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The following fails and I don't understand why:

$ echo "#!"

the following also fails with the same error message:

$ echo "\#!"

the error message:

-bash: !": event not found

Why does it fail? How should the echo be done instead?

F. Hauri - Give Up GitHub
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erikbstack
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    [rici's answer to a duplicate](http://stackoverflow.com/a/25021905/874188) is really stellar if you want a detailed, long exposition. – tripleee Sep 17 '15 at 05:50
  • I can't reproduce the error with that code in Bash 5.0. Maybe it's been patched. But I can reproduce it with similar commands, like `echo "#!foobar"` – wjandrea Aug 04 '22 at 18:55

5 Answers5

26

The ! character is used for csh-style history expansion.

If you do not use this feature, set +o histexpand (aka set +H) turns off this behavior. It is turned off for scripts, but often enabled for interactive use. In such cases, my personal recommendation is to turn it off permanently by adding set +o histexpand to your .bash_profile (or .bashrc if you don't have a .bash_profile; this is more complex than I want to try to fit into a parenthetical).

As a workaround, if for some reason you cannot or don't want to turn off and forget about this legacy csh feature, you can use single quotes instead of double quotes -- keeping in mind, of course, their different semantics. If you need to combine quoting with variable interpolation, for example, you can change

echo "#!$SHELL"  # oops, history expansion breaks this

into

 echo '#!'"$SHELL"

(notice the adjacent single-quoted and double-quoted strings; after the shell is done with this, the quotes will be removed and the string #! will be output next to the value of the variable SHELL with no space between them) or a number of other common workarounds like

 printf '#!%s\n' "$SHELL"

... though as remarked in a comment, even single quotes don't necessarily prevent this from happening when they are at the end of a pipeline.

tripleee
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  • Rolled back the edit with @GaborJozan's answer as that was [posted separately](/a/37741294/874188) anyway, but has the obvious flaw that it adds a space where none was present before. – tripleee Jun 10 '16 at 07:24
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    You can use single instead of double quotes _except_ in some odd circumstances, such as where the single quotes are in the downstream part of a pipeline, as noted in [this SO answer](https://stackoverflow.com/a/37917491/785213). Yes, this can be exasperating sometimes; you could momentarily `set +o histexpand` if you can't work around it another way. – TheDudeAbides Nov 09 '17 at 23:40
  • Thanks for the remark. I added a section emphasizing that there are obviously differences between single and double quotes and included a couple of workarounds. – tripleee Nov 10 '17 at 04:43
  • ... or even: `echo -e "#\041$SHELL"` – F. Hauri - Give Up GitHub Mar 05 '20 at 07:50
  • Best answer on [`[bash] "event not found"`](https://stackoverflow.com/search?q=%5Bbash%5D+%22event+not+found%22) unfortunely not at 1st place – F. Hauri - Give Up GitHub Mar 05 '20 at 07:52
  • @F.Hauri Depends on how you sort the search results, but the top result by relevance wasn't so great; I have now marked it as a duplicate of this one. – tripleee Mar 05 '20 at 08:17
9

By default, bash supports csh compatible history-expansion.

In bash

echo #!

will only print a newline, as # starts a comment.

In

echo "#!"

the # is part of the string started with ". Such strings are still inspected by bash for special characters. ! is a special character iff it is followed by any other text.

-bash: !": event not found

In this case, bash expects the !" token to refer to a previous command in shell history starting with ", and does not find one. All by itself, ! will not trigger this behaviour:

$ echo \# !
# !

$ echo fee ! fie
fee ! fie

Finally,

$ echo !echo

produces two lines, the first line is printed by the shell to show how the pattern above expands to:

echo echo '# !'

while the second line is just the result of executing the expanded command: echo # !

See Also: The Bash man page on History Expansion

Henk Langeveld
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8
echo '#!'

Basically, with double quotes (") aka "weak quoting", Bash does some wacky things with the string, like variable substitution. With single (') aka "strong quoting" the string is taken literally.

See e.g. here for a more in-depth explanation of quoting.

Staven
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4

Another workaround may be to put an extra space after the "!" :

# echo "#! "
#!
JG_
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  • For years this is how I've worked around the error message (which I never _really_ understood until today) for one-liners like `grep -q pattern somefile.txt && echo "YAY! "`. And probably still will, because double-quoting strings in the shell is just in my muscle memory. – TheDudeAbides Nov 09 '17 at 23:56
2

In my case all commands are working. Maybe you can specify your environment.

$ echo "\#\!"
\#\!
$ echo "#!"
echo "#"
#
$ echo "#!"
echo "#"
#
$ echo $BASH_VERSION
3.2.48(1)-release
devanand
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