how do i feed printf as input to a C program using a file?

Question

I know you can give input to a C program in linux like this

me $ printf "some input" | ./someProgram

I want to do the same thing using input from a file, like this

me $ myProgram < myFile.txt

myProgram has two gets statements that I want to fill w/printfs as input.

  printf("...."); fflush(stdout);
  gets(var1);
  printf("...."); fflush(stdout);
  gets(var2);

The program behaves as expected when I fill the vars from an input file like this

"12345"  //expect to fill var1 w/ 12345 and it does
"12345"  //expect to fill var2 w/ 12345 and it does

But the program does not behave as expected when my input file looks like this

printf "12345"  //expect to fill var1 w/ 12345 but it does not
printf "12345"  //expect to fill var2 w/ 12345 but it does not

Clearly, C is not interpreting the print command in the same was as if I gave the command as input and then piped it into the program.

What's going on? What do I fix? How can I give printf input from a file?

Answer 1

C isn't interpreting any commands at all. When you use ... < file to redirect stdin , the contents of that file become the input to your program - verbatim. So if the file contains

"12345"  
"abc"

, then var1 will be "12345" (not 12345 ) and var2 will be "abc" (not abc ). Similarly, if the file contains

printf "12345"
printf "12345"

, then var1 will be printf "12345" and var2 will be printf "12345" .

As I see it, you have two choices: Either you put

12345
12345

in the file and use ./someProgram < myFile.txt , or you put

printf "12345\n"
printf "12345\n"

in the file and use sh myFile.txt | ./someProgram sh myFile.txt | ./someProgram (executing the contents of myFile.txt as a shell script).

Answer 2

You are missing the trailing newline.

printf "12345\\n" should work well.