Unix command to generate cryptographically secure random string

Is the following Unix command cryptographically secure to randomly generate 20 characters (a-zA-Z0-9 only)? dd if=/dev/urandom bs=256 count=1 2> /dev/null | LC_ALL=C tr -dc 'A-Za-z0-9' | head -c20 Is there a better or more secure way to go about this in Unix?
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