EOF is an acronym that stands for “End of file”

In shell, EOF is commonly used as a terminator in what is called a heredoc (“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

In C programming language, EOF is a constant defined -1 which indicates an end of file.

// 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;
}

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