README.md

Q1. Shell

Run

make run

Design

Commands separated by single pipe |

Sample command: ls -l | wc | cat >> out.txt

Commands separated by double or triple pipes (|| or |||)

Sample command: ls -l || wc, cat >> out.txt

In this case, both the wc and cat >> out.txt commands should get an input as the output of ls -l command. Since reads in pipe are destructive, when wc command is run, we first buffer the data in the pipe (output of ls) into a char array. Then, we write that data back to two pipes, one for wc and one for cat >> out.txt. The figure below depicts the implementation. Sample command: ls -l | wc | cat >> out.txt

Features

• For each command, shell creates a new process group and gives foreground control to that group. Upon completion of the command, the foreground control is given back to the main process.
• Shell also supports background commands. In that case, the command is not given foreground control and continues running in the background.
• Shell supports input and output redirection operations
• Shell supports pipelining (|, ||, and |||)
• Shell supports adding, deleting and running a short-cut command. While insertion, if a short-cut command with the index already exists, the shell confirms whether to replace it. The index can be any integer.

Example Commands

• Simple shell commands

    shell> ls
    
    shell> pwd
  

• Piped commands

    shell> ls -l | wc
    
    shell> ls -l ||  grep ^-, grep ^d
    
    shell> ls -l ||| grep ^-, grep ^d, cat >> out.txt
  

• Background commands

    shell> ls -l | wc&
  

• Short-cut commands

    shell> sc -i 1 ls -l | wc
    
    shell> Ctrl+C
    Enter command index: 1
    
    shell> sc -d 1 ls -l | wc
  

• Quitting the shell

    shell> exit
  

Assumptions

For simplicity, several assumptions were made -

• The maximum number of arguments in a command allowed is 20
• The size of each argument should not exceed 30
• Each identifier in a command should be separated by a space. eg: ls | cat >> out.txt is valid but ls | cat>>out.txt is not valid.
• It is assumed that the index of each shortcut command would be unique. Hence, while deleting a short-cut command, only the index is required. eg: both sc -d 1 and sc -d 1 ls | cat are valid commands.

Screenshots

Simple shell commands

Piped commands

Background commands

Short-cut commands