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Use $(...) notation instead of legacy backticked `...`.

Problematic code

echo "You are running on `uname`"

Correct code

echo "You are running on $(uname)"

Rationale

Backtick command substitution `...` is legacy syntax with several issues.

1. It has a series of undefined behaviors related to quoting in POSIX.
2. It imposes a custom escaping mode with surprising results.
3. It's exceptionally hard to nest.

$(...) command substitution has none of these problems, and is therefore strongly encouraged.

Exceptions

• Some legacy sh implementations (like Solaris) do not support $(...), it is necessary to use backtick command substitution there. See [mc-devel] [PATCH] Prefer $() to backticks in sh script and follow-ups.

Related resources:

• BashFaq: Why is $(...) preferred over `...` (backticks)?
• StackOverflow: What is the difference between $(command) and `command` in shell programming?
• shfmt will automatically convert the legacy syntax

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