> ## 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 Here Document
The `<<` operator, known as a **Here-Document** (or heredoc), is an I/O redirection mechanism in Bash. It instructs the shell to read a block of string literals from the current input stream and redirect it to the standard input (`stdin`) of a specified command. The shell continues reading until it encounters a predefined, user-specified delimiter token on a line by itself.
```bash theme={"dark"}
command << DELIMITER
String content line 1
String content line 2
DELIMITER
```
## Parsing and Expansion Mechanics
By default, the shell parses the body of a Here-Document similarly to a double-quoted string. It subjects the content to three specific types of expansion before passing it to the command:
1. **Parameter Expansion** (`$VAR`)
2. **Command Substitution** (`$(command)` or `` `command` ``)
3. **Arithmetic Expansion** (`$((expression))`)
To prevent the shell from evaluating a specific character (like `$`, `` ` ``, or `\`), it must be escaped using a backslash (`\`).
## Delimiter Quoting (Literal Mode)
If any character of the opening delimiter is quoted (using single quotes, double quotes, or a backslash), the shell alters its parsing behavior. It disables all expansions within the Here-Document body, treating the entire block as a raw, literal string.
```bash theme={"dark"}
# Single quotes
command << 'EOF'
$VAR will not be expanded here.
EOF
# Double quotes
command << "EOF"
$(command) will not execute here.
EOF
# Backslash escape
command << \EOF
$((1 + 1)) remains literal text.
EOF
```
## The `<<-` Variant (Tab Stripping)
Appending a hyphen to the operator (`<<-`) modifies the shell's whitespace handling. When `<<-` is used, the shell strips all leading tab characters (`\t`) from every line within the Here-Document body, as well as from the line containing the closing delimiter.
This variant exists strictly for source code formatting, allowing the Here-Document to be indented to match the surrounding script structure without passing the indentation to the command's `stdin`.
```bash theme={"dark"}
if true; then
command <<- EOF
This line has leading tabs stripped.
So does this one.
EOF
fi
```
*Note: The `<<-` operator strictly strips tab characters. Leading spaces are preserved and passed to the command.*
## Termination Constraints
The shell enforces strict lexical rules for the closing delimiter:
* It must be the exact character sequence specified by the opening delimiter.
* It must reside on its own line.
* It cannot be preceded by any characters (except tabs if `<<-` is used).
* It cannot be followed by any characters, including trailing spaces, semicolons, or comments.
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