If testing reveals a problem the next step is debugging.

1. Isolate

   Isolate the problem area by commenting out sections of code

   ``` bash 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

   bash 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.

   ``` bash

   !/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

   bash [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

     bash [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

     ``` bash

     !/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

   bash 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