Often one of the two files is either a hard link or a symbolic link to the other, so that either name refers to the same improved version of the C shell. On many systems, such as macOS and Red Hat Linux, csh is actually tcsh. Tcsh is very stable but new releases continue to appear roughly once a year, consisting mostly of minor bug fixes. Though it started as a side branch from the original csh source tree that Bill Joy had created, tcsh is now the main branch for ongoing development. Because it only added functionality and did not change what was there, tcsh remained backward compatible with the original C shell. Tcsh added filename and command completion and command line editing concepts borrowed from the TENEX operating system, which is the source of the “t”. It is the native root shell for BSD-based systems such as FreeBSD. Unlike the other common shells, functions cannot be defined in a tcsh script and the user must use aliases instead (as in csh). It is essentially the C shell with programmable command-line completion, command-line editing, and a few other features.
