In centos,how to switch to default gcc after switched to a higher version of gcc with devtoolset

Question

I want to install several gcc with different versions in centos. The default version of gcc in centos 6 is 4.9.3. So I use devtoolset install a higher version of gcc. Then I switch to the higher version of gcc by executing "source /opt/rh/devtoolset-5/enable". But now if I want to switch back to the default gcc, how should I do? By the way, is there any solution to install multiple gcc with different versions in centos 5?

Answer 1

The version of gcc that's distributed with CentOS 6 is actually 4.4.7.

You can install as many versions of gcc either by installing devtoolset-# via yum or by compiling then from source.

The first way is the easiest. Make sure that you are installing the devtoolset packages via the scl repo. I figure that you already did as you have installed one already but in case you didn't:

yum install centos-release-scl

You can then use the below command to set the gcc version to whichever one you want. Using 5 for this example and assuming that your shell is bash:

scl enable devtoolset-5 bash

If you want to change to 6:

scl enable devtoolset-6 bash

If you want to change back to the default then any of the following will work assuming bash is your shell:

bash

source ~/.bash_profile

The first will start a new shell session and set any aliases/variables/commands in ~/.bashrc. The second will set it with the variables/commands in ~/.bash_profile. (Without the devtoolset enabled).

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. 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. 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. 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.

Answer 2

I'm no expert on scl but I do have years of linux experience.

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.

You can see the new bash process by:

• first starting a new shell and checking your shell's pid via echo $$
• second enabling the new devtoolset via scl enable devtoolset-9 bash
• then check your pid again via echo $$
• for bonus points you can do pstree -p to see that your new bash pid has a parent pid of your old bash process

So to finally answer your question: To return to the default g++ compiler all you need to do is exit your current bash process and then you should have the old g++ compiler.

Important note regarding your ~/.bashrc:

• my solution won't work if you have somehow modified your ~/.bashrc
• i.e. if you have something in there that always does the scl enable devtoolset-9
• see the other solutions on this page because the other solutions talk more in-depth about your ~/.bashrc and how to modify or unmodify it

Answer 3

It's probably just a matter of changing some environment variables, like PATH.  How big is that ⁠/opt/rh/devtoolset-5/enable file?  Can you look at it, see what it does, and adapt it for the older compiler that you want to use?  The obvious things to look for would be the names of the directories where the new compiler's files are installed (change them to the corresponding pathnames for the old compiler) and the version number.  I don't know what else is likely to be there.