> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Bash Left Shift

The `<<` operator, known as a **Here Document** or **Heredoc**, is an I/O redirection mechanism that instructs the Bash parser to read a block of multi-line text from the current source and feed it to a command's standard input (`stdin`). The shell continues reading until it encounters a line containing only a predefined, user-specified delimiter.

```bash theme={"dark"}
command <<DELIMITER
document_body
DELIMITER
```

## Lexical Rules and Parsing Mechanics

* **Delimiter Constraints:** The terminating `DELIMITER` must appear on a line by itself. It cannot be preceded by any characters (including spaces), cannot be followed by any characters (including comments), and must match the opening identifier exactly.
* **Standard Expansion:** By default, the shell subjects the `document_body` to parameter expansion (`$VAR`), command substitution (`$(command)`), and arithmetic expansion (`$((expression))`). The backslash (`\`) acts as an escape character to prevent specific expansions.
* **Literal Evaluation (Quoted Delimiter):** If any character of the opening `DELIMITER` is quoted (using single quotes `'EOF'`, double quotes `"EOF"`, or escaping `\EOF`), the shell disables all expansions. The entire `document_body` is passed to the command as raw, literal text.

## The Tab-Stripping Modifier (`<<-`)

Appending a hyphen to the operator alters the parsing behavior to ignore leading tab characters.

```bash theme={"dark"}
command <<-DELIMITER
	document_body
	DELIMITER
```

When `<<-` is used, Bash strips all leading tab characters (`\t`)—but not spaces—from every line within the `document_body` as well as from the line containing the terminating `DELIMITER`. This mechanism exists strictly to allow the Here Document to be indented naturally within the source code without passing the indentation whitespace to the command's `stdin`.

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