The host command environment replaceable routine is called to process all host commands for a specific host command environment (see Host commands and host command environments for the definition of "host commands"). A REXX exec may contain host commands to be processed. When the language processor processes an expression that it does not recognize as a keyword instruction or function, it evaluates the expression and then passes the string to the active host command environment. A specific environment is in effect when the command is processed. The host command environment table (SUBCOMTB table) is searched for the name of the active host command environment. The corresponding routine specified in the table is then called to process the string. For each valid host command environment, there is a corresponding routine that processes the command.

In an exec, you can use the ADDRESS instruction to route a command string to a specific host command environment and therefore to a specific host command environment replaceable routine.

The names of the routines that are called for each host command environment are specified in the ROUTINE field of the host command environment table. Host command environment table describes the table.

You can provide your own replaceable routine for any one of the default environments provided. You can also define your own host command environment that handles certain types of "host commands" and provide a routine that processes the commands for that environment.