1

Bash offers the ${PATH,,} and ${PATH^^} in its more recent incarnations to change a the case of a variable value ad hoc. When were those introduced? The Bash Cookbook from 2007 does not make any mention of that string substitution and it is not listed on the Bash Hackers Wiki page.

Makes me think it was introduced with 4.x in 2009, but I'm looking for an authoritative source.

2

They were introduced in bash 4.0, as indicated in the NEWS file:

This is a terse description of the new features added to bash-4.0 since the release of bash-3.2. As always, the manual page (doc/bash.1) is the place to look for complete descriptions.

[...]

hh. There are new case-modifying word expansions: uppercase (^[^]) and lowercase (,[,]). They can work on either the first character or array element, or globally. They accept an optional shell pattern that determines which characters to modify. There is an optionally- configured feature to include capitalization operators.

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