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That's the distance used for instance by zsh approximate matching (as in [[ abcd = (#a2)acbe ]]) and is common when it comes to consider human misspellings or DNA mutations.

That's the distance used for instance by zsh approximate matching (as in [[ abcd = (#a2)acbe ]] to check that abcd is the same of acbe within a maximum distance of 2) and is common when it comes to consider human misspellings or DNA mutations.

A different metric that you may want to consider is the Damerau-Levenshtein distance (Text::Levenshtein::Damerau module in perl). That's the same as the the Levenshtein distance, except that transposition of contiguous characters (as in ab vs ba) counts as 1 instead of 2.

That's the distance used for instance by zsh approximate matching (as in [[ abcd = (#a2)acbe ]]) and is common when it comes to consider human misspellings or DNA mutations.

The Levenshtein distance is a useful metric to give an idea of the amount of difference between two strings. It measures the number of insertions, deletions and substitutions needed to get from one string to the other.

For instance, if you compare abcdef and bcdef, all characters are different if you compare them one to one, but only one deletion is need to get from one to the other.

So you could make your percentage like: distance / max_length:

perl -MList::Util=max -MText::LevenshteinXS -le '
  ($x, $y) = @ARGV
  print 100 * distance($x, $y) / max(length $x , length $x)
  ' -- "$string1" "$string2"

Or in awk:

awk '
    function min(x, y) {
      return x < y ? x : y
    }
    function max(x, y) {
      return x > y ? x : y
    }
    function lev(s,t) {
      m = length(s)
      n = length(t)

      for(i=0;i<=m;i++) d[i,0] = i
      for(j=0;j<=n;j++) d[0,j] = j

      for(i=1;i<=m;i++) {
        for(j=1;j<=n;j++) {
          c = substr(s,i,1) != substr(t,j,1)
          d[i,j] = min(d[i-1,j]+1,min(d[i,j-1]+1,d[i-1,j-1]+c))
        }
      }

      return d[m,n]
    }

    BEGIN {
      print 100 * lev(ARGV[1], ARGV[2]) / max(length(ARGV[1]), length(ARGV[2]))
      exit
    }' "$string1" "$string2"

The Levenshtein distance is a useful metric to give an idea of the amount of difference between two strings. It measures the number of insertions, deletions and substitutions needed to get from one string to the other.

For instance, if you compare abcdef and bcdef, all characters are different if you compare them one to one, but only one deletion is need to get from one to the other.

So you could make your percentage like: distance / max_length:

perl -MList::Util=max -MText::LevenshteinXS -le '
  ($x, $y) = @ARGV
  print 100 * distance($x, $y) / max(length $x , length $x)
  ' -- "$string1" "$string2"

Or in awk:

awk '
    function min(x, y) {
      return x < y ? x : y
    }
    function max(x, y) {
      return x > y ? x : y
    }
    function lev(s,t) {
      m = length(s)
      n = length(t)

      for(i=0;i<=m;i++) d[i,0] = i
      for(j=0;j<=n;j++) d[0,j] = j

      for(i=1;i<=m;i++) {
        for(j=1;j<=n;j++) {
          c = substr(s,i,1) != substr(t,j,1)
          d[i,j] = min(d[i-1,j]+1,min(d[i,j-1]+1,d[i-1,j-1]+c))
        }
      }

      return d[m,n]
    }

    BEGIN {
      print 100 * lev(ARGV[1], ARGV[2]) / max(length(ARGV[1]), length(ARGV[2]))
      exit
    }' "$string1" "$string2"

That would give 100 for a vs b or bc, but 50 for ab vs ac or a or b or abcd. Beware you'll get a division-by-zero error if you try to compare the empty string against itself.

Those are limited by the maximum length of a command argument (128KiB on modern Linux systems), though you could work around that by getting the strings some other way (like reading them from a file) if need be.