Why does "$<space><tab><tab>" perform filename completion?

Question

While trying to configure the programmable completion, I came across a behavior I don't understand.

Given those three completion scenarios, this is the way I understand them:

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$<TAB><TAB>

Beginning of the line --> use complete -E --> if it doesn't exist, use complete -D --> if it doesn't exist, use the default Bash completion --> there are no words on the line --> use command completion.

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$com<TAB><TAB>

Trying to complete first word --> use command completion.

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$command argu<TAB><TAB>

There is at least one word on the line --> trying to complete an argument --> use complete for the given command --> if it doesn't exist, use complete -D --> if it doesn't exist, use the default Bash completion --> there is already at least one word on the line so don't use command completion --> doesn't start with $, ~ or @ so perform filename completion.

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My question is why is the filename completion performed instead of command completion in this scenario even though there are no words on the line yet:

$<SPACE><TAB><TAB>

It's even more confusing since the command completion below is performed as expected:

$<SPACE>com<TAB><TAB>

I most likely misunderstood the manual and I would be most grateful for any explanation.

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NOTE: $ denotes the prompt, not a literal dollar sign.

Answer 1

When a line starts with a blank in a Bash shell, this tells the shell that the command should not go into the command history. This can be configured by setting the HISTCONTROL env variable in your .bashrc, .profile or whatever file you want and should be the default behavior (see also How to prevent commands to show up in bash history?).

So command execution is the same when the line has a leading blank except that this command doesn't go into history. Thus, code completion also behaves the same.