Difference between revisions of "What does "unary operator expected" mean"

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When you work with Linux scripts on the command line, you will sometimes get an error message saying
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The bash message
 
  unary operator expected
 
  unary operator expected
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And you may wonder what this means. To give you an example, let's write a short bash script.
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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
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if [ = "foo" ]
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and this is not a valid expression.
  
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= The script =
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The solution is to quote variable names like this:
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Just copy and paste the lines below into a Linux Shell:
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  if [ "$name" = "foo" ]
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  [[cat]] >test.sh<<EOF
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Then it will work.
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[[echo]] "how is your name? "
 
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read name
 
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if [ $name = "Thorsten" ]; then echo "I know you"; fi
 
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EOF
 
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chmod 777 test.sh
 
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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]]
 
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./test
 
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Now if you don't enter a name and just press enter you will get this:
 
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# ./test.sh
 
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how is your name?
 
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./test.sh: line 3: [: =: unary operator expected
 
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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
 
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if [ = "Thorsten" ]; then echo "I know you"; fi
 
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Which does not work because you cannot compare ''nothing'' with "Thorsten".
 
  
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= The solution =
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= Shell scripting tutorial =
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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:
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Try the tutorial [[BaBE - Bash By Examples]] to avoid this and similar mistakes in the future.
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if [ x$name = "xThorsten" ]; then echo "I know you"; fi
 
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The shell may still replace $name by ''nothing'', but then the x will stay and the command will be after evaluation:
 
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  if [ x = "xThorsten" ]; then echo "I know you"; fi
 
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And there will not be an error message any longer.
 
  
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= If this happens to you ... =
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= Debugging bash scripts =
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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.
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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:
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= Outlook =
 
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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>
 
<pre>
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tstaerk@ubuntu:~$ bashdb test.sh  
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tweedleburg:~ # bash -x test.sh  
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bashdb debugger, release 4.2-0.6
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+ echo 'how is your name? '
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how is your name?
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Copyright 2002, 2003, 2004, 2006, 2007, 2008, 2009, 2010 Rocky Bernstein
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+ read name
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This is free software, covered by the GNU General Public License, and you are
 
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welcome to change it and/or distribute copies of it under certain conditions.
 
  
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(/home/tstaerk/test.sh:1):
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+ '[' = foo ']'
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1:      echo "how is your name? "
 
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bashdb<0> step
 
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how is your name?
 
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(/home/tstaerk/test.sh:2):
 
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2:      read name
 
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bashdb<1> step
 
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Thorsten
 
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(/home/tstaerk/test.sh:3):
 
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3:      if [ = "Thorsten" ]; then echo "I know you"; fi
 
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bashdb<2> step
 
 
test.sh: line 3: [: =: unary operator expected
 
test.sh: line 3: [: =: unary operator expected
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(/usr/bin/bashdb:1):
 
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1:      #!/bin/bash
 
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bashdb<3> step
 
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Debugged program terminated normally. Use q to quit or R to restart.
 
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bashdb<4>
 
 
</pre>
 
</pre>
  

Latest revision as of 19:15, 24 December 2014

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

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