-------------------------------------------------------------------------------
URLencoding or Percent Encoding

Encoding data so it can be placed within a URL, or in post data
of the form

   https://.../?var1=value1&var2=value2&var3=value3

Where the var and values are seperatally URLencoded
so as to preserve the '&' and '=' seperators

Basically characters that could be missinterpreted need to be converted
to the for  "%xx"  where xx is the character code in hex.

An exception is space which is encoded as either "+" (URL form) or as "%20"

Not all characters need to be encoded (RFC3986)...
And depends on how 'special' the character is in the application.
  See https://en.wikipedia.org/wiki/Percent-encoding

  Reserved Characters (must be encoded)
     "!#$&'()*+,/:;=?@[]"
  Unreserved Characters (optionally encoded)
     [-_.~a-zA-Z0-9]

There exists a non-standard encoding for Unicode characters: %uxxxx

However RFC3987   Internationalized Resource Identifier
specifies Unicode should be UTF-8 and encoded as binary %xx codes.

In POST data the values (after the '?' is set in the body of the request
using the data type  "application/x-www-form-urlencoded"

-------------------------------------------------------------------------------
Encoding URLencoding

A Test string
   string='My data_+/?=&%_-!@#$^*().~'

Unicode test (should become "%C3%BC")
   string="ü"

---
BASH...

WARNING: Neither bash method handles unicode or binary input

# Pure Bash urlencode (looped)
urlencode() {
  # WARNING: does not handle unicode!
  for (( pos=0 ; pos<"${#1}" ; pos++ )); do
    local c="${1:$pos:1}"
    case "$c" in
      [-_.~a-zA-Z0-9]) printf "%c" "$c" ;;
      \;)              printf "%c" ';'  ;;  # optional - make more readable
      \ )              printf "%c" '+'  ;;  # optional
      * )              printf '%%%02x' "'$c" ;;
    esac
  done
}
urlencode "$string"

# Pure Bash urlencode (series - minimal)
urlencode_series() {
  o=${1//%/%25}   # must be first
  o=${o//&/%26}   # variable seperator
  o=${o////%2F}   # URI path seperator
  o=${o//</%3C}   # XML open brace (optional)
  o=${o//=/%3D}   # var=value separator
  o=${o//>/%3E}   # XML close brace (optional)
  o=${o//\?/%3F}  # start GET data
  o=${o//+/%2B}   # before space
  o=${o// /+}     # spaces, could be "%20"
  printf "%s" "$o"
}
urlencode_series "$string"

---
External Command Alturnatives

NOTE: All this methods substitute "%20" instead of "+" for spaces
and coverts almost all non alpha-numerics to percent form

  perl -MURI::Escape -e 'print uri_escape($ARGV[0]);' "$string"

  php -r 'echo urlencode("'"$string"'");'

Python

  Just encode a individual string...

    python3 -c "import urllib.parse; print(urllib.parse.quote(input()))" \
      <<<"$string"

  Pyhton can do the urlencoding while putting together the query string,
  But then uses '+' for spaces.

    import urllib.parse
    queryString = { 'name' : 'Jason Smith', 'age_group' : '(18-24)'}
    urllib.parse.urlencode( queryString )
    # => 'name=Jason+Smith&age_group=%2818-24%29'

  A parameter 'safe' can be used to specify characters that are safe to
  not URLencode into percents.

    import urllib.parse
    queryString = { 'name' : 'Jason Smith', 'age_group' : '(18-24)'}
    urllib.parse.urlencode( queryString , safe='()')
    # => 'name=Jason+Smith&age_group=(18-24)'


JQ (watch the EOL)...
Does not encode characters "_-!*().~"   encodes space as %20
Always adds a newline to output (not to encoded string)

    jq -Rr @uri <<<"$string"

  OR if there are linefeeds in the data to be encoded...

    printf '%s' "$string" | jq -sRr @uri

  OR
    jq -rn --arg x "$string" '$x|@uri'


Just encode ALL bytes to %xx  (3 times bigger, handles unicode)

  printf "%s" "$string" |
    { xxd -p |tr -d \\n | sed 's/../%&/g'; }
  echo

-------------------------------------------------------------------------------
Decoding URLencoding

  perl -pe 'tr/+/ /; s/%([a-fA-F0-9][a-fA-F0-9])/chr hex $1/eg;'

  # Example....
  # Input:  %24argon2i%24v%3D19%24m%3D4096%2Ct%3D3%2Cp%3D1%24%2FFzf
  # Output: $argon2i$v=19$m=4096,t=3,p=1$/Fzf

BASH

  string='%24argon2i%24v%3D19%24m%3D4096%2Ct%3D3%2Cp%3D1%24%2FFzf'

  partial="${string/+/ /}"
  printf '%b' "${partial//%/\\x}"

-------------------------------------------------------------------------------