How to use sqlcl with / as sysdba

Creds to the findings of Rodrigo Jorge:
Make sure the environment variable LD_LIBRARY_PATH is set, and if so, includes $ORACLE_HOME/lib:

oracle@testserver01.oric.no:[testdb01]# export LD_LIBRARY_PATH=$ORACLE_HOME/lib

The following files are important for sqlcl to work:

cd $ORACLE_HOME/lib

ls -la *ocijdbc19*
-rw-r--r-- 1 oracle dba  166082 Apr  5  2019 libocijdbc19.a
-rw-r--r-- 1 oracle dba 1812344 Apr  6 17:24 libocijdbc19_g.so
-rw-r--r-- 1 oracle dba  153648 Apr  6 17:24 libocijdbc19.so

Connect with sqlcl as sysdba, just the way you used to with sqlplus:

 sql / as sysdba


SQLcl: Release 21.4 Production on Tue Jun 06 08:46:27 2023

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Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.19.0.0.0