Here-Documents (<<EOF)

A Basic Here-Document

<<DELIM starts a here-document; everything until a line containing exactly DELIM becomes the stdin of the preceding command. This is a clean way to embed multi-line text without a pile of echo calls.

Example: A Basic Here-Document

#!/bin/bash
cat <<EOF
Line one
Line two
Line three
EOF

Variable Expansion Inside a Here-Document

By default, variables and command substitutions inside a here-document are expanded just like in double quotes. Quoting the delimiter, e.g. <<EOF, disables that expansion and treats the block as literal text.

Example: Variable Expansion Inside a Here-Document

#!/bin/bash
name="Ada"
cat <<EOF
Hello, $name!
Today is a scripted day.
EOF

echo "---"
cat <<'EOF'
This $name will NOT be expanded.
EOF

Generating a File with a Here-Document

Redirecting a here-document into a file is a common way to generate configuration files or templates from within a script.

Example: Generating a File with a Here-Document

#!/bin/bash
cat > config.txt <<EOF
host=localhost
port=8080
EOF
cat config.txt
rm -f config.txt

Indented Here-Documents with <<-

<<-DELIM strips leading tab characters (not spaces) from each line, including the closing delimiter, so you can indent a here-document to match the surrounding code inside a function.

Warning: <<- strips leading tabs only, not spaces, so mixing spaces and tabs will break the trick.

Example: Indented Here-Documents with <<-

#!/bin/bash
show_message() {
	cat <<-EOF
	This line was indented with a tab.
	So was this closing delimiter.
	EOF
}
show_message