Bash – What does typing a single exclamation mark do in Bash

! invokes history expansion, a feature that originally appeared in the C shell, back in the days before you could count on terminals to have arrow keys. It's especially useful if you add the current command number to the prompt (PS1="\!$ ") so you can quickly look at your screen to get numbers for past commands.

Now that you can use arrow keys and things like Ctrl-R to search the command history, I don't see much use for the feature.

One variant of it you might still find useful is !!, which re-executes the previous command. On its own, I don't find !!Enter any faster than just ↑ Enter, but it can be helpful when combined into a larger command.

Example: A common pilot error on sudo based systems is to forget the sudo prefix on a command that requires extra privileges. A novice retypes the whole command. The diligent student edits the command from the shell's command history. The enlightened one types sudo !!.

Processing ! in this way is enabled in Bash by default in interactive shells and can be disabled with set +o histexpand or set +H. You can disable it in Zsh with set -K.

The ! character invokes bash's history substitution (enabled by default in interactive shells). When followed by a string (as in your failing example) it tries to expand to the last history event that began with that string. Just like $var gets expanded to the value of that string, !echo would expand to the last echo command in your history.

Space is a breaking character in such expansions. First note how this would work with variables:

# var="like"
# echo "$var"
like
# echo "$"
$
# echo "Do you $var frogs?"
Do you like frogs?       <- as expected, variable name broken at space
# echo "Do you $varfrogs?"
Do you?                  <- $varfrogs not defined, replaced with blank
# echo "Do you $ var frogs?"
Do you $ var frogs?      <- $ not a valid variable name, ignored

The same thing will happen for history expansion. The bang character (!) starts off a history replacement sequence, but only if followed by a string. Following it with a space make it literal bang instead of part of a replace sequence.

You can avoid this kind of replacement for both variable and history expantion by using single quotes. Your first examples used single quotes and so ran fine. Your last examples are in double quotes and thus bash scanned them for expantion sequences before it did anything else. The only reason the first one didn't trip is that that the space is a break character as shown above.