What is it "!!" and how it connected to `$_` or `$?`?

Question

I was playing with string substitution while learning a bit more of bash, and I have add !! into some dummy example echo ${TEST/hey/!!}...

I was surprised that !! was replaced with last entered command... what is it? is it somehow connected to magick codes link $? or $_ or even -. Is there anything else I can use in same manner in bash?

Related (but doesn’t address the relationship with parameter expansion): Understanding the exclamation mark (!) in bash. — Stephen Kitt, Dec 28 '18 at 09:46
They're not related, and they're not performed at the same stage (even if $_ and !$ will get you the same with simple commands). ! will get you the "bang" / history expansion, a mechanism inherited from csh. Go read the "HISTORY EXPANSION" chapter from the bash manual. — , Dec 28 '18 at 10:03

score 1 · Answer 1 · answered Dec 28 '18 at 12:35

Bash performs different kinds of automatic replacements called expansions. For example, some of them are:

History Expansion
Path Expansion
Parameter Expansion
and others...

When you include !! bash automatically replaces by previous executed command. The character ! performs history expansion. For example !10 is replaced by the 10th command in the command history. $ does parameter and variable expansion. All of those characters: -, _, and ? are special parameter for bash.

The best source to learn about it is the bash manual: Bash Reference Manual: Top

The example you mention echo ${TEST/hey/!!} include both parameter and history expansions.

What is it "!!" and how it connected to `$_` or `$?`?

1 Answers1