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grep.c
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grep.c
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// 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 - 1)) > 0)
{
m += n;
buf[m] = '\0';
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;
}