This command is really only of any use with the TYPE_IN command. It selects the SuperBASIC command line (#0) so that anything passed with TYPE_IN is automatically entered into that channel (as if it were typed).
Note that COMMAND_LINE cannot have any effect if SuperBASIC is doing something or if the job which uses the command was started with EXEC_W or similar.
COMMAND_LINE pre v3c27 does not seem to work correctly on all versions of the QL ROM.
Two files called TurboFix_bin and MiniCommdLin_bin can be used to allow COMMAND_LINE to select the command line of a Minerva MultiBASIC - this relies on the MultiBASIC being the job which uses the COMMAND_LINE command. Some early versions of TurboFix_bin have bugs in it.
A similar version is available called SMSQCommdLin_BIN which works in the same way, except for SMS SBASIC interpreters. Some versions of TurboFix_BIN also support SBASIC but it is currently recommended that this file is used instead.
See TYPE_IN for an example.