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