A source of confusion when using heredocs often arises from using variables. By default, variables are resolved in a heredoc, as can be seen in the following example:

reader@ubuntu:~/scripts/chapter_12$ cat << EOF
> Hi, this is $USER!
> EOF
Hi, this is reader!

However, this might not always be desirable functionality. You might want to use this to write to a file in which the variables should be resolved later.

In this case, we can quote the delimiting identifier EOF to prevent variables being substituted:

reader@ubuntu:~/scripts/chapter_12$ cat << 'EOF'
> Hi, this is $USER!
> EOF
Hi, this is $USER!