[Excerpted from the README:]  "scponly" is an alternative "shell" (of sorts)
for system administrators who would like to provide access to remote users to
both read and write local files without providing any remote execution
privileges.  Functionally, it is best described as a wrapper to the
tried-and-true ssh suite.

scponly validates remote requests by examining the third argument passed to the
shell upon login.  (The first argument is the shell itself, and the second is
-c.)  The only commands allowed are "scp", "sftp-server" and "ls".  Arguments
to these commands are passed along unmolested.

WWW: https://github.com/scponly/scponly/wiki