创建用于Shell输出重定向的文件

Question

This is a school assignment.

So basically I have written a C code for a shell that handles terminating the command with & , and input / output redirection using > or <. using pipes and fork() , execvp() .

The problem is my input/output redirection only handles that for files that already exist .

What i need to know is how would I go about redirecting output to a file that doesn't exist - I know I would have to create the file, but I'm not sure how that works.

For example: ls -a < test.txt

If test.txt is not in the directory, i need to create it and redirect output to that. So how do I create this file?

here is some basic example code which does not create a new file:

        else if( '>' ==  buff[i] ){
           i++;
           j=0;

           if( ' ' == buff[i] ) 
              i++;

           while( ' ' != buff[i] && i < len )
              out_file[j++]=buff[i++];
           out_file[j]='\0';


           if ( ( ofd = open(out_file,1) ) < 0 ){
              perror("output redirected file");
              exit( 1 );
           }

           close(1);
           dup(ofd);
        }

Any help with how I can output and create a new file would be much appreciated! Thanks!

Answer 1

You need to tell open to create the file, if necessary:

if ( ( ofd = open(out_file, O_WRONLY | O_CREAT) ) < 0 ) {

Note that your code will be more readable if you use the named constants for the flags to open . Compare with the functionally equivalent

if ( ( ofd = open(out_file, 513) ) < 0 ) {

or even

if ( ( ofd = open(out_file, 0x0201) ) < 0 ) {