Question

I’m writing my own shell in C and I need to detect EOF (for when I run ./myshell < commands.txt)

commands.txt contains:

ls
pwd

These both run fine separately from within the program. But when I run it with the text file, I get an infinite loop.

In my while(1) loop for the shell, the first thing I do is this:

if (feof(stdin)) { my_exit(); }

my_exit is simply:

void my_exit() {
    printf("End of file! Bye\n");
    exit(0);
}

Doesn’t exit(0) end the program (and the loop)? Why am I getting

End of File! ByeEnd of File! ByeEnd of File! ByeEnd of File! ByeEnd of File! ByeEnd of File! Bye…. etc

I have also tried doing the fgets == NULL way. Same loop

Answer 1

The problem is that feof() tells you if the LAST input operation ran into EOF. But you’re not checking this until the next iteration. So when you’re at EOF, you do fgets() and then try to use the empty result that it returned.

What’s happening is that you fork() a child process, and then call execvp() with an empty command name. This is failing, so the child process returns to the beginning of the loop, and does the same thing. Meanwhile, the parent process calls my_exit(). So each child process forks another child of its own, and then exits.