Shell Scripting Arithmetic Operators for Comparison

Question

I am trying to learn shell scripting and following the tutorials on tutorialspoint when I came across this problem with arithmetic comparison.

$VAL1=10
$VAL2=20
$VAL3=10

if [ $VAL1 == $VAL2 ]
then
    echo "equal"
else
    echo "not equal"
fi

but I got a [: ==: unexpected operator I am not sure why the comparison operator did not work. I know I can also use rational operators, but I want to know why '==' is not defined.

as your title says ksh (but your tag says bash) , you can use == inside of (( ... == ... )) tests (which I also believe are OK in bash). Good luck. — shellter
– shellter, Commented Dec 3, 2012 at 2:03
@Jack: Did the answer solved the problem? If so, can you please accept it to mark the question as solved? — MERose
– MERose, Commented Jan 23, 2016 at 22:22

sampson-chen · Accepted Answer · 2012-12-03 00:28:00Z

11

You want to change it to:

VAL1=10
VAL2=20
VAL3=10

if [ "$VAL1" -eq "$VAL2" ]
then
    echo "equal"
else
    echo "not equal"
fi

Explanations:

Don't add the $ for the lvalue (variable being assigned) in an assignment.
Always wrap your variables with double-quotes in tests. The [: ==: unexpected operator error you got is because, since VAL1 / VAL2 were not assigned properly earlier, ksh expansion of your test actually ends up resolving to this: if [ == ] - (but you see that it's actually not a problem about == being undefined)
Use the following for numeric comparisons instead of the == notation:
- -eq (==)
- -ne (!=)
- -gt (>)
- -ge (>=)
- -lt (<)
- -le (<=)

edited Dec 3, 2012 at 0:28

answered Dec 3, 2012 at 0:21

sampson-chen

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1 Comment

sampson-chen Over a year ago

@Jack np =) if you've found the answer useful, you can upvote it / accept it!

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