Heredoc

A Here document (Heredoc) is a type of redirection that allows you to pass multiple lines of input to a command.

The syntax of writing HereDoc takes the following form:

[COMMAND] <<[-] 'DELIMITER'
  HERE-DOCUMENT
DELIMITER

• The first line starts with an optional command followed by the special redirection operator << and the delimiting identifier.
• You can use any string as a delimiting identifier, the most commonly used are EOF or END.
• If the delimiting identifier is unquoted, the shell will substitute all variables, commands and special characters before passing the here-document lines to the command.
• Appending a minus sign to the redirection operator <<-, will cause all leading tab characters to be ignored. This allows you to use indentation when writing here-documents in shell scripts. Leading whitespace characters are not allowed, only tab.

See more here

Examples

Heredoc is most often used with the cat command.

An example with environment variables:

cat << EOF
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF

Output:

The current working directory is: /home/john
You are logged in as: john

+++

What if we enclose the delimeter with quotes?

cat << "EOF"
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF

Output:

The current working directory is: $PWD
You are logged in as: $(whoami)

Notice that no paramter expansion and command substitution is done by the shell

+++

Instead of displaying the output, we can redirect to file with > or >> operators:

cat << EOF > file.txt
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF

> overwrites the file whilst >> appends to file.