ip netns: Identify netns for the current process

As 'ip' util will share the same netns from the caller
process then we can just look at /proc/self/.. to show
the netns of the current process by:

    ip netns id
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

ip/ipnetns.c

@@ -59,7 +59,7 @@ static int usage(void)
 	fprintf(stderr, "Usage: ip netns list\n");
 	fprintf(stderr, "       ip netns add NAME\n");
 	fprintf(stderr, "       ip netns delete NAME\n");
-	fprintf(stderr, "       ip netns identify PID\n");
+	fprintf(stderr, "       ip netns identify [PID]\n");
 	fprintf(stderr, "       ip netns pids NAME\n");
 	fprintf(stderr, "       ip netns exec NAME cmd ...\n");
 	fprintf(stderr, "       ip netns monitor\n");
@@ -299,19 +299,17 @@ static int netns_identify(int argc, char **argv)
 	struct dirent *entry;
 
 	if (argc < 1) {
-		fprintf(stderr, "No pid specified\n");
-		return -1;
-	}
-	if (argc > 1) {
+		pidstr = "self";
+	} else if (argc > 1) {
 		fprintf(stderr, "extra arguments specified\n");
 		return -1;
-	}
-	pidstr = argv[0];
-
-	if (!is_pid(pidstr)) {
-		fprintf(stderr, "Specified string '%s' is not a pid\n",
-			pidstr);
-		return -1;
+	} else {
+		pidstr = argv[0];
+		if (!is_pid(pidstr)) {
+			fprintf(stderr, "Specified string '%s' is not a pid\n",
+					pidstr);
+			return -1;
+		}
 	}
 
 	snprintf(net_path, sizeof(net_path), "/proc/%s/ns/net", pidstr);

man/man8/ip-netns.8

@@ -21,7 +21,7 @@ ip-netns \- process network namespace management
 
 .ti -8
 .BR "ip netns identify"
-.I PID
+.RI "[ " PID " ]"
 
 .ti -8
 .BR "ip netns pids"
@@ -85,10 +85,11 @@ persists until it has no more users.  ip netns delete may fail if
 the mount point is in use in another mount namespace.
 
 .TP
-.B ip netns identify PID - Report network namespaces names for process
+.B ip netns identify [PID] - Report network namespaces names for process
 .sp
 This command walks through /var/run/netns and finds all the network
-namespace names for network namespace of the specified process.
+namespace names for network namespace of the specified process, if PID is
+not specified then the current process will be used.
 
 .TP
 .B ip netns pids NAME - Report processes in the named network namespace