How to check a public RSA key file

Inside a shell script I want verify a public RSA file public.pem.

All I want is to check that this file is a genuine public RSA key file, nothing else, like a regular file or a corrupted key.

I will be using this public key file in the future to validate an incoming encrypted gzip file but that is out of scope for now.

Please note that I do not have any other files with me (e.g., a private key).

I’m already checking that file is not zero-sized and I use MD5.

Another possible check I found is that the file contains the text BEGIN PUBLIC KEY and END PUBLIC KEY.

Also I found this command with Google:

openssl rsa -noout -text -inform PEM -in pubkey.pem -pubin

Is there a better way to do this using openssl?

It's possible to use any public key format parser, including openssl or even parse key yourself as the format is not that difficult.

Command line tools set a non-zero exit code, when parsing fails:

openssl rsa -inform PEM -pubin -in pubkey.pem -noout &> /dev/null
if [ $? != 0 ] ; then
    echo "this was definitely not a RSA public key in PEM format"
    exit 1
fi

Just to check any public key:

openssl pkey -inform PEM -pubin -in pubkey.pem -noout &> /dev/null
if [ $? != 0 ] ; then
    echo "this was definitely not a public key in PEM format"
    exit 1
fi