A here document (also called a here-document, a heredoc, a hereis, a here-string or a here-script) is a way of specifying a string literal in command line shells including Unix shells (sh, csh, ksh, Bash and zsh) and in programming or scripting languages such as Perl, PHP, Python, PowerShell and Ruby. It preserves the line breaks and other whitespace (including indentation) in the text. Some languages allow variable substitution and command substitution inside the string.
The most common syntax for here documents is << followed by a delimiting identifier, followed, starting on the next line, by the text to be quoted, and then closed by the same identifier on its own line. Under the Unix shells, here documents are generally used as a way of providing input to commands.
Read more about Here Document: Specific Implementations
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