fs rewrite stage 3: nested directories, paths, cwd (cd/mkdir/ls)

- directories generalized: dir_find/dir_add/dir_remove take any dir inode;
  every dir carries '.'/'..'. Path resolution (resolve / resolve_parent) walks
  absolute or cwd-relative paths; open/mkdir/chdir/remove/opendir take paths.
- per-process cwd: PROC_CWD in the PCB (root by default, inherited on fork);
  sys_chdir sets it, sys_opendir points ls at any directory.
- shell: 'cd' builtin + cwd-aware prompt; 'mkdir'/'ls <dir>' tools; 'exit'/'quit'
  and EOF call poweroff() (clean shutdown via the $ED opcode -> reliable save).
- three HL/buffer-clobber bugs fixed along the way: resolve didn't preserve the
  path cursor across dir_find; cur_cwd clobbered HL; resolve_parent set the final
  name before resolve() overwrote wFsNameBuf (now copied after).
- verified: nested mkdir/cd/save, multi-level paths (cat docs/sub/b), and the
  whole tree persists across a reboot.
- README: document directories + the poweroff opcode.

1 files changed, 36 insertions, 2 deletions

diff --git a/c/sh.c b/c/sh.c
index 8aa619e..b1bf630 100644
--- a/c/sh.c
+++ b/c/sh.c
@@ -18,6 +18,30 @@ static unsigned char tokenize(char *s, char **tok, unsigned char max) {
 }
 
 static unsigned char isop(char *t, char c) { return t[0] == c && t[1] == 0; }
+static unsigned char iscmd(char *t, char a, char b) {
+    return t[0] == a && t[1] == b && t[2] == 0;
+}
+
+/* keep the prompt's path string roughly in sync with chdir(). */
+static void set_cwd(char *cwd, char *arg) {
+    char *d, *s, *last;
+    if (arg[0] == '/') {                 /* absolute */
+        d = cwd; s = arg;
+        while (*s) *d++ = *s++;
+        *d = 0;
+    } else if (arg[0] == '.' && arg[1] == '.' && arg[2] == 0) {
+        last = cwd; d = cwd;
+        while (*d) { if (*d == '/') last = d; d++; }
+        if (last == cwd) { cwd[0] = '/'; cwd[1] = 0; } else *last = 0;
+    } else if (arg[0] == '.' && arg[1] == 0) {
+        /* stay */
+    } else {                             /* relative: append */
+        d = cwd; while (*d) d++;
+        if (!(cwd[0] == '/' && cwd[1] == 0)) *d++ = '/';
+        s = arg; while (*s) *d++ = *s++;
+        *d = 0;
+    }
+}
 
 /* build the child command line "name\0arg1 arg2\0" at 0xA000 */
 static void set_cmdline(char **tok, unsigned char start, unsigned char end) {
@@ -74,22 +98,32 @@ done:
 void main(void) {
     char line[80];
     char *tok[16];
+    char cwd[40];
+    cwd[0] = '/'; cwd[1] = 0;
     for (;;) {
         unsigned char nt, i, pipepos, c, bg;
         /* reap finished background jobs */
         { unsigned char p; while ((p = reap())) { putc('['); putu(p); puts(" done]"); nl(); } }
-        puts("$ ");
+        puts(cwd); puts("$ ");
         /* read a line (echoing), stop at newline; exit on EOF */
         i = 0;
         for (;;) {
             c = readc();
-            if (c == EOF) return;
+            if (c == EOF) poweroff();
             if (c == '\r' || c == '\n') { nl(); break; }
             if (i < 79) { line[i++] = c; putc(c); }
         }
         line[i] = 0;
         nt = tokenize(line, tok, 16);
         if (nt == 0) continue;
+        if (iscmd(tok[0], 'c', 'd')) {              /* cd builtin */
+            char *arg = (nt >= 2) ? tok[1] : "/";
+            if (chdir(arg) == 0) set_cwd(cwd, arg);
+            else { puts("cd: no such directory"); nl(); }
+            continue;
+        }
+        if ((tok[0][0] == 'e' && tok[0][1] == 'x') ||
+            (tok[0][0] == 'q' && tok[0][1] == 'u')) poweroff();
         bg = 0;
         if (isop(tok[nt - 1], '&')) { bg = 1; nt--; if (nt == 0) continue; }
         pipepos = 0;