I want to create a step in a Job for Merge Replication. I basically want to know which command should I write so that a particular Subscription starts synchronizing?
When you create your replication, it will automatically create a job with all of the steps needed for replication if you use the GUI.
Step 1: Merge Agent Startup Message. Type T-SQL ran on the distribution database
Step 2: Run Agent. Type Replication Merge. The command will be something like: -Publisher  -PublisherDB  -Publication  -Subscriber  -SubscriberDB  -Distributor  -DistributorSecurityMode 1
Step 3: Detect nonlogged agent shutdown. Type T-SQL sp_MSDetect_nonlogged_shutdown @ subsystem='Merge', @agentid=