SHELL=C:\DOS\COMMAND.COM C:\DOS /E:512 /P
In CONFIG.SYS the SHELL command is used to specify the primary command processor (C:\DOS\COMMAND.COM), the path to COMMAND.COM (C:\DOS, needed to reload the "transient" part of COMMAND.COM), the environment size in bytes (/E:512), and finally the /P parameter makes the primary command interpreter permanent (without the /P parameter, it would be possible to remove the primary command interpreter from memory by using the EXIT command, thus leaving the operating system without command interpreter and without the means to reload it).
You may add any parameter that is valid for the specified command interpreter.