V6 Shell

Tsh by the same author would appear to be that

xv6 has a shell in modern C:

xv6 is a re-implementation of Dennis Ritchie’s and Ken Thompson’s Unix Version 6 (v6). xv6 loosely follows the structure and style of v6, but is implemented for a modern x86-based multiprocessor using ANSI C.

https://github.com/mit-pdos/xv6-public/blob/master/sh.c#L100

I like to look at how they implement pipelines, which is:

  case PIPE:
    pcmd = (struct pipecmd*)cmd;
    if(pipe(p) < 0)
      panic("pipe");
    if(fork1() == 0){
      close(1);
      dup(p[1]);
      close(p[0]);
      close(p[1]);
      runcmd(pcmd->left);
    }
    if(fork1() == 0){
      close(0);
      dup(p[0]);
      close(p[0]);
      close(p[1]);
      runcmd(pcmd->right);
    }

and then let’s look at Thompson shell - https://v6sh.org/src/sh.c

	case TFIL:
		f = t[DFLG];
		pipe(pv);
		t1 = t[DLEF];
		t1[DFLG] =| FPOU | (f&(FPIN|FINT|FPRS));
		execute(t1, pf1, pv);
		t1 = t[DRIT];
		t1[DFLG] =| FPIN | (f&(FPOU|FINT|FAND|FPRS));
		execute(t1, pv, pf2);
		return;

So they both do 2 recursive invocations – to runcmd() or execute(), on the left and right node of the AST.

---

Oh weird, but actually now I notice a difference … xv6 is less efficient?

• It has a pipe() and 2 fork() calls for every PIPE node
• original Thompson shell has only a pipe() call, and two execute() calls, which may or may not fork ???

So basically in xv6:

cat | cat | cat | cat

is executed like:

cat | (cat | (cat | cat)))

That is, you would expect there to be 4 cat processes to run, but there are at least 2 more processes.

But not Thompson shell? I’m curious if I’m reading that right …

---

BTW https://oils.pub/ stores all the pipeline components in a list, so it doesn’t have this left/right recursion. I’m pretty sure most shells, like bash and dash, are like that – you can have a variable number of children in an AST node, not just 2