> I believe the oldest program that uses subcommand is the version control system SCCS, from 1972 IIUC, other non-Unix operating systems commonly used the subc

> I believe the oldest program that uses subcommand is the version control system SCCS, from 1972 IIUC, other non-Unix operating systems commonly used the subcommand style for normal interactive use; it was the style used by the normal command shell, and all commands in those operating systems therefore followed this style. The operating system (or its shell) was responsible for enabling easy use of this style by adding completion support (commonly using the ESC key, IIRC). This older style can still today be seen in Unix tools like ftp and telnet, which uses this style for its internal command line interface. Another tool using this style, which many might be familiar with, is the multi-platform Kermit. Unix therefore has been rather late in adopting this style, first in gradually adopting tools which uses the subcommand style (ip, git, etc.), and later with support in shells for programmable completion support where a program can optionally register completion hooks for the user shell to use, making it finally come up to the level of those older operating systems.