I wanted to install gcc-6 alongside my existing installation of gcc-9 and this is how I did it. First off, sudo apt install gcc-6 didn't work because the package wasn't found so I had to add a new repository that contained gcc-6. To do this, I first found a repository that contains gcc-6 from Google and ended up at: https://packages.ubuntu.com/bionic/gcc-6

From there, I chose an architecture (amd64) which took me to a page with all the mirrors. I added the first mirror (mirrors.kernel.org/ubuntu) to /etc/apt/sources.list and did sudo apt update and then installed gcc-6 with sudo apt install gcc-6.

To switch between gcc versions, I used the following:

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 6
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 9

sudo update-alternatives --config g++

gcc-10 is now available in the Ubuntu tool chains.

Add the PPA and install gcc-10:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt install gcc-10

For g++-10:

   sudo apt install g++-10

At the end you need to update the alternatives for compilers (reference):

#Remove the previous alternatives
sudo update-alternatives --remove-all gcc
sudo update-alternatives --remove-all g++

#Define the compiler
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 30
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 30

sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc 30
sudo update-alternatives --set cc /usr/bin/gcc

sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++ 30
sudo update-alternatives --set c++ /usr/bin/g++

#Confirm and update (You can use the default setting)
sudo update-alternatives --config gcc
sudo update-alternatives --config g++

Tested on Ubuntu 20.04:

sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
sudo apt update
sudo apt install g++-10

This is a common pattern in linux. When there are multiple versions of the same program installed, though the executables are all present in the /usr/bin/ directory, only one of them is "visible" as that program. For example, if you install gcc-9 and gcc-10, both executables are present as /usr/bin/gcc-9 and /usr/bin/gcc-10 but only one of them is visible as gcc. This happens by symlinking a preferred version to the same directory as /usr/bin/gcc. In ubuntu 20.04, the preferred version is gcc-9 and so, gcc-9 is symlinked as gcc.

You can check this by running the following command.

Copy$ which gcc | xargs file

The output will be

Copy/usr/bin/gcc: symbolic link to gcc-9

There are a few things you can do to use gcc-10 as your c compiler.

1. Directly call the gcc-10 executable. Instead of using gcc <code.c>, call gcc-10 <code.c>.
2. You can manually symlink gcc-10 as the preferred gcc. Assuming you did not modify the system paths, the following command can be used.

Copy# ln -s /usr/bin/gcc-10 /usr/local/bin/gcc

This works because, by default, the executables in /usr/local/bin/ take precedence over /usr/bin/.

3. If you are using bash, you can create an alias for gcc as gcc-10. Add the following line to your .bashrc.

Copyalias gcc="gcc-10"

Remember to relaunch bash or source ~/.bashrc.

4. Using update-alternatives (Thanks to @ted-lyngmo for pointing it out). Debian based distributions supply a separate program, that can make symlinking easier / more functional. Read more using man update-alternatives. To use gcc-10 as the preferred gcc, use the following command.

Copy# update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 60

The above command says, /usr/bin/gcc is the link needed and the name is gcc, the target executable is /usr/bin/gcc-10 and it has a priority of 60.

This links gcc to /etc/alternatives/gcc, which itself is a symlink to /usr/bin/gcc-10. If a higher priority program is added to update-alternatives, /etc/alternatives/gcc points to the higher priority program.

If you don't have any specific reason, I would also recommend to upgrade to a newer ubuntu version, so that the default gcc is a newer one.

I read on the gcc website that version 9.4 is more up-to-date than some 10.x versions.

With newer gcc versions, new features are added. Support for newer c/c++ standards are also added. Eg. You can read the changes for gcc-10 here. But people still need gcc-9 because some programs only build with gcc-9. So, GNU maintains gcc-9 (and much older versions) for a long time. Bugs are fixed, and newer releases are made. This can happen after the release of a newer gcc version. So, it is very much possible that a version of gcc-9 is newer than a version of gcc-10.

It is absolutely expected.

Ubuntu 18.04 LTS does not have gcc-10 package. It is available only since 20.04 LTS.

See https://packages.ubuntu.com/focal/gcc-10 for details and make a bookmark for https://packages.ubuntu.com/ to ease package search next time.

If you do not care about package origin, then you can try to install the package from some third-party PPA.