Generate an alpha numeric password

$ pwgen -s 20 1                                           # => 66jTuJXUYOrnqoOpfOWE
$ </dev/urandom tr -dc [:alnum:] | head -c20 && echo      # => Zg0OFvg0wOEc4hdEwawZ

Generate an numeric only password

$ </dev/urandom tr -dc [:digit:] | head -c20 && echo      # => 91331194034454954513
$ </dev/urandom tr -dc 0-9 | head -c20 && echo            # => 00166785542509259371

Generate an alpha only password

$ pwgen -s0 20 1                                          # => JqNGGWcaJyamSAyXkqnG
$ </dev/urandom tr -dc a-zA-Z | head -c20 && echo         # => PAnRPpXuPuYNrNpGtKrN
$ </dev/urandom tr -dc [:alpha:] | head -c20 && echo      # => QlSBFAAmqTXjJLDyBZoN