Why Bash can't ignore case when tab completion variable names? [closed]

Question

When I want to input the command echo $bash_, then I press Tab key, the autocompletion can't occur.
But when I input the command echo $BASH_, then I press Tab, the completion list will output like this:

$BASH_ALIASES                $BASH_COMMAND                $BASH_SOURCE
$BASH_ARGC                   $BASH_COMPLETION_COMPAT_DIR  $BASH_SUBSHELL
$BASH_ARGV                   $BASH_LINENO                 $BASH_VERSINFO
$BASH_CMDS                   $BASH_REMATCH                $BASH_VERSION

And my .inputrc file contains the readline ignorecase option, set completion-ignore-case on, and the filename completion case insensitive is okay.
So, I want the variable name tab completion can ignore the variable case.

Answer 1

There are case-insensitive filesystems, and on such systems it makes sense for filename completion to be case-insensitive. In an ideal world, bash could tell if a component of a filepath were case insensitive, but in practice there is no standard interface which provides this information, so bash falls back to letting you explicitly configure case-insensitive filename completion: (quote from bash manual, emphasis added)

completion-ignore-case
  If set to on, readline performs filename matching and completion in a case-insensitive fashion. The default value is ‘off’.

Similarly, you can configure glob expansion and/or case patterns to be case-insensitive.

But you cannot make bash variables case-insensitive. $bash and $BASH are different variables. Similarly with bash function names, bash builtin names and bash keywords. Bash can do tab-completion with all of those, but regardless of the configuration of filename case sensitivity, the tab-completion is case sensitive.

While that seems like a reasonable justification, it turns out that bash tab-completion is always case sensitive except for filenames (including directories and bash commands, which map onto filenames), which can be configured to be case-insensitive. Even things which are typically case-insensitive, like signal names and host names can only be completed case-sensitively.