This assumes you have already created a public certificate with the .crt extension, such as example.com.crt. If not, refer to our article on creating a public certificate using OpenSSL on Linux. 

Then OpenSSL command with the x509 -in -out and -outform options is used to convert a crt certificate to a pem certificate, like this.

openssl x509 -in example.com.crt -out example.com.pem -outform PEM

Or, to convert a pem certificate to a crt certificate, like this.

openssl x509 -in example.com.pem -out example.com.crt -outform CRT