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