Is the "tick" or "acute accent" character ´ (not `) used for anything in Linux shell?

The character sets used historically with Unix, including ASCII, don’t have a tick character, so it wasn’t used. As far as I’m aware no common usage for that character has been introduced since it’s become available; nor would it, since it’s not included in POSIX’s portable character set.

` was apparently originally included in ASCII (along with ^ and ~) to serve as a diacritic. When ASCII was defined, the apostrophe was typically represented by a ′-style glyph (“prime”, as used for minutes or feet) rather than a straight apostrophe ', and was used as a diacritic acute accent too.

Historically, in Unix shell documentation, ` was referred to as a grave accent, not a backtick. The lack of a forward tick wouldn’t have raised eyebrows, especially since ' was used as the complementary character (see roff syntax).

Although the ASCII committee thought of ` as a diacritic and ' as an apostrophe, the Unix tradition was to treat them as left and right single quotes (the characters that are now in Unicode as U+2018 and U+2019). This is most obvious from the TeX convention that you write ``quoted text'' in the markup source and you get “quoted text” in the formatted document. This can also be seen in older monospace fonts, such as the bitmap fonts shipped with X11R5 and X11R6, where the two characters were given mirror-image glyphs.

As such, ` and ' were considered each others' inverse, and ´ was not felt to be missing.