Understanding the exclamation mark (!) in bash

! is 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 !!.

Bash lets you disable ! processing in the shell with set +o histexpand or set +H. You can disable it in Zsh with set -K.

If there isn't a longer answer here there's certainly one on Super User, since I've read one recently. In the bash man page you can find a huge section titled HISTORY EXPANSION on the matter.

You can do a whole host more than just run the last command, or command number X. You can do things like !cat to run the last command that started with cat. Or !?bash?:s/bash/csh/ runs the last command containing bash but replaces it with csh.

A lot more can be done with ! such as:

• execute a command which is typed before 3 commands: !-3
• execute a command that starts with !ls

and a lot more. See 15 Linux Bash History Expansion Examples You Should Know