How to export certificate in PEM format?

I have an Ubuntu LDAP server, with STARTTLS using a self-signed certificate (following these instructions).

When setting up a CentOS client (following these instructions), the authconfig-tui command asks me for the certificate in PEM format:

│ To connect to a LDAP server with TLS       │
│ protocol enabled you need a CA certificate │
│ which signed your server's certificate.    │
│ Copy the certificate in the PEM format to  │
│ the '/etc/openldap/cacerts' directory.     │
│ Then press OK.                             │

How do I get the certificate in PEM format? I found these command, but the /etc/dirsrv/ folder does not even exist on my ubuntu system.

certutil -L -d /etc/dirsrv/slapd-SOMETHING
certutil -L -d /etc/dirsrv/slapd-SOMETHING -n "SOMETHING_ELSE" -a > cacert.asc

I have also seen the command certutil -L -d /etc/openldap/certs, which gives me this error message (I have also tried with /etc/ssl/certs):

certutil: function failed: SEC_ERROR_LEGACY_DATABASE: The certificate/key database is in an old, unsupported format.

Note that for obvious reasons, I would like to export only the public part of the certificate, not the private part (being used to the easy-to-recognize SSH keys, where id_rsa is private and id_rsa.pub is public, I always find it difficult to know if a certificate I exported is public or private…)

Solution

If you follow the instructions you linked to, then you've got the certificate in PEM format already. From these instructions:

Create the self-signed CA certificate:

sudo certtool --generate-self-signed \
   --load-privkey /etc/ssl/private/cakey.pem \ 
   --template /etc/ssl/ca.info \
   --outfile /etc/ssl/certs/cacert.pem

The certificate is /etc/ssl/certs/cacert.pem and it is in PEM format.

...I would like to export only the public part of the certificate, not the private part

A certificate (cacert.pem) contains only the public key. The private key (cakey.pem) is not included in the certificate.