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rsh is a limiting version of the standard command interpreter sh, used to restrict logins to execution environments whose capabilities are more controlled than
those of sh (see sh(1) for complete description
and usage).
When the shell is invoked, it scans the environment for the value of the environmental variable, SHELL. If it is found and rsh is the file name part of its value,
the shell becomes a restricted shell.
The actions of rsh are identical to those of sh, except that the following are disallowed:
- changing directory (see cd(1)),
- setting the value of $PATH,
- pecifying path or command names containing /,
- redirecting output (> and >>).
The restrictions above are enforced after .profile is interpreted.
A restricted shell can be invoked in one of the following ways:
-
rsh is the file name part of the last entry in the /etc/passwd file (see passwd(4));
- the environment variable SHELL exists and rsh is the file name part of its value; the environment variable SHELL needs to be set in the .login file;
- the shell is invoked and rsh is the file name part of argument 0;
- the shell is invoke with the -r option.
When a command to be executed is found to be a shell procedure, rsh invokes sh to execute it. Thus, it is possible to provide to the end-user
shell procedures that have access to the full power of the standard shell, while imposing a limited menu of commands; this scheme assumes that the end-user does not have write and execute permissions in
the same directory.
The net effect of these rules is that the writer of the .profile (see profile(4)) has complete control over user actions by performing guaranteed setup actions and leaving the user in an appropriate directory (probably not the login directory).
The system administrator often sets up a directory of commands (that is, /usr/rbin) that can be safely invoked by a restricted shell. Some systems also provide a restricted editor, red.
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