pipe.c

#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>

#define STDOUT 1
#define STDERR 2

int main(int argc, char **argv)
{
        pid_t pid;

        if (argc < 2) {
                fprintf(stderr, "Usage: %s command args ...\n", argv[0]);
                exit(EXIT_FAILURE);
        }

        pid = fork();
        if (pid == 0) {         /* child */
                int bout, berr; /* backup stdout and stderr */
                int rval;
                int fd = open("output", O_APPEND | O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
                if (fd < 0) {
                        fprintf(stderr, "Error: open %s\n", strerror(errno));
                        exit(EXIT_FAILURE);
                }

                bout = dup(STDOUT);
                berr = dup(STDERR);
                if (bout == -1 ||
                    berr == -1)
                {
                        fprintf(stderr, "Error: dup (STDOUT|STDERR) %s\n", strerror(errno));
                        exit(EXIT_FAILURE);
                }

                rval = dup2(fd, STDOUT);
                if (rval < 0) {
                        fprintf(stderr, "Error: dup2 (STDOUT) %s\n", strerror(errno));
                        exit(EXIT_FAILURE);
                }

                rval = dup2(fd, STDERR);
                if (rval < 0) {
                        fprintf(stderr, "Error: dup2 (STDERR) %s\n", strerror(errno));
                        exit(EXIT_FAILURE);
                }

                argv++;
                rval = execv(argv[0], argv);
                if (rval == -1) {
                        char message[256];

                        snprintf(message, 256, "Error: execv %s\n", strerror(errno));
                        write(berr, message, strlen(message));
                        exit(EXIT_FAILURE);
                }
                return 0;

        } else if (pid > 0) {   /* parent */
                return waitpid(pid);
        } else {                /* fork error */
                fprintf(stderr, "Error: %s\n", strerror(errno));
                exit(EXIT_FAILURE);
        }

        return 0;
}