如何将标准输入（特定文件名）重定向到标准输出（特定文件名）

Question

I am creating a shell code. Basically, I want to redirect stdin file to stdout file. For instance when I enter a command like sort < hello.c > t.txt, then the hello.c file should be copied in new mentioned file called t.txt.

Here is my code, I am able to redirect output of other commands, when I type ls > t.txt. However, I don't have any idea about redirecting one file's input to other file using dup2.

Here is my code, I am only posting the loop, as this is where I have to create the logic.

int in, out;

for (i = 0; i < arridx; i++) {
    if(strcmp( array[i],"<")==0)
    {    
             in = open(array[i+1], O_RDONLY);        
             array[i]=0;
            // array[i+1]=0;
    }
    if(strcmp( array[i],">")==0)
    {
              out = open(array[i+1], O_WRONLY | O_TRUNC | O_CREAT, S_IRUSR | S_IRGRP | S_IWGRP | S_IWUSR);
              array[i]=0;
             // array[i+1]=0;
    }
}
dup2(in, 0);
dup2(out, 1);

// close unused file descriptors
close(in);
close(out);

Input array would be like

array[0]="sort"
array[1]="<"
array[2]="hello.c"
array[3]=">"
array[4]="t.txt"

Answer 1

In fact, whenever you run something like :

command < hello.c > t.txt

The redirection will take place presuming command is your argv[0] with no of arguments as 1, and redirection taking place by the shell.

However, on another point, going through your program, if redirection is used not from command prompt but by array contents only, int dup2(int oldfd, int newfd); - creates a copy of the file descriptor oldfd. In your case,

dup2(in, 0);
dup2(out, 1);

0 and 1 stands for stdin and stdout file descriptors respectively. So, if you would like to redirect your input to be taken from the stdin instead of hello.c (file opened as in) and output to be taken from the stdout instead of t.txt (file opened as out), then shouldn't be other way round ie

dup2(0, in);
dup2(1, out);