Question

Code goes below:

#!/bin/bash

wd1="hello"
wd2="world"

cat >> log.txt <<<"$wd1\t$wd2\n\n"

When I run the above script, '\t','\n' were not expanded at all. So I altered it to this:

cat >> log.txt <<<$(echo -e "$wd1\t$wd2\n\n")

But '\t','\n' are still not expanded. Why?

Answer 1

From info bash:

3.6.7 Here Strings
------------------

A variant of here documents, the format is:
     <<< WORD

   The WORD is expanded and supplied to the command on its standard
input.

1. <<<"$wd1\t$wd2\n\n" is subject to bash expansions but there is no standard expansion for \t or \n. That’s why it doesn’t happen.
2. <<<$(echo -e "$wd1\t$wd2\n\n") doesn’t work because it is unquoted. echo outputs the special characters but then bash does field splitting and they got replaced by spaces.

You just need to quote it:

cat >> log.txt <<<"$(echo -e "$wd1\t$wd2\n\n")"