unary operator expected in shell script when comparing null value with string

Question

I have two variables

var=""
var1=abcd

Here is my shell script code

if [ $var == $var1 ]; then
  do something
else
  do something
fi

If I run this code it will prompt a warning

[: ==: unary operator expected

How can I solve this?

To format code in a post, just highlight it and click the "{}" icon (or indent it by 4 spaces). Your code was marked with > (which denotes quoted text, not formatted code) and unnecessary <br> tags. I fixed it. — Keith Thompson
– Keith Thompson, Commented Mar 22, 2013 at 15:16
There are numerous other questions on StackOverflow dealing with the need to quote variables in case they are unset or null-valued. — chepner
– chepner, Commented Mar 22, 2013 at 15:19
Just a reminder: To indicate that an answer solved your problem, you can click the green arrow to "accept" it. Which answer to accept, and whether to accept an answer at all, is entirely up to you. — Keith Thompson
– Keith Thompson, Commented Mar 22, 2013 at 18:39

Keith Thompson · Accepted Answer · 2014-08-08 17:38:12Z

110

Since the value of $var is the empty string, this:

if [ $var == $var1 ]; then

expands to this:

if [ == abcd ]; then

which is a syntax error.

You need to quote the arguments:

if [ "$var" == "$var1" ]; then

You can also use = rather than ==; that's the original syntax, and it's a bit more portable.

If you're using bash, you can use the [[ syntax, which doesn't require the quotes:

if [[ $var = $var1 ]]; then

Even then, it doesn't hurt to quote the variable reference, and adding quotes:

if [[ "$var" = "$var1" ]]; then

might save a future reader a moment trying to remember whether [[ ... ]] requires them.

edited Aug 8, 2014 at 17:38

answered Mar 22, 2013 at 15:13

Keith Thompson

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1 Comment

sugunan Over a year ago

Same error coming on greater than or less than operator. How to solve that as number comparison. if [ $var -gt $var1 ]

Anatoly · Accepted Answer · 2017-08-14 06:18:22Z

1

Why all people want to use '==' instead of simple '=' ? It is bad habit! It used only in [[ ]] expression. And in (( )) too. But you may use just = too! It work well in any case. If you use numbers, not strings use not parcing to strings and then compare like strings but compare numbers. like that

let -i i=5 # garantee that i is nubmber
test $i -eq 5 && echo "$i is equal 5" || echo "$i not equal 5"

It's match better and quicker. I'm expert in C/C++, Java, JavaScript. But if I use bash i never use '==' instead '='. Why you do so?

answered Aug 14, 2017 at 6:18

Anatoly

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1 Comment

hagello Over a year ago

Well, there are more convincing arguments than I'm an export in… Why you do so?. See for example: unix.stackexchange.com/a/16110/59743. It tells you that == is an alias for =. = is a standard operator, whereas == is widely implemented but it's not POSIX: pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html

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