CODEWITHSUNDEEP
bash
Varun Rao
Varun Rao

Reputation: 433

Redirect multiple files to stdin

I am running a C Program ( on Ubuntu - bash terminal) that expects input from 3 files - f1, f2, f3.

for(...)
    scanf(....)//reads input from f1
for(...)
    scanf(....)//reads input from f2
for(...)
    scanf(....)//reads input from f3

I want to provide the input to the C Program using the redirection operator on bash.

However if I run my program as follows I am able to read from only 1 file:

./a.out < f1

How do I redirect the input of f1, f2,andf3 to my C Program?

Upvotes: 0

Views: 5039

Answers (2)

ilkkachu
ilkkachu

Reputation: 6527

Usually, you'd only use redirections for standard input (fd 0, with the < redirection operator), standard output (fd 1, >) and standard error (fd 2, 2>), which seems to limit you to only one input stream.

(Of course you could do something like cat f1 f2 f3 | program, but telling the files apart would be hard.)

But, if you actually really want to, you can use other file descriptor numbers for inputs too. This is done in the same way as when redirecting stderr, just add the number before the redirection operator. So, running

./prog 3<file1 4<file2 5< file3

would open the given files in fd's 3, 4 and 5 and pass those on to ./prog. The program can then use fdopen to associate stdio streams (FILE *) with them. The biggest issue here is that you'll need some way of knowing which fd's to use in the program, and that would quickly require using something like command line arguments. The toy example below is fixed to use those three fd's, which of course does not scale in the least.

Instead, you'd be better off doing what all other programs do, and passing the file names as command line arguments. Even in the situations where you need to refer to an already-open file descriptor, many Unixes have a way of referring to them by name: /dev/fd/1 etc. (Process substitution, which is something like a generalized pipe, is one of those things that need to do this.)

Silly fdopen() example: 

#include <stdio.h>

void read_a_line(int fd)
{
    char buf[64];
    FILE *f = fdopen(fd, "r");
    fgets(buf, 64, f);
    printf("fd %d: %s\n", fd, buf);
    fclose(f);
}

int main(void)
{
    read_a_line(3);
    read_a_line(4);
    read_a_line(5);
    return 0;
}

Upvotes: 10

user7851115
user7851115

Reputation:

The function I think you might be looking for is getopt. That way you can take command line arguments instead of redirecting (which is only for one input stream) three files to your program. Then just call it like this:

./a.out f1 f2 f3

Additionally, I just saw this question, which uses a structure like this:

#include <stdio.h>
int main(int argc, const char* argv[]) {
    for(int i = 1; i < argc; i++) {
       //work with each file name
       //note that argv[0] is: ./a.out
    }
}

Upvotes: 1

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