Bernhard Fischer provided a mountpoint(1) applet. This is apparently something

sysvinit provides, and which is used by the debian init scripts.

include/applets.h

@@ -435,6 +435,9 @@
 #ifdef CONFIG_MOUNT
 	APPLET(mount, mount_main, _BB_DIR_BIN, _BB_SUID_NEVER)
 #endif
+#ifdef CONFIG_MOUNTPOINT
+	APPLET(mountpoint, mountpoint_main, _BB_DIR_BIN, _BB_SUID_NEVER)
+#endif
 #ifdef CONFIG_MSH
 	APPLET_NOUSAGE("msh", msh_main, _BB_DIR_BIN, _BB_SUID_NEVER)
 #endif

include/usage.h

@@ -1922,6 +1922,20 @@
 	"$ mount /dev/fd0 /mnt -t msdos -o ro\n" \
 	"$ mount /tmp/diskimage /opt -t ext2 -o loop\n"
 
+#define mountpoint_trivial_usage \
+	"[-q] <[-d] DIR | -x DEVICE>"
+#define mountpoint_full_usage \
+	"mountpoint checks if the directory is a mountpoint\n\n" \
+	"Options:\n"  \
+	"\t-q:\t\tBe more quiet\n" \
+	"\t-d:\t\tPrint major/minor device number of the filesystem\n" \
+	"\t-x:\t\tPrint major/minor device number of the blockdevice"
+#define mountpoint_example_usage \
+	"$ mountpoint /proc\n" \
+	"/proc is not a mountpoint\n" \
+	"$ mountpoint /sys\n" \
+	"/sys is a mountpoint\n"
+
 #define mt_trivial_usage \
 	"[-f device] opcode value"
 #define mt_full_usage \

miscutils/Config.in

@@ -194,6 +194,12 @@ config CONFIG_FEATURE_MAKEDEVS_TABLE
 
 endchoice
 
+config CONFIG_MOUNTPOINT
+	bool "mountpoint"
+	default n
+	help
+	  mountpoint checks if the directory is a mountpoint.
+
 config CONFIG_MT
 	bool "mt"
 	default n

miscutils/Makefile.in

@@ -34,6 +34,7 @@ MISCUTILS-$(CONFIG_EJECT)       += eject.o
 MISCUTILS-$(CONFIG_HDPARM)      += hdparm.o
 MISCUTILS-$(CONFIG_LAST)        += last.o
 MISCUTILS-$(CONFIG_MAKEDEVS)    += makedevs.o
+MISCUTILS-$(CONFIG_MOUNTPOINT)	+= mountpoint.o
 MISCUTILS-$(CONFIG_MT)          += mt.o
 MISCUTILS-$(CONFIG_RX)          += rx.o
 MISCUTILS-$(CONFIG_SETSID)      += setsid.o

miscutils/mountpoint.c

@@ -0,0 +1,70 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * mountpoint implementation for busybox
+ *
+ * Copyright (C) 2005 Bernhard Fischer
+ *
+ * Licensed under the GPL v2, see the file LICENSE in this tarball.
+ *
+ * Based on sysvinit's mountpoint
+ */
+
+#include <sys/stat.h>
+#include <errno.h> /* errno */
+#include <string.h> /* strerror */
+#include <getopt.h> /* optind */
+#include "busybox.h"
+
+int mountpoint_main(int argc, char **argv)
+{
+	int opt = bb_getopt_ulflags(argc, argv, "qdx");
+#define OPT_q (1)
+#define OPT_d (2)
+#define OPT_x (4)
+
+	if (optind != argc - 1)
+		bb_show_usage();
+	{
+		char *arg = argv[optind];
+		struct stat st;
+
+		if ( (opt & OPT_x && stat(arg, &st) == 0) || (lstat(arg, &st) == 0) ) {
+			if (opt & OPT_x) {
+				if (S_ISBLK(st.st_mode))
+				{
+					bb_printf("%u:%u\n", major(st.st_rdev),
+								minor(st.st_rdev));
+					return EXIT_SUCCESS;
+				} else {
+					if (opt & OPT_q)
+						putchar('\n');
+					else
+						bb_error_msg("%s: not a block device", arg);
+				}
+				return EXIT_FAILURE;
+			} else
+			if (S_ISDIR(st.st_mode)) {
+				dev_t st_dev = st.st_dev;
+				ino_t st_ino = st.st_ino;
+				char *p;
+				bb_xasprintf(&p, "%s/..", arg);
+				if (stat(p, &st) == 0) {
+					short ret = (st_dev != st.st_dev) ||
+						(st_dev == st.st_dev && st_ino == st.st_ino);
+					if (opt & OPT_d)
+						bb_printf("%u:%u\n", major(st_dev), minor(st_dev));
+					else if (!(opt & OPT_q))
+						bb_printf("%s is %sa mountpoint\n", arg, ret?"":"not ");
+					return !ret;
+				}
+			} else {
+				if (!(opt & OPT_q))
+					bb_error_msg("%s: not a directory", arg);
+				return EXIT_FAILURE;
+			}
+		}
+		if (!(opt & OPT_q))
+			bb_perror_msg(arg);
+		return EXIT_FAILURE;
+	}
+}