\n in variable in heredoc

Question

Is there any way to for a Bash heredoc to interpret '\n\' in a heredoc?

I have an iteratively built string in a loop, something like

for i in word1 word2 word3
do
        TMP_VAR=$i
        ret="$ret\n$TMP_VAR"
done

and then I want to use the created string in a heredoc:

cat <<EOF > myfile
HEADER
==
$ret
==
TRAILER
EOF

however I would like to interpret the "\n" character as newline, so that the output is

HEADER
==
word1
word2
word3
==
TRAILER

instead of

HEADER
==
\nword1\nword2\nword3
==
TRAILER

Is it possible? Or should I perhaps build my initial string somehow otherwise?

Answer 1

In bash you can use $'\n' to add a newline to a string:

ret="$ret"$'\n'"$TMP_VAR"

You can also use += to append to a string:

ret+=$'\n'"$TMP_VAR"

Answer 2

As others (and other answers to other questions) have said, you can put encoded characters into a string for the shell to interpret.

x=$'\n' # newline
printf -v x '\n' # newline

That said, I don't believe there is any way to directly put an encoded newline into a heredoc.

cat <<EOF
\n
EOF

just outputs a literal \n

cat <<$'EOF'
…
EOF

is nothing special, nor is <<'EOF'

The best you can do is to preencode the newline, and include the expansion in the heredoc:

nl=$'\n'
cat <<EOF
foo bar $nl baz
EOF

outputs

foo bar
 baz