Reading text file from terminal line?

Question

So I understand that to read and print out a line of text you can just use printf, scanf and type it out. However what if I want to print out a text file without typing them out in terminal? And I don't mean using fopen(filename, "r") where you can only open a specific file. I think this is called redirection but I'm having trouble understanding it. Something along the line as the below input example:

./myprogram < input.txt

Answer 1

Here is a redirection cheat sheet. The line that interest us is:

cmd < file: Redirect the contents of the file to the standard input (stdin) of cmd.

Here a simple example that will print the content of your input.txt file. Compared to manual input, the program will never wait and will loop until the end of the file is reached (Note: there are cases where there is no end, you might want to add alternative break condition).

#include <stdio.h>

int main(void)
{
    char    buffer[100];

    while (fgets(buffer, 100, stdin))
        printf("%s", buffer);
    return (0);
}

• ./myprogram < input.txt will print your input.txt
• ./myprogram will wait for your manual input and print what you just typed.

Answer 2

This is not exactly what you asked but you can put the filename as argument and get it in argv[1] and then use fopen