Is there a way to run parameterized update statements using only DB2 commandline and bash?
If you are on 10.1 or later you can use INGEST. It is almost as fast as load, more feature rich and has less impact on a running system. First, you need to enable it by creating control tables (if you haven't already):

db2 'call sysproc.sysinstallobjects ( 'INGEST' ,'C' , null, null )';


Now you can ingest the data like:

db2 "ingest from file tab1.del 
 format delimited (
     $colb ...,
     $colc ...
 ) update tableA set colA = null 
   where (colb, colc) = ($colb, $colc)"


Here is an developerworks article:

https://www.ibm.com/developerworks/data/library/techarticle/dm-1304ingestcmd/index.html

and here is the documentation:

https://www.ibm.com/support/knowledgecenter/SSEPGG_11.1.0/com.ibm.db2.luw.admin.cmd.doc/doc/r0057198.html