userland: add "uname" command

userland/Makefile

@@ -39,6 +39,7 @@ BIN_TARGET_NAMES := \
 	sh \
 	sleep \
 	touch \
+	uname \
 	wc \
 	xv6-usertests
 OUTDIR := ../base/bin

userland/uname.c

@@ -0,0 +1,62 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/utsname.h>
+#include <unistd.h>
+
+static void usage(void) {
+    dprintf(STDERR_FILENO, "Usage: uname [-asnrvm]\n");
+    exit(EXIT_FAILURE);
+}
+
+int main(int argc, char* const argv[]) {
+    bool all = false;
+    bool nodename = false;
+    bool release = false;
+    bool version = false;
+    bool machine = false;
+    for (int i = 1; i < argc; ++i) {
+        const char* arg = argv[i];
+        if (arg[0] != '-' || arg[1] == '\0' || arg[2] != '\0')
+            usage();
+        switch (arg[1]) {
+        case 'a':
+            all = true;
+            break;
+        case 's':
+            break;
+        case 'n':
+            nodename = true;
+            break;
+        case 'r':
+            release = true;
+            break;
+        case 'v':
+            version = true;
+            break;
+        case 'm':
+            machine = true;
+            break;
+        default:
+            usage();
+        }
+    }
+
+    struct utsname buf;
+    if (uname(&buf) < 0) {
+        perror("uname");
+        return EXIT_FAILURE;
+    }
+
+    printf("%s ", buf.sysname);
+    if (all || nodename)
+        printf("%s ", buf.nodename);
+    if (all || release)
+        printf("%s ", buf.release);
+    if (all || version)
+        printf("%s ", buf.version);
+    if (all || machine)
+        printf("%s", buf.machine);
+    puts("");
+
+    return EXIT_SUCCESS;
+}