How to Use Heredoc in Bash: Syntax and Examples

In bash, use heredoc to create multiline strings or input by typing <<DELIMITER followed by the text and ending with the same DELIMITER on a line alone. This lets you easily pass blocks of text to commands or variables without quotes or escapes.

Syntax

The basic syntax of a heredoc in bash is:

• <<DELIMITER: Starts the heredoc, where DELIMITER is a word you choose to mark the end.
• Lines of text: The content you want to include.
• DELIMITER: The same word on a line by itself ends the heredoc.

You can use quotes around DELIMITER to control variable expansion inside the heredoc.

bash

cat <<EOF
This is line 1
This is line 2
EOF

Example

This example shows how to use heredoc to print multiple lines with cat and how variable expansion works inside it.

bash

name="Alice"
cat <<EOF
Hello, $name!
Welcome to heredoc in bash.
EOF

Output

Hello, Alice!
Welcome to heredoc in bash.

Common Pitfalls

Common mistakes include:

• Not placing the ending DELIMITER alone on its line (no spaces or tabs allowed).
• Using quotes around DELIMITER changes variable expansion behavior.
• Indentation before the ending DELIMITER will cause the heredoc to not end properly.

Example of wrong and right usage:

bash

# Wrong: ending delimiter indented
cat <<EOF
Line 1
  EOF

# Right: ending delimiter alone
cat <<EOF
Line 1
EOF

Quick Reference

Feature	Description
<	Start heredoc, DELIMITER marks end
DELIMITER	Must be alone on line to end heredoc
"DELIMITER"	No variable expansion inside heredoc
'DELIMITER'	No variable expansion inside heredoc
<<-DELIMITER	Allows tab-indented end delimiter

Key Takeaways

Use <

Variable expansion happens unless DELIMITER is quoted.

The ending delimiter must not have spaces or tabs before it unless using <<-.

Heredoc is useful for multiline strings or feeding input to commands.

Use <<-DELIMITER to allow tabs before the ending delimiter for cleaner indentation.