The restricted shell is a Unix shell that restricts some of the capabilities available to an interactive user session, or to a shell script, running within it. It is intended to provide an additional layer of security, but is insufficient to allow execution of entirely untrusted software. A restricted mode operation is found in the original Bourne shell and its later counterpart Bash, and in the Korn shell. In some cases a restricted shell is used in conjunction with a chroot jail, in a further attempt to limit access to the system as a whole.
Invocation
The restricted mode of the Bourne shellsh, and its POSIX workalikes, is used when the interpreter is invoked in one of the following ways:
sh -rnote that this conflicts with the "read" option in some sh variants
rshnote that this may conflict with the remote shell command, which is also called rsh on some systems
The restricted mode of Bash is used when Bash is invoked in one of the following ways:
Similarly the Korn shell's restricted mode is produced by invoking it thus:
rksh
ksh -r
Setting Up rbash
For some systems, the invocation through is not enabled by default, and the user obtains a error if invoked directly, or a login failure if the /etc/passwd file indicates as the user's shell. It suffices to create a link named pointing directly to. Though this invokes Bash directly, without the or options, Bash does recognize that it was invoked through and it does come up as a restricted shell. This can be accomplished with the following simple commands : root@host:~# cd /bin root@host:/bin# ln bash rbash
limitations on the use of slash-ed filenames in Bash builtins
Restrictions in the restricted Korn shell are much the same as those in the restricted Bourne shell.
Weaknesses of a restricted shell
The restricted shell is not secure. A user can break out of the restricted environment by running a program that features a shell function. The following is an example of the shell function in vi being used to escape from the restricted shell: user@host:~$ vi
Or by simply starting a new unrestricted shell, if it is in the, as demonstrated here: user@host:~$ rbash user@host:~$ cd / rbash: cd: restricted user@host:~$ bash user@host:~$ cd / user@host:/$
