Bash Command Line – Understanding the Exclamation Mark (!)

bashcommand historycommand line

I used

history | less

to get the lines of previous commands and from the numbers on the left hand side I found the line I wanted repeated (eg. 22) and did

!22

at the command prompt and it worked — executing the set of commands on the line I did at that time. I cannot figure out where the exclamation mark is used, what does it represent in terms of actions taken by bash, and where to use it. From the documentation I do not see an explanation that is 'tangible'.

Best Answer

! 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.

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Command-Line Bash History – Put History Command onto Command Line Without Executing It

To request that the command be printed rather than executed after history substitution, add the :p modifier, e.g. !42:p. The resulting command will also be entered in the history, so you can press Up to edit it.

If you have the histverify option set (shopt -s histverify), you will always have the opportunity to edit the result of history substitutions.

The fc builtin gives limited access to history expansion (no word designators), and lets you edit a previous command in an external editor.

You can use !prefix to refer to the last command beginning with prefix, and !?substring to refer to the last command beginning with substring. When you know what you're looking for, this can save a lot of time over history | less.

Another way to search through previous history is incremental search: press Ctrl+R and start entering a substring of what you're looking for. Press Ctrl+R to go to the previous occurence of the search string so far and Ctrl+S if you've gone too far. Most keys other than Ctrl+R, Ctrl+S, Backspace and ordinary characters terminate the incremental search and have their usual effect (e.g. arrow keys to move the cursor in the line you've reached, Enter to run the command).

Why Does the Exclamation Mark `!` Sometimes Upset Bash?

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.

Best Answer

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Command-Line Bash History – Put History Command onto Command Line Without Executing It

Why Does the Exclamation Mark `!` Sometimes Upset Bash?

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