in the shell this would be different)
• Use execvp to execute echo Hello! in the child process
• Do wait to wait for child to finish
• Use scanf in the parent process to read the child's output, and use printf to print it to the screen

Use pipe.c to begin.

Extra Questions

• How does the shell implement background computations? For example, observe the difference between $ sleep 2 ; ls and $ sleep 2 & ls ($ sleep 2 & ; ls in msh's syntax).

Parsing

Module 0 of the shell requires parsing the input strings.

Coding Exercise

See the parsing.c file for sample code that parsing a command and its argument using strtok. Write code to print out the following:

• Print the commands in ls -l ~ | grep msh | wc -l.

Example outputs:

Parsing command: ls /bin/ /usr/bin/
    Arguments:
        ls
        /bin/
        /usr/bin/
Parsing command: ls -l ~ | grep msh | wc -l
    Sub-command: ls -l ~
        Arguments:
            ls
            -l
            ~
    Sub-command:  grep msh
        Arguments:
            grep
            msh
    Sub-command:  wc -l
        Arguments:
            wc
            -l