A restricted shell limits what a user account can do on Linux. A restricted user cannot change their directory, and you control which commands they have access to. Here’s how to set up a restricted shell on Linux.
A restricted shell isn’t a different shell. It’s a different mode of a standard shell. The Bash, Korn, Fish, and other shells can all be started in restricted shell mode. We’ll be using Bash in this article, but the same principles apply to the other shells.
Because restricted shells are just another way of using your standard shell, they are easy to set up. There’s nothing to install, and they’re available wherever Linux is.
Restricted shells can be applied to scripts, too. That ensures that any damage they may cause if they’ve been written incorrectly is limited to the confines of their restricted world and that they don’t have access to your entire computer.
Be aware, though, that restricted shells are not completely escape-proof. Someone with enough knowledge can escape a restricted shell. They’re great for putting safe boundaries on a casual user, but don’t rely on restricted shells for any real-world security on a production system.
When you run Bash as a restricted shell, the user has some capabilities removed from them. Specifically, the user cannot:
cdto change the working directory.
- Change the values of the
$ENVenvironmental variables (but they can read the current values).
- Read or change
$SHELLOPTSshell environmental options.
- Redirect the output of a command.
- Invoke commands that require a path to locate them. That is, you can’t issue a command that has one or more forward slashes “
/” in it.
execto substitute a different process for the shell.
- Use any of the restricted features in a script.
You can invoke a restricted Bash shell by using the
-r (restricted) option. Trying to do a simple task like changing the working directory is forbidden. A terse message tells you that
cd is restricted.
The Bash shell can also detect when it has been invoked using “rbash” instead of “bash.” This causes it to start as a restricted shell, too. This provides a convenient way to set the default shell for a particular user, which we’ll use soon.
If we use the
whereis command on Ubuntu to look for the
rbash files, we’ll see that the executable is in the “usr/bin” directory. The man page is in “/usr/share/man/man1” directory.
ls command with the
-l (long) option reveals that
rbash is actually a symbolic link to
ls -l /usr/bin/rbash
On Manjaro and Fedora, the
rbash symbolic link had to be created. This works on both distributions:
sudo ln -s /bin/bash /bin/rbash
The second time we use the
whereis command, it finds
rbash in the “/usr/bin” directory.
Restricting a User
Let’s create a new user account named “Minnie.” We’ll set their shell to be the restricted shell using the
-s (shell) option of the
useradd command. We’ll also set the account’s password using the
passwd command, and we’ll create a home folder for them.
-p (parents) flag in the
mkdir command tells
mkdir to create the target directory and any parent directories that it needs to create, too. So by creating the “/home/minnie/bin” directory, we create the “/home/minnie” directory at the same time.
sudo useradd minnie -s /bin/rbash
sudo passwd minnie
sudo mkdir -p /home/minnie/bin
When minnie logs in, she will be running in a restricted shell.
She cannot invoke commands that need to include a forward slash “
However, she can still execute commands that are found in the path.
That’s not the behavior you might have expected, and it certainly isn’t what we want. To tighten the restrictions further, we need to change the path that minnie’s shell will use to look for commands.
Tightening the Restrictions
When we created minnie’s home directory “/home/minnie”, we also created a “/home/minnie/bin” directory. This is where that directory comes into play.
We’re going to edit minnie’s “.bash_profile” file and set her path to point to that directory only. We’ll also restrict minnie’s “.bash_profile” file so that only root can edit it. That means that no other user can edit that file and change her path.
sudo gedit /home/minnie/.bash_profile
Either edit the existing “PATH=” or add the following line:
sudo chown root:root /home/minnie/.bash_profile
sudo chmod 755 /home/minnie/.bash_profile
ls -l /home/minnie/.bash_profile
The next time user minnie logs in, her path points to a single folder.
Our restricted user minnie can only use Bash built-in commands like
logout. She can’t even use
We’ll need to slacken our stranglehold a little if we want them to be able to do anything useful at all. We’ll create some symbolic links from minnie’s “bin” directory to the commands that we want minnie to be able to use.
sudo ln -s /bin/ls /home/minnie/bin
sudo ln -s /bin/top /home/minnie/bin
sudo ln -s /bin/uptime /home/minnie/bin
sudo ln -s /bin/pinky /home/minnie/bin
When minnie next logs in, she’ll find that she can use the Bash built-in commands, plus those commands that have been linked to.
Restricting Existing Users
We created minnie as a new user. To change the shell of an existing user, we can use the
-s (shell) option of the
sudo usermod -s /bin/rbash mary
You can use the
less command on the “/etc/passwd” file to quickly see what shell is set as a user’s default shell.
We can see that user mary will use the restricted shell when she next logs in.
Remember to apply the other changes to restricts their
$PATH environment variable and to set the commands you want the user mary to be able to execute.
A regular, unrestricted user can launch scripts that are executed in a restricted shell. Copy the following lines and paste them into an editor. Save the file as “restricted.sh” and close the editor.
#!/bin/bash # script starts in normal Bash shell echo "## In UNrestricted mode! ##" echo echo "Current directory: `pwd`" echo "Changing directory" cd /usr/share echo "Now in directory: `pwd`" echo "Changing to home directory" cd ~ echo "Now in directory: `pwd`" # Setting restricted mode set -r echo echo "## In restricted mode! ##" echo echo "Current directory: `pwd`" echo "Changing directory to /home/" cd /home echo "Still in directory: `pwd`" echo echo "Trying to start another shell" /bin/bash echo echo "Trying to redirect command output" ls -l $HOME > my_files.txt cat my_files.txt echo exit 0
We need to use the
chmod command with the
+x (execute) flag to make the script executable.
chmod +x restricted.sh
The first part of the script runs in a normal shell.
The second portion of the script—the bit after the “set -r” line—runs in a restricted shell.
None of the attempted actions succeed in the restricted portion of the script.
An entire script can be made to run in a restricted shell by adding
-r to the first line:
Restricted shells are useful, but not completely infallible. A sufficiently skilled user may be able to escape them. But when used judiciously, they are a useful way to establish a set of limitations for a particular account.
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