shell - Compare a string to a variable(string or number)

Question

I want to find out if the string variable is empty or not. I do this by comparing the variable to a literal empty string ("").

Here is my code:

var=$1

if [$var == ""]; then
    echo "\$var is $var"
fi

It gives me this error when $1 is "" (No command line argument(s)):

./script.sh: line 5: [: ==: unary operator expected

When $1 has a value, it works fine.

I've tried the following things and they still have given me an error:

I want to be able to compare a string variable (empty or not) with "".

Anton Kovalenko · Accepted Answer · 2013-02-15 16:15:00Z

2

You should always have a space after the opening bracket ([), because it's a command name.

The way to do it closest to your own: if [ "$var" = "" ]; then...

Another way to do it (-n predicate which test for non-emptiness): if [ -n "$var" ]; then...

Double quotes around $var are required.

answered Feb 15, 2013 at 16:15

Anton Kovalenko

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1 Comment

To keep the same sense of the original test you'd want to use [ -z "$var" ] (-z: true if the argument is a zero-length string)