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>

1 files changed, 42 insertions, 34 deletions

diff --git a/shell/cttyhack.c b/shell/cttyhack.c
index 6241c76a9..640f5b1fb 100644
--- 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);
 }