SQUISH

Superior Quality Unbeatable Interprocess (communication) Shell, or “squish”, is a shell that supports running processes, piping between processes, handle redirection, variable globbing, cd and exit.

How to Compile and Run

make
./squish

How errors are handled

• When entering a command that doesn’t exist, squish will ouput “name: command not found”.
• The exit status of the command will be outputted if running a single command.

  Piping algorithm

  Processes are piped recursively backwards from the last command. The recursive base case is when there is only one command left to be exec’d.

For ex. p1 | p2 | p2 can be represented as:

       squish
         /\
        /  \
       /\   p3
      /  \
     p1  p2

Pseudo-code follows as such:

ipc (tokens, nTokens) 
{
  if ( nTokens == 1 ) {
    fork new process
    exec ( tokens[0] )
    return
  }
  fork new process
  if ( pid == 0 ) {
    replace stdout with write end of pipe
    ipc (tokens, --ntokens)
  }
  fork new process
  if ( pid == 0 ) {
    replace stdin with read end of pipe
    exec ( tokens[nTokens] )
  }
}

File Structure

Tokenization Files by AHW

squish_main.c
squish_run.c
squish_tokenize.c

“Built-in” CD and exit

w_change_dir.c
w_exit.c

Globbing

w_glob.c

Piping and redirection

w_pipe.c
w_redirection.c

Input parser

w_run.c

Exec wrapper

w_run_command.c

Works Cited

Some of the code is copied from the cis-3050 c examples from the linux.socs server. Wherever that is the case, a comment block will be included above the code snippit.