Dash Requirements

Dash Shell has two Modes:
1. Interactive Prompt - "dash> " It allows the user to type commands directly.
2. Batch prompt: > ./dash batch.txt Reads input from a batch file and executes commands from therein.
Dash shell creates a new process for every command, except the built-in commands - cd, exit and path - which are implemented internally.

Shell should support standard output redirection: ls -la /tmp > output

  - Here the standard output of ls program should be redirected to the file output. 
  - The standard error should also be rerouted to the file output.
  - If output file exists before running the program, overwrite it.
  - The exact format of redirection is a command (and possibly some arguments) followed by the redirection symbol followed by a filename.
  - Multiple redirection operators or multiple files to the right of the redirection sign are errors.
  - After extracting each command, we need to look for ">"

User can launch parallel Commands - &
Print only one error message:

char error_message[30] = "An error has occurred\n";

write(STDERR_FILENO, error_message, strlen(error_message));

parse the input
- for batch mode, need to read file line by line
Extract the tokens
1. Check for parallel commands (&)

For every command

Extract the tokens with space delimiter
- store in array
check whether the command is a built-in command or not.
- implement exit, cd(using chdir()) and path.
- invoke the implementation
if it is not a built-in command,
- scan all the paths and check using access().
- if it is present in the particular path
  - fork
  - In the child process
    
    -If user requested for redirection - close std out file desc - open file with name given by user (eg "output") - execv(path, arg)
    - if no redirection requested
      - execv(path, arg)
- if not, it is an error

create child process (fork + exec) - parent waits for child to finish (wait()) When process is done prompt for next command

Repeat above till user types exit

phsaikiran/dash

Dash Requirements