How to redirect using pipelines

I am trying to implement the command

Code:

---

sort myshell.c  > 2

---

in C.

But I am having difficulties understanding how to use pipelines to do so.

Code:

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int main(){
  char *args[] = {"sort", "myshell.c", ">", "2"};
  int size = sizeof(args)/sizeof(*args);
  int rw[2]//pipes
  int i;
  pipe(rw);
  FILE* file;


  for(i = 0; i < size; i++){
        if(!strcomp(args[i], ">")){
                file = args[i+1];//getting the file name (in this case 2)
        }
  }


  if((pid = fork()) == 0){//child


        int fd = open(file, O_CREAT | O_RDWR);


  }else{//parent


  }




return 0;
}

---

Are you trying to redirect to descriptor 2 (stderr), or just a file called "2" ?

You're half-way there based on your previous thread.
All you need now is to dup() fd onto descriptor 1 (stdout) or 2 (stderr) as you like.

Hey salem lol. glad to see you again. well the file is literally called "2". I just dont know how to sort "myshell.c" and putting that output in a file called "2"...

hum,
it's not much but hope its helps a little, that compiles, and runs.

Code:

---


#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
     
     
int main(){
  char *args[] = {"sort", "myshell.c", ">", "2"};
  int size = sizeof(args)/sizeof(args[0]);
  int rw[2];//pipes
  int i;
  pipe(rw);
  //FILE* file;
 
  char file[30];
 
 
  for(i = 0; i < size; i++){
      // if(!strcomp(args[i], ">")){
        if(!strcmp(args[i], ">")){ //Linux not windows
                strcpy(file,args[i+1]);
                printf("%s\n", file);
                //file = args[i+1];//getting the file name (in this case 2)
        }
  }
 
 /*
 // not really sure what you're doing here
  if((pid = fork()) == 0){//child
 
 
        int fd  = open(file, O_CREAT | O_RDWR);
 
 
  }else{//parent
    //printf("Parent\n");
 
 }
 */
 
 
return 0;
}

---

fork
The fork() System Call

open
http://www.techytalk.info/linux-syst...nd-write-file/

redirect

Code:

---

userx@slackwhere:~
$ touch 2
userx@slackwhere:~
$ echo "hey stdout" > 2
userx@slackwhere:~
$ cat 2
hey stdout
userx@slackwhere:~
$ sort ~/bin/term2.c >> 2
userx@slackwhere:~
$ cat 2 
hey stdout
    //printf("Parent\n");
 
 
 
 
 
 
 
 
 
 
 
     
     
                //file = args[i+1];//getting the file name (in this case 2)
                printf("%s\n", file);
                strcpy(file,args[i+1]);
        if(!strcmp(args[i], ">")){ //Linux not windows
        int fd  = open(file, O_CREAT | O_RDWR);
        }
      // if(!strcomp(args[i], ">")){
  if((pid = fork()) == 0){//child
  //FILE* file;
  char *args[] = {"sort", "myshell.c", ">", "2"};
  char file[30];
  for(i = 0; i < size; i++){
  int i;
  int rw[2];//pipes
  int size = sizeof(args)/sizeof(args[0]);
  pipe(rw);
  }
  }else{//parent
 */
 /*
 // not really sure what you're doing here
 }
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
int main(){
return 0;
}

---

First, you do this

Code:

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        if(!strcomp(args[i], ">")){
                file = args[i+1];//getting the file name (in this case 2)
                args[i] = NULL;
        }

---

This is so you now have an argv compatible with the exec() functions.

Then you call dup() to map stdout to the fd you just opened (see manual pages posted previously)

Then finally, something like
execvp( argv[0], argv );

Code:

---

term2.c:26:22: error: assignment to expression with array type
                file = args[i+1];//getting the file name (in this case 2)

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so if it is C then how can that be valid in windows? just wondering.

To redirect stdout just close(1) before opening the file. Newly opened files always use the lowest available file descriptor, so it will be opened with fd 1.

I got it working guys. Thanks for all the help!