Why is 'while.. read.. <<EOL' performing variable expansion, yet <file and | don't?

Question

Aside from the variable expansion mentioned in the question's heading, I also experience another alarming issue when reading inline data to <<EOL... When the data contains a backtick `, it causes an error.

Both are substitution issues, but what and where is the relevant option? ...(and why is the inline read behaving differently?)

Here is the script

#!/bin/bash
echo '==$BASH' | \
while IFS= read -r line ; do echo "# output 1: $line" ;done

echo '==$BASH'>junk
while IFS= read -r line ; do echo "# output 2: $line" ;done <junk

while IFS= read -r line ; do echo "# output 3: $line" ;done <<EOL
==$BASH
EOL

while IFS= read -r line ; do echo "# output 4: $line" ;done <<EOL
`my backtick test
EOL

This is the output

# output 1: ==$BASH
# output 2: ==$BASH
# output 3: ==/bin/bash
...: bad substitution: no closing "`" in `my backtick test

Answer 1

Shell expansion is one of the main benefits of 'here documents'. Good news is you can turn it off.

Try:

while IFS= read -r line ; do echo "# output 3: $line" ;done <<'EOL'
==$BASH
EOL

while IFS= read -r line ; do echo "# output 4: $line" ;done <<'EOL'
`my backtick test
EOL

For details see:

> info bash

<...>

3.6.6 Here Documents
--------------------

This type of redirection instructs the shell to read input from the
current source until a line containing only WORD (with no trailing
blanks) is seen.  All of the lines read up to that point are then used
as the standard input for a command.

   The format of here-documents is:
     <<[-]WORD
             HERE-DOCUMENT
     DELIMITER

   No parameter expansion, command substitution, arithmetic expansion,
or filename expansion is performed on WORD.  If any characters in WORD
are 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.  In the latter case,
the character sequence `\newline' is ignored, and `\' must be used to
quote the characters `\', `$', and ``'.

   If the redirection operator is `<<-', then all leading tab
characters are stripped from input lines and the line containing
DELIMITER.  This allows here-documents within shell scripts to be
indented in a natural fashion.