--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/usr/grep.c	Sun Feb 04 17:54:49 2018 +0900
@@ -0,0 +1,106 @@
+// Simple grep.  Only supports ^ . * $ operators.
+
+#include "types.h"
+#include "stat.h"
+#include "user.h"
+
+char buf[1024];
+int match(char*, char*);
+
+void
+grep(char *pattern, int fd)
+{
+    int n, m;
+    char *p, *q;
+    
+    m = 0;
+    while((n = read(fd, buf+m, sizeof(buf)-m)) > 0){
+        m += n;
+        p = buf;
+        while((q = strchr(p, '\n')) != 0){
+            *q = 0;
+            if(match(pattern, p)){
+                *q = '\n';
+                write(1, p, q+1 - p);
+            }
+            p = q+1;
+        }
+        if(p == buf)
+            m = 0;
+        if(m > 0){
+            m -= p - buf;
+            memmove(buf, p, m);
+        }
+    }
+}
+
+int
+main(int argc, char *argv[])
+{
+    int fd, i;
+    char *pattern;
+    
+    if(argc <= 1){
+        printf(2, "usage: grep pattern [file ...]\n");
+        exit();
+    }
+    pattern = argv[1];
+    
+    if(argc <= 2){
+        grep(pattern, 0);
+        exit();
+    }
+    
+    for(i = 2; i < argc; i++){
+        if((fd = open(argv[i], 0)) < 0){
+            printf(1, "grep: cannot open %s\n", argv[i]);
+            exit();
+        }
+        grep(pattern, fd);
+        close(fd);
+    }
+    exit();
+}
+
+// Regexp matcher from Kernighan & Pike,
+// The Practice of Programming, Chapter 9.
+
+int matchhere(char*, char*);
+int matchstar(int, char*, char*);
+
+int
+match(char *re, char *text)
+{
+    if(re[0] == '^')
+        return matchhere(re+1, text);
+    do{  // must look at empty string
+        if(matchhere(re, text))
+            return 1;
+    }while(*text++ != '\0');
+    return 0;
+}
+
+// matchhere: search for re at beginning of text
+int matchhere(char *re, char *text)
+{
+    if(re[0] == '\0')
+        return 1;
+    if(re[1] == '*')
+        return matchstar(re[0], re+2, text);
+    if(re[0] == '$' && re[1] == '\0')
+        return *text == '\0';
+    if(*text!='\0' && (re[0]=='.' || re[0]==*text))
+        return matchhere(re+1, text+1);
+    return 0;
+}
+
+// matchstar: search for c*re at beginning of text
+int matchstar(int c, char *re, char *text)
+{
+    do{  // a * matches zero or more instances
+        if(matchhere(re, text))
+            return 1;
+    }while(*text!='\0' && (*text++==c || c=='.'));
+    return 0;
+}
+