usr: touch, tr, rev, cut, uniq

    /# ls -l /bin > b
    /# cut -d \s -f 1 b | uniq -c
    52 -

- touch FILE...: gbfs has no timestamps (an MBC5 cart has no RTC), so touch
  does the half that means something here - make an empty file. O_APPEND is
  exactly right: it creates, and it does not truncate what is already there.
- tr [-d] SET1 [SET2]: ranges (a-z), -d to delete, and a short SET2 padded
  with its last character, like tr(1). ASCII only - a 128-entry map.
- rev, uniq [-c]: one line each, from a file or stdin like the other filters.

Two things the tests caught. A bare "-" is a SET, not a flag, so `tr . -`
has to skip flag-matching only when a letter follows the dash. And uniq
compared an empty "line" against the last run at EOF and flushed a phantom
blank one - a stray "1" in uniq -c output.

tr and cut also take \s \t \n \r escapes, because the shell has no quoting:
`-d ' '` arrives as three tokens, so a space or tab delimiter was simply
untypeable. Shell quoting would be the better fix; this makes the tools
usable today without touching the parser.

Banks 49-53; 10 program slots left in the 1 MB ROM.

1 files changed, 43 insertions, 0 deletions

diff --git a/usr/rev.c b/usr/rev.c
new file mode 100644
index 0000000..2932190
--- /dev/null
+++ b/usr/rev.c
@@ -0,0 +1,43 @@
+#include "gbos.h"
+/* rev [file]: reverse the characters of each line, from a file or stdin. */
+
+#define MAXLINE 80
+
+void main(void) {
+    char *argv[4];
+    char line[MAXLINE];
+    unsigned char argc = argv_parse(argv, 4);
+    unsigned char len = 0, fd = NOFD, i;
+    int ch;
+
+    if (argc > 0 && argv[0][0] != '-') {
+        fd = open(argv[0], O_READ);
+        if (fd == EISDIR) {
+            puts("rev: is a directory");
+            nl();
+            sexit(2);
+        }
+        if (fd == NOFD) {
+            puts("rev: no such file");
+            nl();
+            sexit(2);
+        }
+    }
+    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 < MAXLINE) line[len++] = (char)ch;
+            continue;
+        }
+        if (ch < 0 && len == 0) break;
+        for (i = len; i--;) putc(line[i]);
+        nl();
+        len = 0;
+        if (ch < 0) break;
+    }
+    if (fd != NOFD) close(fd);
+}