Parameter expansion
Quote the word in here-documents to inhibit any expansion.
<<'word'
If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence
\<newline>is ignored, and\must be used to quote the characters \, $, and `.