Shell Programming and Scripting

 
if [ $4 != "-f" ]
then
PREVIEW="/usr/bin/echo PREVIEW MODE:"
else
PREVIEW=""
fi

# command1 and command2 will still be executed
$PREVIEW grep needle haystack | command1 | command2
# command4 will still execute
$PREVIEW command3 `command4`
# N will still be incremented
$PREVIEW echo $((N++))

TMP=`mktemp`

# Open FD 5 writing to temp file
exec 5>"$TMP"
# Open FD 6 reading from temp file
exec 6<"$TMP"
# Delete temp file.  FD 5 and 6 will remain valid until closed.
rm "${TMP}"

cat <<EOF >&5
        # This won't run at all
        grep needle haystack | command1 | command2
        # command4 will still execute
        command3 `command4`
        # command4 won't execute
        command3 \`command4\`
        # N will still be incremented
        echo $((N++))
        # N will not be incremented
        echo \$((N++))
EOF

if [[ "${DRY_RUN}" == "Y" ]]
then
# Just print the contents of the temp file to the screen
        cat <&6
else
# Execute the script inside a new shell
        bash <&6
fi

if [ $4 != "-f" ]
then
set -v
set -o noexec
fi