How can I print only variables defined inside my shell script?

Here's a portable means of getting a sorted list of all currently defined varnames:

v(){ set "${IFS+IFS=\$2}" "$IFS"; unset IFS
     set  $(set|sed -n '/^[_[:alpha:]][[:alnum:]]*=/s/=.*//p'|sort -u) "$@"
     while [ "$#" -gt 2 ] 
     do    eval '[ "${'$1'+1}" = 1 ]' && 
           echo "$1"; shift
     done; eval "$1"
}

That list is echoed to v()'s stdout - one name per line. Because it is sorted it is a good candidate for later comparison with a similarly generated list as you might do with comm or similar. The list intentionally excludes $IFS - and this is because - set or not - $IFS is always in effect and so must be a given anyway in such a list. That could be altered like:

set "${IFS+IFS=\$2}" "$IFS"; IFS='
'; set $(set|... 

...but that would only always include it in a list whether it were set or not - which brings me back to my previous point...

You can store the list of variables at the beginning of the script and compare it with the value somewhere during the script. Beware that the output of commands like set isn't built to be unambiguous: something like set | sed 's/=.*//' doesn't work with variables whose values contain newlines. (In bash, it actually does for string variables, but not for arrays, and it also displays function code.) I think that the following snippet reliably lists the currently defined variables (in alphabetical order, to boot):

variables=$(tmp=$(declare -p +F); declare () { echo "${2%%=*}"; }; eval "$tmp")

Thus, set initial_variables=… at the beginning of the script, and compare with the later value. You can use something other than echo to act on the list directly.

initial_variables=" $(tmp=$(declare -p +F); declare () { echo "${2%%=*}"; }; eval "$tmp") "
…
( tmp=$(declare -p +F)
  declare () {
    case "$initial_variables" in
      *" $2 "*) :;; # this variable was present initially
      *) eval "set -- \"\$$2\" \"\$2\""; echo "locally defined $2=$1";;
    esac
  }
)

A solution using comm based on storing all the variables before and after:

#!/bin/bash 

# Storing variables before:
set -o posix
set > $TMP/VariablesBefore.txt
sort $TMP/VariablesBefore.txt -o $TMP/VariablesBeforeSorted.txt

a=3
b=7

# Storing variables after:
set > $TMP/VariablesNow.txt
sort $TMP/VariablesNow.txt -o $TMP/VariablesNowSorted.txt

# Computing differences:
echo "The variables inside the script are: "
comm -3 $TMP/VariablesBeforeSorted.txt $TMP/VariablesNowSorted.txt

Well, there are still a few variables remaining like _ or BASH_LINENO, but this is the bet I have reached.

Note that this solution could not work in all cases, for example variables very long (KB), with new line codes, or variables including variable names. Thanks, MikeServ, for pointing.