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urlencode_od_awk () {
  echo "$1" | od -t d1 | awk '{
      for (i = 2; i <= NF; i++) {
        printf(($i>=48 && $i<=57) || ($i>=65 &&$i<=90) || ($i>=97 && $i<=122) ||
                $i==45 || $i==46 || $i==95 || $i==126 ?
               "%c" : "%%%02x", $i)
      }
    }'
}

urlencode_od_awk () {
  echo -n "$1" | od -t d1 | awk '{
      for (i = 2; i <= NF; i++) {
        printf(($i>=48 && $i<=57) || ($i>=65 && $i<=90) || ($i>=97 && $i<=122) ||
                $i==45 || $i==46 || $i==95 || $i==126 ?
               "%c" : "%%%02x", $i)
      }
    }'
}

Note that the code above may fail in fancy locales (not likely on a router). You may need export LC_ALL=C if you use a non-default locale.

Awk can do most of the job, except that it annoyingly lacks a way to convert from a character to its number. If od is present on your device, you can use it to convert all characters (more precisely, bytes) into the corresponding number (written in decimal, so that awk can read it), then use awk to convert valid characters back into literals and quoted characters into the proper form.

Depending on compilation options, [ (a.k.a. test) may be an external utility. We're only using it for string matching which can also be done within the shell with the case construct. Here are the last two approaches rewritten to avoid the test builtin.

[TL,DR: use the urlencode_grouped_case version in the last code block.]

Awk can do most of the job, except that it annoyingly lacks a way to convert from a character to its number. If od is present on your device, you can use it to convert all characters (more precisely, bytes) into the corresponding number (written in decimal, so that awk can read it), then use awk to convert valid characters back into literals and quoted characters into the proper form.

Depending on compilation options, [ (a.k.a. test) may be an external utility. We're only using it for string matching which can also be done within the shell with the case construct. Here are the last two approaches rewritten to avoid the test builtin, first going character by character:

and copying each literal segment in a batch: