# EOF
**EOF** is an acronym that stands for "End of file"

## Shell 
In shell, EOF is commonly used as a terminator in what is called a [heredoc](https://en.wikipedia.org/wiki/Here_document) ("here document").
It's when you want to supply a multiline text to a command to its standard output, using `<<`.
```
 $ cat > hello.txt << EOF
Here, you can write anything you like.
It's not over until you actually type E-O-F.
You can cancel with Ctrl + C.
EOF
```
But this is just a convention. You can use any string you like in heredocs.
```
 $ cat > hello.txt << whatever
Now you can type EOF EOF EOF as much as you want
until you type it
whatever
```

## C
In C programming language, EOF is a constant defined `-1` which indicates an end of file.
```c
// Compile: gcc main.c -o program
// Run:     echo 'Hello world' | ./program
// Output:  HELLO WORLD
#include <stdio.h>
#include <ctype.h>

int main()
{
    int ch;
    while ((ch = fgetc(stdin)) != EOF) {
        putchar(toupper(ch));
    }
    return 0;
}
```

## Links

 - https://en.wikipedia.org/wiki/End-of-file
 - https://stackoverflow.com/questions/1782080/what-is-eof-in-the-c-programming-language