Using "and" in Bash while loop

Question

Okay, essentially this is what the script looks like:

echo -n "Guess my number: "
read guess

while [ $guess != 5 ]; do
echo Your answer is $guess. This is incorrect. Please try again.
echo -n "What is your guess? "
read guess
done

echo "That's correct! The answer was $guess!"

What I want to change is this line:

while [ $guess != 5 ]; do

To something like this:

while [ $guess != 5 and $guess != 10 ]; do

In Java I know "and" is " && " but that doesn't seem to work here. Am I going about this the right way using a while loop?

The question title was originally "or" instead of "and". Of course, per de Morgan's law, it's logically all the same whether you phrase the requirement as "I want 5 or 10" or with a double negation "I don't want an answer which isn't 5 and isn't 10". — tripleee
– tripleee, Commented Jul 8, 2019 at 3:51

Matvey Aksenov · Accepted Answer · 2011-11-23 10:48:10Z

63

There are 2 correct and portable ways to achieve what you want.
Good old shell syntax:

while [ "$guess" != 5 ] && [ "$guess" != 10 ]; do

And bash syntax (as you specify):

while [[ "$guess" != 5 && "$guess" != 10 ]]; do

answered Nov 23, 2011 at 10:48

Matvey Aksenov

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1 Comment

Smitty Over a year ago

Thanks for taking the time to let me know, I noticed now I wasn't bracketing off the two statements and that was why it wasn't working.

thiton · Accepted Answer · 2011-11-23 08:39:53Z

34

The [] operator in bash is syntactic sugar for a call to test, which is documented in man test. "or" is expressed by an infix -o, but you need an "and":

while [ $guess != 5 -a $guess != 10 ]; do

answered Nov 23, 2011 at 8:39

thiton

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2 Comments

Smitty Over a year ago

yep, my mistake "and" was the one I should be using. That'll explain why "-o" didn't work when I tried it earlier; I just thought it was applicable only to if statements when it gave me an error. Thank you very much for your prompt response!

Matvey Aksenov Over a year ago

Note: this is not POSIX and so not portable.

tripleee · Accepted Answer · 2011-11-23 10:56:13Z

2

The portable and robust way is to use a case statement instead. If you are not used to it, it might take a few looks just to wrap your head around the syntax.

while true; do
    case $guess in 5 | 10) break ;; esac
    echo Your answer is $guess. This is incorrect. Please try again.
    echo -n "What is your guess? "
    read guess  # not $guess
done

I used while true but you could in fact use the case statement there directly. It gets hairy to read and maintain, though.

while case $guess in 5 | 10) false;; *) true;; esac; do ...

answered Nov 23, 2011 at 10:56

tripleee

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