usr: ls -l, df, mv, tail, tee, sleep, help, poweroff

Filling in the core utilities the box was missing:

- ls -l: type, links and size, now that stat() exists. The screen is 40
  columns, so it is "d  1   256 name", not the GNU sprawl. Sizes were
  previously unknowable except one file at a time with wc -c.
- df: blocks and inodes, used and free, on the 32 KiB cart disk.
- mv: no rename syscall, so it is cp + rm - and the copy has to finish before
  the source goes, or a failure loses the file.
- tail [-n N]: no seek syscall, so it streams the input through a ring of N
  lines rather than jumping to the end. N is capped by that ring.
- tee: the missing half of a pipeline - `cat big | tee copy | wc -l`.
- sleep: the thing every script needs; the kernel counts 1/64 s ticks.
- help: lists every command the shell knows, walked out of the kernel's own
  name table. The box previously had no way to tell you what it could do.
- poweroff: the only safe way to stop a gbos that has data on it. The cart's
  battery RAM is written when the emulator exits cleanly, so killing it loses
  the whole filesystem - it comes back "formatted". Found while testing /rc.

1 files changed, 75 insertions, 0 deletions

diff --git a/usr/tail.c b/usr/tail.c
new file mode 100644
index 0000000..1fd5304
--- /dev/null
+++ b/usr/tail.c
@@ -0,0 +1,75 @@
+#include "gbos.h"
+/* tail [-n N] [N] [file]: last N lines (default 10) of a file or stdin.
+ *
+ * There is no seek syscall, so this cannot jump to the end: it streams the
+ * whole input through a ring of N lines and prints the ring at EOF. The ring
+ * is the memory budget - N lines of up to LINEW chars each - so N is capped. */
+
+#define MAXN  12 /* 12 * 42 = 504 bytes of ring, fits a task */
+#define LINEW 42
+
+void main(void) {
+    char *argv[8];
+    char ring[MAXN][LINEW];
+    unsigned char len[MAXN];
+    unsigned char argc = argv_parse(argv, 8);
+    char *nv = optval(argv, argc, 'n');
+    unsigned char n = 10, i, cur = 0, filled = 0, fd = NOFD, l = 0;
+    char *fname = 0;
+    int ch;
+
+    if (nv) n = atou(nv);
+    for (i = 0; i < argc; i++) {
+        if (argv[i][0] == '-') continue;
+        if (nv == argv[i]) continue;
+        if (!nv && argv[i][0] >= '0' && argv[i][0] <= '9') {
+            n = atou(argv[i]);
+            continue;
+        }
+        fname = argv[i];
+        break;
+    }
+    if (n == 0) return;
+    if (n > MAXN) n = MAXN;
+
+    if (fname) {
+        fd = open(fname, O_READ);
+        if (fd == EISDIR) {
+            puts("tail: is a directory");
+            nl();
+            sexit(2);
+        }
+        if (fd == NOFD) {
+            puts("tail: no such file");
+            nl();
+            sexit(2);
+        }
+    }
+    for (i = 0; i < MAXN; i++) len[i] = 0;
+
+    for (;;) {
+        if (fd == NOFD) {
+            char c = readc();
+            ch = (c == EOF) ? -1 : c;
+        } else ch = fgetc(fd);
+        if (ch == '\r') continue;
+        if (ch >= 0 && ch != '\n') {
+            if (len[cur] < LINEW) ring[cur][len[cur]++] = (char)ch;
+            continue;
+        }
+        if (ch < 0 && len[cur] == 0) break;   /* EOF, nothing pending */
+        cur = (unsigned char)((cur + 1) % n); /* keep only the last n */
+        if (filled < n) filled++;
+        len[cur] = 0;
+        if (ch < 0) break;
+    }
+    if (fd != NOFD) close(fd);
+
+    /* oldest kept line first: cur points at the (empty) next slot */
+    i = (unsigned char)((cur + n - filled) % n);
+    while (filled--) {
+        writes(ring[i], len[i]);
+        nl();
+        i = (unsigned char)((i + 1) % n);
+    }
+}