What does "unary operator expected" mean

When you work with Linux scripts on the command line, you will sometimes get an error message saying

unary operator expected

And you may wonder what this means. To give you an example, let's write a short bash script.

The script

Just copy and paste the lines below into a Linux Shell:

cat >test.sh<<EOF
echo "how is your name? "
read name
if [ $name = "Thorsten" ]; then echo "I know you"; fi
EOF
chmod 777 test.sh

After you did this, you have a script test.sh that will ask you for your name and say "I know you" if your name is Thorsten. You can call the script using the command

./test

Now if you don't enter a name and just press enter you will get this:

# ./test.sh 
how is your name? 

./test.sh: line 3: [: =: unary operator expected

This is clearly a problem in line 3. $name is replaced by nothing when the shell executes the line. So the remainder of the line reads

if [ = "Thorsten" ]; then echo "I know you"; fi

Which does not work because you cannot compare nothing with "Thorsten".

The solution

There is a simple trick to avoid this kind of error messages already when programming. For example if you add an "x" left and right next to the equal sign in line 3:

if [ x$name = "xThorsten" ]; then echo "I know you"; fi

The shell may still replace $name by nothing, but then the x will stay and the command will be after evaluation:

 if [ x = "xThorsten" ]; then echo "I know you"; fi

And there will not be an error message any longer.