What does "unary operator expected" mean

The bash message

unary operator expected

means that you do a comparison where one site is empty for example

if [ $name = "foo" ]

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

if [ = "foo" ]

and this is not a valid expression.

The solution is to quote variable names like this:

if [ "$name" = "foo" ]

Then it will work.

Shell scripting tutorial

Try the tutorial BaBE - Bash by Examples to avoid this and similar mistakes 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

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

See also

• troubleshooting
• error messages
• shell scripting tutorial