Example of debugging a shell script

To print commands and their arguments as they are executed:

   cat example
   #!/bin/sh
   TEST1=result1
   TEST2=result2
   if [ $TEST1 = "result2" ]
   then
     echo $TEST1
   fi
   if [ $TEST1 = "result1" ]
   then
     echo $TEST1
   fi
   if [ $test3 = "whosit" ]
   then
     echo fail here cos it's wrong
   fi

This is a script called example which has an error in it; the variable $test3 is not set so the 3rd and last test [command will fail.

Running the script produces:

   example
   result1
   [: argument expected

The script fails and to see where the error occurred you would use the -x option like this:

   sh -x example
   TEST1=result1
   TEST2=result2
   + [ result1 = result2 ]
   + [ result1 = result1 ]
   + echo result1
   result1
   + [ = whosit ]
   example: [: argument expected

The error occurs in the command [ = whosit ] which is wrong as the variable $test3 has not been set. You can now see where to fix it.