Bash bc calculator evaluation, multiple conditions with NOT evaluations

Question

Long story short, is it possible to make bc evaluate the following expression?

echo "!(2.2 >= 0) && !(2.2 < 10)" | bc
bash: !: event not found

I know this works:

echo "(2.2 >= 0) && (2.2 < 10)" | bc
1

So what am I doing wrong? The bc man page said that it has support for !expr, but I cannot seem to invoke it. I know it's possible to work around the issue by negating the comparison signs, but I'd like to know how should the !expr be invoked in this case.

Much appreciated.

score 4 · Accepted Answer · answered Jan 14 '13 at 23:42

bash: !: event not found

There is a feature of bash, called "history expansion" (see man bash): it handles ! specially, even within double quotes.

Replacing double quotes with single quotes should help (history expansion doesn't happen there).

Also, history expansion doesn't happen in non-interactive shells (that is, your example will work as is in a script).

score 1 · Answer 2 · answered Jan 14 '13 at 23:40

1

It seems like the bash is interpreting your "!" as some kind of command. Replacing " for ' has worked for me.

It also happens when just outputing something to the terminal:

$ echo "!"
-bash: !: event not found
$ echo '!'
!

answered Jan 14 '13 at 23:40

GabrielF

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score 1 · Answer 3 · answered Jan 14 '13 at 23:47

1

a walk-around in bash with double quote:

kent@7pLaptop:/tmp$ echo  -e "\x21(3>5)"|bc
1

tested under:

kent@7pLaptop:/tmp$ bash -version|head -1
GNU bash, version 4.2.42(2)-release (i686-pc-linux-gnu)

answered Jan 14 '13 at 23:47

Kent

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ASCII hex to prevent '!' evaluation? Interesting. – IDDQD Jan 15 '13 at 14:42
@S.T.A.L.K.E.R. I use this trick to avoid to escape ' or " when I wrote sed lines. \' \" pairs are annoying. – Kent Jan 15 '13 at 15:13

Bash bc calculator evaluation, multiple conditions with NOT evaluations

3 Answers3