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Here Documents

Passing Multi-Line Input with Here Documents and Herestrings

When scripting, you often need to pass multi-line input to commands without relying on external files. Bash provides two mechanisms for this: here documents and herestrings. Both allow you to embed input directly into a command’s standard input stream, but they differ in how they handle variable expansion and syntax.

Here Documents

A here document (also called a "here doc") sends multi-line text to a command’s standard input. It begins with << followed by a delimiter (e.g., EOF), and ends when the delimiter is encountered on a line by itself. The delimiter is case-sensitive and must match exactly.

Syntax

command <<DELIMITER
input line 1
input line 2
...
DELIMITER

Example: Using cat with a Here Document

cat <<EOF
This is a
multi-line input.
EOF
Output: 
This is a
multi-line input.

Key Notes

  • The delimiter line must not have leading spaces.
  • Variables in the here document are expanded (e.g., $HOME becomes the home directory path).
  • A newline is added after the last line of input unless explicitly suppressed.

Herestrings

A herestring is similar to a here document but uses <<'DELIMITER' or <<"DELIMITER"`" to disable variable expansion. This makes it ideal for passing literal text, such as configuration snippets or raw data, without unintended substitutions.

Syntax

command <<'DELIMITER'
input line 1
input line 2
...
DELIMITER

Example: Using echo with a Herestring

echo <<'EOF'
This is a
literal string with $VAR and `command`.
EOF
Output: 
This is a
literal string with $VAR and `command`.

Key Notes

  • The delimiter is enclosed in quotes (' or "), which prevents variable expansion and command substitution.
  • Herestrings are specific to Bash and not supported in all shells.

When to Use Each

Feature Here Document Herestring
Variable Expansion ✅ Enabled ❌ Disabled
Syntax <<DELIMITER <<'DELIMITER'
Use Case Dynamic input Literal data

Key takeaways

  • Use here documents for multi-line input that requires variable expansion (e.g., generating config files).
  • Use herestrings for literal text where variables and commands should remain unprocessed (e.g., logging or debugging).
  • Always ensure the delimiter matches exactly and is placed on a line by itself.
  • Herestrings are Bash-specific; avoid them in scripts intended for other shells.