Here document (heredoc)

Here document (heredoc) redirects a multiline string literal to the preceding command while preserving line breaks. Unix syntax for it is:

[command] <<DELIMITER

    First line.

    Second line.

    Third line.

    Fourth line.

DELIMITER

<< is Redirection Operator

- is optional Tab Suppression

DELIMITER - an arbitrary string, Delimiter Token; must be the same at the beginning and at the end

Appending a minus sign to the redirection operator <<- causes all leading tab characters to be ignored. This allows you to use indentation when writing heredocs in shell scripts. We can then indent both the here-doc and the delimiter with tabs (not spaces!):

#! /bin/bash

cat <<-EOF

    indented

    EOF

echo Done

---

References:

Here document - Wikipedia