If testing reveals a problem the next step is debugging.

1. Isolate

   Isolate the problem area by commenting out sections of code

   if [[ -d $dir_name ]]; then
       if cd $dir_name; then
           rm *
       else
           echo "cannot cd to '$dir_name'" >&2
           exit 1
       fi
   # else
   #    echo "no such directory: '$dir_name'" >&2
   #    exit 1
   fi
   

2. Tracing

   Tracing by placing informative messages in the script

   echo "preparing to delete files" >&2
   if [[ -d $dir_name ]]; then
       if cd $dir_name; then
   echo "deleting files" >&2
           rm *
       else
           echo "cannot cd to '$dir_name'" >&2
           exit 1
       fi
   else
       echo "no such directory: '$dir_name'" >&2
       exit 1
   fi
   echo "file deletion complete" >&2
   

   Send messages to standard error to separate them from normal output. Don’t indent so easier to find when time to remove.

   Implement tracing by the -x options and the set command with the -x option.

   #!/bin/bash -x
   # trouble: script to demonstrate common errors
   number=1
   if [ $number = 1 ]; then
       echo "Number is equal to 1."
   else
       echo "Number is not equal to 1."
   fi
   

   example

   [me@linuxbox ~]$ trouble
   + number=1
   + '[' 1 = 1 ']'
   + echo 'Number is equal to 1.'
   Number is equal to 1.
   

   The + is the default char for trace output, contained in the PS4 shell variable.

   • You can change the PS4 variable to print line numbers

     [me@linuxbox ~]$ export PS4='$LINENO + '
     [me@linuxbox ~]$ trouble
     5 + number=1
     7 + '[' 1 = 1 ']'
     8 + echo 'Number is equal to 1.'
     Number is equal to 1.
     

   • To trace a section of the script, use set -x set +x

     #!/bin/bash
     
     # trouble: script to demonstrate common errors
     number=1
     
     set -x # Turn on tracing
     if [ $number = 1 ]; then
         echo "Number is equal to 1."
     else
         echo "Number is not equal to 1."
     fi
     set +x # Turn off tracing
     

3. Examine values during execution

   Echo variables to see internal workings during execution

   echo "number=$number" # DEBUG
   set -x # Turn on tracing
   if [ $number = 1 ]; then
       echo "Number is equal to 1."
   else
       echo "Number is not equal to 1."
   fi
   set +x # Turn off tracing