echo "$(whoami)"

whoami

ShellCheck found the unnecessary construct echo "$(somecommand here)".

This is generally due to a misunderstanding about what echo does. It has no role in "showing on screen" or similar, but simply writes a string to standard output. This is also how all other programs output data.

echo "$(somecommand)" will capture the output somecommand writes to standard output and write it to standard output, where it was already going. At best this is a no-op, but it may have several other negative effects:

• It disables parallel processing in pipelines, such as echo "$(find . -name '*.iso')" | xargs sha1sum which does not allow iterating files and checksumming at the same time. Similarly, users don't see incremental updates as programs run.
• It introduces shell and echo related pitfalls like being unable to output the string -n, stripping NUL bytes and trailing linefeeds, and expanding escape sequences in some shells but not others.
• It suppresses the exit code of the command, so that echo "$(grep '^user:' /etc/passwd)" no longer returns with failure when the user is not found.
• It does not allow programs to tailor their output for terminals, such as ls vs echo "$(ls)" where the former outputs columns and colors according to user preferences, while the latter doesn't.
• It uses unnecessary memory to buffer up the data before writing it where it was already going.

To avoid all this, simply replace echo "$(somecommand)" with somecommand as in the example. It's shorter, faster, and more correct.

If you are relying on one of the otherwise detrimental effects for correctness, you can consider one of:

# Suppress exit code without the other negative effects
cmd || true

# Disable tty specific output without the other negative effects
cmd | cat

# Buffer up potentially large output without using more memory or modifying the content in any way
cmd > file.tmp
cat file.tmp

# Exactly like `echo "$(cmd)"`, but allows output like `-n` and works the same across shells
printf '%s\n' "$(cmd)"