Using cat and execvp - cat

Question

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?

Answer 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;
}