Rbash is a restricted version of the Bash shell that limits what commands can be run for security purposes. It prevents actions like changing directories, modifying shell variables, using redirection operators, or replacing the shell with another command. Rbash can be invoked by running bash with the -r option or by linking /bin/bash to /bin/rbash. To disable rbash, one can execute bash directly or use ssh to run a command in a non-restricted shell.

1. RBASH

2. What is rbash?
If Bash is started with the name rbash, or the --restricted or -r option is supplied at invocation, the shell becomes restricted. A
restricted shell is used to set up an environment more controlled than the standard shell.
A restricted shell behaves identically to bash with the exception that the following are disallowed or not performed:
➢ Changing directories with the cd builtin.
➢ Setting or unsetting the values of the SHELL, PATH, HISTFILE, ENV, or BASH_ENV variables.
➢ Specifying command names containing slashes.
➢ Specifying a filename containing a slash as an argument to the builtin command.
➢ Specifying a filename containing a slash as an argument to the history builtin command.
➢ Specifying a filename containing a slash as an argument to the -p option to the hash builtin command.
➢ Importing function definitions from the shell environment at startup.

3. And more
➢ Parsing the value of SHELLOPTS from the shell environment at startup.
➢ Redirecting output using the ‘>’, ‘>|’, ‘<>’, ‘>&’, ‘&>’, and ‘>>’ redirection operators.
➢ Using the exec builtin to replace the shell with another command.
➢ Adding or deleting builtin commands with the -f and -d options to the enable builtin.
➢ Using the enable builtin command to enable disabled shell builtins.
➢ Specifying the -p option to the command builtin.
➢ Turning off restricted mode with ‘set +r’ or ‘shopt -u restricted_shell’.

5. How to enable rbash?
Restricted bash shell can be invoked using below command:
# bash -r
The ‘/bin/rbash‘ represents the restricted bash shell and can be created by creating as a soft link to ‘/bin/bash’ as shown below:
# ln -s /bin/bash /bin/rbash
Once soft link for /bin/rbash is created, one can launch restricted bash shell using below command:
# rbash

6. How to disable rbash?
# perl -e ‘exec(“/bin/bash”);’
Or using ssh:
ssh user@ip “command”
ssh user@ip -t "bash --noprofile"