openssl-genrsa
OpenSSL command to generate RSA private keys.
TLDR
Generate an RSA private key of 2048 bits to stdout
$ openssl genrsa
Save an RSA private key of an arbitrary number of bits to the output file
$ openssl genrsa -out [output_file.key] [1234]
Generate an RSA private key and encrypt it with AES256 (you will be prompted for a passphrase)
$ openssl genrsa [-aes256]