Though it may seem like a fundamental tool for SQL programmers and DBAs, the method for describing the columns in a RDBMS table from the SQL command line varies from vendor to vendor. This is how some of the major vendors do it.
INFO COLUMNS FOR tablename;
There is no built-in command. Instead, you need to query from the system catalogue for this information.
SELECT COLNAME,TABSCHEMA,TABNAME FROM SYSCAT.COLUMNS WHERE TABSCHEMA='owner' AND TABNAME='tablename';