What is the syntax I would use to start a particular job step in UNIX for background processing? And also a restart syntax if that's available.

Thanks.

Create a shell script in unix :
this should contains the following
"
BAMODE=1; export BAMODE
USER=jobXXX; export USER
BSH_JOB=JOBNAME; export BSH_JOB
DS_SERVER=ba; export DS_SERVER
ba6.1 ttaad5203m000 2> errorjob
"

Start this script via cron.

You can insert more unix stuff in the shell script to do the following :
assure unique job,
assure that preceding job correctly ended
etc..