This is a Unicode single quote. Delete and retype it.

Problematic code:

echo ‘hello world’

Correct code:

echo 'hello world'

Rationale:

Some software, like OS X, Word and WordPress, may automatically replace your regular quotes with slanted Unicode quotes. Try deleting and retyping them, and/or disable “smart quotes” in your editor or OS.

Status:

This error was retired after 0.4.5. In this version and earlier, ShellCheck parsed slanted quotes as a valid double quote. This meant that the warning could not simply be ignored. It has since been replaced by SC1110 (outside quotes) and SC1112 (inside single quotes).

Exceptions

If you want to use typographic single quotes, you can put them in double quotes (or typographic double quotes in single quotes) to make shellcheck ignore them, e.g.,

printf "Warning: ‘wakeonlan’ is not installed.\n"