openssl req -x509

# interactive
openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -sha256 -days 365

# non-interactive and 10 years expiration
openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -sha256 -days 3650 -nodes -subj "/C=XX/ST=StateName/L=CityName/O=CompanyName/OU=CompanySectionName/CN=CommonNameOrHostname"

Generating a 4096 bit RSA private key
.............................................................................++++
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writing new private key to 'key.pem'

file cert.pem
cert.pem: PEM certificate

file key.pem
key.pem:  ASCII text

Related

• ssh-keygen

See also

• CSR: openssl req [ -x509 ]
• X.509, ASN.1, openssl x509, .pem, der, PFX, PKCS, SAN, openssl x509, CSR