Using cat and execvp | 易学教程

问题

Trying to understand why this section of code using the cat command isn't working with execvp in C.

char *in[5] ={"cat", "file1.txt", ">>", "file2.txt", 0};
execvp(in[0], in);

When I run it displays the contents of file1.txt but then says:

cat: >> No such file or directory.

Then displays the contents of file2.txt Why wouldn't it recognize the >> operator in this instance?

回答1:

You can read the "man tee" command which it read from standard input and write to standard output and files. You could achieve this with below example.

#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

/*
Implementation of below command:
cat file1.txt > file2.txt
*/

char *cmd1[] = { "/bin/cat", "file1.txt", 0 };
char *cmd2[] = { "tee", "file2.txt", 0 };

static void sigchld_hdl (int sig)
{
    int status;
    while (waitpid(-1, &status, 0) > 0) {       
        if(WIFEXITED(status))
            printf("Child exited with code %d\n", WEXITSTATUS(status)); }
}

int runcmd(int pfd[])
{
    int i=0;

    switch (fork()) {
        case -1:
            perror ("fork");
            return 1;
        case 0:
            dup2(pfd[0], 0);
            close(pfd[1]);  /* the child does not need this end of the pipe */
            execvp(cmd2[0], cmd2);
            perror(cmd2[0]);
            exit(10);
        default: /* parent */               
            dup2(pfd[1], 1);
            close(pfd[0]);  /* the parent does not need this end of the pipe */
            execvp(cmd1[0], cmd1);
            perror(cmd1[0]);

    }
    sleep(1);
}

int main (int argc, char *argv[])
{   
    struct sigaction act;   
    int fd[2];

    pipe(fd);

    memset (&act, 0, sizeof(act));
    act.sa_handler = sigchld_hdl;

    if (sigaction(SIGCHLD, &act, 0)) {
        perror ("sigaction");
        return 1;
    }
    runcmd(fd);

    return 0;
}

来源：https://stackoverflow.com/questions/42119440/using-cat-and-execvp

标签

cat

execvp