cttyhack: print detected ctty name when called without parameters

Sometimes there's a need to figure out the controlling tty from a shell
script, for example, to obtain a line for getty. In this case it's easier
to call cttyhack than trying to repeat some of the cttyhack's logic.

function                                             old     new   delta
cttyhack_main                                        283     327     +44
packed_usage                                       28911   28915      +4

Signed-off-by: Alexander Shishkin <virtuoso@slind.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/shell/cttyhack.c b/shell/cttyhack.c
index 6241c76a9166108449bc2f0c6091bff64c621520..640f5b1fb53ec61a0bd030e68f8ec29698a24e3f 100644 (file)
--- a/shell/cttyhack.c
+++ b/shell/cttyhack.c
@@ -50,9 +50,12 @@
 //config:
 //config:        # exec setsid sh -c 'exec sh </dev/tty1 >/dev/tty1 2>&1'
 //config:
+//config:        Starting getty on a controlling tty from a shell script:
+//config:
+//config:        # getty 115200 $(cttyhack)
 
 //usage:#define cttyhack_trivial_usage
-//usage:       "PROG ARGS"
+//usage:       "[PROG ARGS]"
 //usage:#define cttyhack_full_usage "\n\n"
 //usage:       "Give PROG a controlling tty if possible."
 //usage:     "\nExample for /etc/inittab (for busybox init):"
@@ -108,61 +111,66 @@ int cttyhack_main(int argc UNUSED_PARAM, char **argv)
                char paranoia[sizeof(struct serial_struct) * 3];
        } u;
 
-       if (!*++argv) {
-               bb_show_usage();
-       }
-
        strcpy(console, "/dev/tty");
        fd = open(console, O_RDWR);
-       if (fd >= 0) {
-               /* We already have ctty, nothing to do */
-               close(fd);
-       } else {
+       if (fd < 0) {
                /* We don't have ctty (or don't have "/dev/tty" node...) */
                do {
 #ifdef __linux__
-                       int s = open_read_close("/sys/class/tty/console/active",
-                               console + 5, sizeof(console) - 5);
-                       if (s > 0) {
-                               /* found active console via sysfs (Linux 2.6.38+)
-                                * sysfs string looks like "ttyS0\n" so zap the newline:
-                                */
-                               console[4 + s] = '\0';
-                               break;
-                       }
-
+                       int s;
                        if (ioctl(0, VT_GETSTATE, &u.vt) == 0) {
                                /* this is linux virtual tty */
-                               sprintf(console + 8, "S%d" + 1, u.vt.v_active);
+                               sprintf(console + 8, "S%u" + 1, (int)u.vt.v_active);
                                break;
                        }
 #endif
 #ifdef TIOCGSERIAL
                        if (ioctl(0, TIOCGSERIAL, &u.sr) == 0) {
                                /* this is a serial console; assuming it is named /dev/ttySn */
-                               sprintf(console + 8, "S%d", u.sr.line);
+                               sprintf(console + 8, "S%u", (int)u.sr.line);
+                               break;
+                       }
+#endif
+#ifdef __linux__
+                       /* Note that this method is not related to _stdin_ */
+                       s = open_read_close("/sys/class/tty/console/active",
+                               console + 5, sizeof(console) - 5);
+                       if (s > 0) {
+                               /* found active console via sysfs (Linux 2.6.38+)
+                                * sysfs string looks like "ttyS0\n" so zap the newline:
+                                */
+                               console[4 + s] = '\0';
                                break;
                        }
 #endif
                        /* nope, could not find it */
-                       goto ret;
+                       console[0] = '\0';
                } while (0);
+       }
 
+       argv++;
+       if (!argv[0]) {
+               if (!console[0])
+                       return EXIT_FAILURE;
+               puts(console);
+               return EXIT_SUCCESS;
+       }
+
+       if (fd < 0) {
                fd = open_or_warn(console, O_RDWR);
                if (fd < 0)
                        goto ret;
-               //bb_error_msg("switching to '%s'", console);
-               dup2(fd, 0);
-               dup2(fd, 1);
-               dup2(fd, 2);
-               while (fd > 2)
-                       close(fd--);
-               /* Some other session may have it as ctty,
-                * steal it from them:
-                */
-               ioctl(0, TIOCSCTTY, 1);
        }
-
-ret:
+       //bb_error_msg("switching to '%s'", console);
+       dup2(fd, 0);
+       dup2(fd, 1);
+       dup2(fd, 2);
+       while (fd > 2)
+               close(fd--);
+       /* Some other session may have it as ctty,
+        * try to steal it from them:
+        */
+       ioctl(0, TIOCSCTTY, 1);
+ ret:
        BB_EXECVP_or_die(argv);
 }