Read file as input from the command line

Question

I am trying to read a file from the command line passed as input. No not the filename. I'm not expecting the user to input a filename on the command-line, so that I could open it like this : fopen(argv[1], "r");.

I am expecting a file like this : myprogram < file_as_input. So whatever should go into argv is the contents of the file. How do I do this in C/C++ ?

Answer 1

When a program is invoked like this ./a.out < file, the content of the file will be available on the standard input: stdin.

That means that you can read this content by reading the standard input.

For example:

• read(0, buffer, LEN) would read the from your file.
• getchar() would return a char from your file.

Answer 2

On using redirection on the command line, argv does not contain the redirection.

The specified file simply becomes your stdin/cin.

So no need to open it using fopen ,just read from the standard input.

Example :

using namespace std;
int main()
{

vector <string> v;

copy(istream_iterator<string>(cin),
    istream_iterator<string>(),
    back_inserter(v));

          for(auto x:v)
                cout<<x<<" ";

return 0;
}

test <input.txt

Output

Contents of input.txt seperated by space