Shell Programming and Scripting

$ echo "[main] some message [value1] some message [value2] some message [value3]" | sed 's/^\[//;s/\][^\[]*\[/ /g;s/\]$//'
main value1 value2 value3
$

str="some message [value1] some mesage [value2] [value3] some message[value4] some message [value5] some message"

echo ${str} | read -A words

while (( i < ${#words[@]} )); do
   if [[ ${words[i]} =~ \[.*\] ]]; then
      str=${words[i]/*\[/}       # strip left
      output+="${str/\]*/} "     # strip right & append
   fi
   (( i++ ))
done

echo "RESULT: $output"