grep.c

   1 // Simple grep.  Only supports ^ . * $ operators.
   2
   3 #include "types.h"
   4 #include "stat.h"
   5 #include "user.h"
   6
   7 char buf[1024];
   8 int match(char*, char*);
   9
  10 void
  11 grep(char *pattern, int fd)
  12 {
  13   int n, m;
  14   char *p, *q;
  15
  16   m = 0;
  17   while((n = read(fd, buf+m, sizeof(buf)-m)) > 0){
  18     m += n;
  19     p = buf;
  20     while((q = strchr(p, '\n')) != 0){
  21       *q = 0;
  22       if(match(pattern, p)){
  23         *q = '\n';
  24         write(1, p, q+1 - p);
  25       }
  26       p = q+1;
  27     }
  28     if(p == buf)
  29       m = 0;
  30     if(m > 0){
  31       m -= p - buf;
  32       memmove(buf, p, m);
  33     }
  34   }
  35 }
  36
  37 int
  38 main(int argc, char *argv[])
  39 {
  40   int fd, i;
  41   char *pattern;
  42
  43   if(argc <= 1){
  44     printf(2, "usage: grep pattern [file ...]\n");
  45     exit();
  46   }
  47   pattern = argv[1];
  48
  49   if(argc <= 2){
  50     grep(pattern, 0);
  51     exit();
  52   }
  53
  54   for(i = 2; i < argc; i++){
  55     if((fd = open(argv[i], 0)) < 0){
  56       printf(1, "grep: cannot open %s\n", argv[i]);
  57       exit();
  58     }
  59     grep(pattern, fd);
  60     close(fd);
  61   }
  62   exit();
  63 }
  64
  65 // Regexp matcher from Kernighan & Pike,
  66 // The Practice of Programming, Chapter 9.
  67
  68 int matchhere(char*, char*);
  69 int matchstar(int, char*, char*);
  70
  71 int
  72 match(char *re, char *text)
  73 {
  74   if(re[0] == '^')
  75     return matchhere(re+1, text);
  76   do{  // must look at empty string
  77     if(matchhere(re, text))
  78       return 1;
  79   }while(*text++ != '\0');
  80   return 0;
  81 }
  82
  83 // matchhere: search for re at beginning of text
  84 int matchhere(char *re, char *text)
  85 {
  86   if(re[0] == '\0')
  87     return 1;
  88   if(re[1] == '*')
  89     return matchstar(re[0], re+2, text);
  90   if(re[0] == '$' && re[1] == '\0')
  91     return *text == '\0';
  92   if(*text!='\0' && (re[0]=='.' || re[0]==*text))
  93     return matchhere(re+1, text+1);
  94   return 0;
  95 }
  96
  97 // matchstar: search for c*re at beginning of text
  98 int matchstar(int c, char *re, char *text)
  99 {
 100   do{  // a * matches zero or more instances
 101     if(matchhere(re, text))
 102       return 1;
 103   }while(*text!='\0' && (*text++==c || c=='.'));
 104   return 0;
 105 }
 106