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

This means that there is a comparison where one site is empty for example

if [ $name = "Thorsten" ]

and $name is empty. Then the bash shell internally replaces $name by an empty string and it will be interpreted as

if [ = "Thorsten" ]

and this is not a valid expression.

The solution is to quote variable names like this:

if [ "$name" = "Thorsten" ]

Then it will work.

Shell scripting tutorial

Try the shell scripting tutorial to avoid this mistake and similar ones in the future.

Debugging bash scripts

You can also debug the script line-by-line using bash -x. bash -x shows all commands that are being executed, just like gdb or strace, but for bash scripts:

tweedleburg:~ # bash -x test.sh 
+ echo 'how is your name? '
how is your name? 
+ read name

+ '[' = Thorsten ']'
test.sh: line 3: [: =: unary operator expected

See also

• troubleshooting
• error messages
• shell scripting tutorial