first starting a new shell and checking your shell's pid via echo $$.When you do scl enable devtoolset-9 bash what is happening is that a new bash is started and a new environment is set up. I'm no expert on scl but I do have years of linux experience.

The only downside is that any variables that you've set via the export command will no longer be there as the shell session will be new. That will start a new shell session with everything in one of those shell init files except the scl enable command that you commented out. To go back to the system default if you use this method, comment the line out in ~/.bashrc or ~/.bash_profile and then run bash or source ~/.bash_profile, respectively. You can even put scl enable devtoolset-5 bash, for example, in ~/.bashrc or ~/.bash_profile so that it sets the gcc version to one of the devtoolset versions at login. The second will set it with the variables/commands in ~/.bash_profile. The first will start a new shell session and set any aliases/variables/commands in ~/.bashrc. If you want to change back to the default then any of the following will work assuming bash is your shell: If you want to change to 6: scl enable devtoolset-6 bash Using 5 for this example and assuming that your shell is bash: scl enable devtoolset-5 bash

You can then use the below command to set the gcc version to whichever one you want. I figure that you already did as you have installed one already but in case you didn't: yum install centos-release-scl Make sure that you are installing the devtoolset packages via the scl repo. You can install as many versions of gcc either by installing devtoolset-# via yum or by compiling then from source. The version of gcc that's distributed with CentOS 6 is actually 4.4.7.

Centos install gcc