pipe_grep.c

/*
 * usage: ./a.out text_pattern input_file output_file
 * Executes the command "grep text_pattern < input_file > output_file"
 */

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

int main(int argc, char **argv)
{
    int ifp, ofp;

    if (argc == 4)
    {
        // grep on text_pattern
        char *grep_args[] = {"grep", argv[1], NULL};

        // open input and output files
        ifp = open(argv[2], O_RDONLY);
        ofp = open(argv[3], O_WRONLY | O_TRUNC | O_CREAT, S_IRUSR | S_IRGRP | S_IWGRP | S_IWUSR);

        // duplicate input file to stdin
        dup2(ifp, 0);

        // duplicate output file to stdout
        dup2(ofp, 1);

        // close unused input file descriptor
        close(ifp);

        // close unused output file descriptor
        close(ofp);

        // execute grep
        execvp("grep", grep_args);
    }
    else
    {
        printf("usage: %s text_pattern input_file output_file\n", argv[0]);
    }
}