View source for 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.

= If this happens to you ... =
This happened to me with a real script from mldonkey, mldonkey_command. Analyzing the script I could find I did not give any paramters to it.

= Outlook =
You can also debug the script line-by-line using bashdb. Bashdb shows all commands that are being executed, just like [[gdb]] or [[strace]], but for bash scripts:
<pre>
tstaerk@ubuntu:~$ bashdb test.sh 
bashdb debugger, release 4.2-0.6

Copyright 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010 Rocky Bernstein
This is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.

(/home/tstaerk/test.sh:1):
1:      echo "how is your name? "
bashdb<0> step
how is your name? 
(/home/tstaerk/test.sh:2):
2:      read name
bashdb<1> step
Thorsten
(/home/tstaerk/test.sh:3):
3:      if [  = "Thorsten" ]; then echo "I know you"; fi
bashdb<2> step
test.sh: line 3: [: =: unary operator expected
(/usr/bin/bashdb:1):
1:      #!/bin/bash
bashdb<3> step
Debugged program terminated normally. Use q to quit or R to restart.
bashdb<4> 
</pre>

= See also =
* [[troubleshooting]]
* [[error messages]]
* [[shell tutorial]]