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How to redirect stdin to a file? – Debian

I want to redirect stdin to a file, so that I can write to the file and my program prints the character.
Below is a simple c code snippet, which prints stdin.

The programm is compiled with gcc and runs on debian 4.19.0 in virtualbox

//printchar.c

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    
    int c;
    
    while( (c = getchar()) !='.') {
        putchar(c); 
    }

    return EXIT_SUCCESS;
}

I call the programm with ./printchar 0 < testfile.txt

Then i echo efghi > testfile.txt but nothing happens.
If i prefill the file with abcd, abcd is printed instantly after programm start, but again I can’t echo something to the testfile.

Isn’t it possible to redirect stdin in this way?

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2

Answers


  1. 0

    I think you can do it using getcharlike this:

    int main(void) 
{
    int c;

    while(1)
    {
      c = getchar();
      if (c == '.') break;
      if (c != EOF)
      {
        putchar(c);
      }
      else
      {
        usleep(100);
      }
    }

    return 0;
}

    or using read like this:

    #include <stdio.h>
#include <unistd.h>

int main()
{
  char c;
  int n;
  while(1)
  {
    n = read(0, &c, 1);
    if(n > 0)
    {
      if (c == '.') break;
      putchar(c);
    }
    else
    {
        usleep(100);
    }
  }

  return 0;
}

    However, you need to append to the file using >> like:

    touch testfile.txt           // create empty file
./printchar < testfile.txt   // start program
echo hello >> testfile.txt   // append to file
echo world >> testfile.txt   // append to file
echo . >> testfile.txt       // append to file
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  2. 0

    You can also use heredoc << associated with redirection into a file like this :

        << keyCodeToStopWriting >> myFile
    first line
    second line
    keyCodeToStopWriting  //Stop the writing

    In this exemple I use >> to add line at the end of the file but you can overwrite it with >

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