--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/usr/cat.c	Fri May 26 23:11:05 2017 +0900
@@ -0,0 +1,39 @@
+#include "types.h"
+#include "stat.h"
+#include "user.h"
+
+char buf[512];
+
+void
+cat(int fd)
+{
+    int n;
+    
+    while((n = read(fd, buf, sizeof(buf))) > 0)
+        write(1, buf, n);
+    if(n < 0){
+        printf(1, "cat: read error\n");
+        exit();
+    }
+}
+
+int
+main(int argc, char *argv[])
+{
+    int fd, i;
+    
+    if(argc <= 1){
+        cat(0);
+        exit();
+    }
+    
+    for(i = 1; i < argc; i++){
+        if((fd = open(argv[i], 0)) < 0){
+            printf(1, "cat: cannot open %s\n", argv[i]);
+            exit();
+        }
+        cat(fd);
+        close(fd);
+    }
+    exit();
+}