If you are allowing shell commands to be executed from the browser, then what you are effectively doing is allowing the user to log-in and execute commands on your system as the user that the web-server is running as. Ask yourself if that is what you want to do? The answer is definitely no. No-way. No, not never. In fact, hackers spend a lot of time trying to work out how to do what you have effectively designed in to your system.
You can try and "sanitize" the input, but then all you are doing is asking the hackers to try and find ways around your sanitizer. They are probably smarter than you.
You could try and execute the commands as a user with restricted permissions. Then you are effectively allowing anyone to log-in as that user and execute commands on that system. The UNIX permissions mechanism might protect you a bit.
Finally, you could try using chroot to create a very restricted environment to run your bash commands in. That's quite a lot of work, and there is still the possibility for trojans to be planted in to the restricted environment. If you plan this option, you could look up the makejail command: makejail.
The final option is to not allow users to run arbitrary bash commands, but instead provide a list of preconfigured commands that you allow them to run, which you know are safe.
Also, you may be vulnerable to the Shellshock bug Shellshock