Redirection and Pipes

Yesterday, Kamesh wrote a program to redirect the output of the program to a file.

char *ar[]={"ls",">","mylog.dat"};
execv("ls",ar);

I'm going to kill this guy. He dont know what is the difference between passing argument to a command and output redirection. And also dont know who is doing the redirection.

when you say,

ls -l > mylog.dat

this is what shell does

1. Shell forks itself
2. Close the stdout of the child process
3. Open the file and bind it with the stdout of the child process
4. execl "ls -l" on child process
5. parent process (i.e. shell) waits till the complete execution of the child process

Program 1:


#include <unistd.h>
#include <fcntl.h>

int main() {
char *ar[]={"ls","-l"};
int fd;
close(1);
fd=open("/users/revincen/cpp/sample.dat",O_CREAT|O_WRONLY);
write(1,"hello world\n",12);
execv("/bin/ls",ar);
return 0;
}


Program 2:


#include <stdio.h>
#include <stdlib.h>

int main() {
char buf[256];
FILE *inp,*outp;

inp = popen("ls -l","r");
outp=fopen("hello.dat","w");

while ( fgets(buf,sizeof buf,inp) != 0 )
fputs(buf,outp);

pclose(inp);
return 0;
}