What is HERE document?
In computer science, a here document (here-document, heredoc, hereis, here-string or here-script) is a file literal or input stream literal: it is a section of a source code file that is treated as if it were a separate file. The term is also used for a form of multiline string literals that use similar syntax, preserving line breaks and other whitespace (including indentation) in the text.
Here documents originate in the Unix shell, and are found in sh, csh, ksh, Bash and zsh, among others. Here document-style string literals are found in various high-level languages, notably the Perl programming language (syntax inspired by Unix shell) and languages influenced by Perl, such as PHP and Ruby. Other high-level languages such as Python and Tcl have other facilities for multiline strings.