7

I understand the following error is due to ! used for history expansion:

$ echo "Hello!Tim"
bash: !Tim: event not found

However if I put the command into a script and run the script, there is no problem:

$ cat myscript
echo "Hello!Tim"
$ bash myscript 
Hello!Tim

Why is that? Does the bash manual mention the reason?

  • Good question, simply stated. Very well done! – Wildcard Aug 9 '17 at 2:41
  • Based on what I'm reading here: gnu.org/software/bash/manual/html_node/History-Interaction.html I believe you would need to use some sort of shopt modifier to get it to work in your script. I know that's not really the answer you are looking for. – Jesse_b Aug 9 '17 at 2:51
  • What's the use case? What kind of script would you run that would expand your previous history? Or would it expand previous commands from the script? – Jeff Schaller Aug 9 '17 at 10:58
  • @JeffSchaller, numeric history expansion in a script with positive numbers sounds like a really bad idea, but I could easily imagine use cases for !! or !-2 (negative/relative history numbers) or similar. Mostly they would be solved better in other ways, but it's still a good question about how the shell works (and when you do/don't need to escape your exclams!). – Wildcard Aug 23 '17 at 6:27
10

Yes, history expansion is enabled by default only for interactive shells.

To enable it for a shell script, write:

set -H

To disable it in an interactive shell, write:

set +H

To determine whether or not history expansion is currently enabled, use some form of the following code:

case $- in (*H*) echo enabled ;; (*) echo disabled ;; esac

In starting to teach a shell class, I dug through the manual extensively to try to establish what an "interactive shell" really is. It's a whirlpool question, so let me save you some trouble:

The shell has MANY options. Some of these options are initialized in different ways when the shell has a controlling terminal (or when started with -i, blah blah, whatever, see below).

ALL of the shell's options can be individually changed.

An "interactive shell" is a deceptive term when you try to define it precisely. It is really just a collection of option settings.

The question about which settings make a shell interactive or not is impossible to answer; it gets ridiculous. It is precisely the same philosophical question as the Ship of Theseus.

If you start an interactive shell, but then disable history expansion, use the --noediting flag, set --norc, turn off expand_aliases, etc., etc., then in what sense is the shell interactive? Or, when does it become not interactive anymore? You can't answer these questions.

The truth is that "interactive" is just a convenient label for a collection of various shell options. Likewise "non-interactive." Same thing; just a collection of behaviors that can each one be changed individually.

Bottom line: the shell behaves differently when it is started "interactively" versus when it is started "non-interactively." Trying to precisely define these terms after start-up is silly. Just look at each individual option of the shell to understand its behavior.


I had forgotten that in addition to my own research, I posted about it extensively on this very site.

  • Thanks. In the manual, I only find "This option is on by default for interactive shells" for set -H. It is probably your inference that "history expansion is enabled by default only for interactive shells". – Tim Aug 9 '17 at 2:56
  • @Tim, see the rest of my answer. That sentence of the manual is a very slippery one, and trying to get the answer to "what IS an interactive shell?" out of the man page is like talking in circles. It goes nowhere. – Wildcard Aug 9 '17 at 3:00
  • What you CAN say is that the following code snippet will always show you whether history expansion is enabled in the current shell: case $- in (*H*) echo enabled ;; (*) echo disabled ;; esac – Wildcard Aug 9 '17 at 3:01
  • @Tim, by the way, I have no idea what you mean that it's my "inference," because that's the clear meaning of set -H. That IS the histexpand option. – Wildcard Aug 9 '17 at 3:03
  • I meant it is you not the manual which used the word "only" – Tim Aug 9 '17 at 3:05

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