How do I use the latest GCC on Ubuntu?

The best way to correctly install gcc-4.9 and set it as your default gcc version use:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-4.9 g++-4.9
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 60 --slave /usr/bin/g++ g++ /usr/bin/g++-4.9

The --slave, with g++, will cause g++ to be switched along with gcc, to the same version. But, at this point gcc-4.9 will be your only version configured in update-alternatives, so add 4.8 to update-alternatives, so there actually is an alternative, by using:

sudo apt-get install gcc-4.8 g++-4.8
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 60 --slave /usr/bin/g++ g++ /usr/bin/g++-4.8

Then you can check which one that is set, and change back and forth using:

sudo update-alternatives --config gcc

NOTE: You could skip installing the PPA Repository and just use /usr/bin/gcc-4.9-base but I prefer using the fresh updated toolchains.

---

For GCC 5.X or 6, the packages (and correspondingly, the commands) are just called gcc-5, gcc-6, etc. This is due to the change in GCC's version scheme, where 5.1 is the first GCC 5 release, and future 5.X releases are for bug fixes.

Use the Toolchain Test Builds PPA:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-4.9

I don't think GCC 4.9 is fully available for Ubuntu 14.04 yet. The base package (gcc-4.9-base) and the GCC Go 4.9 compiler (gccgo-4.9) are available, but the other frontends are not. I don't know why.

Ultimate mega master compatibility table

OK let's do this:

               GCC            clang
        +----------------+-----------------+
        | 10 9 8 7 6 5 4 | 11 10 9 8 7 6 5 |
+-------+----------------+-----------------+
| 20.10 |  D M M M       |  D  M M M M M   |
| 20.04 |  P D M M       |     D M M M M   |
| 19.10 |    D M M       |                 |
| 19.04 |    M D M M     |                 |
| 18.10 |      D M M M   |                 |
| 18.04 |  P P M D M M   |       M M M D M |
| 16.04 |    P P P P D M |                 |
+-------+----------------+-----------------+

• D: Default GCC

  sudo apt-get update
  sudo apt-get install gcc g++
  gcc --version
  

  Whatever the gcc package aliases to: https://packages.ubuntu.com/search?keywords=gcc and also present in manifests: How do I list the default installed packages?

• M: Present in Main repo

  sudo apt-get update
  sudo apt-get install gcc-X g++-X
  gcc-X --version
  

  All Ubuntu versions that have a hit for a given GCC version, e.g. for GCC 7: https://packages.ubuntu.com/search?keywords=gcc-7

  The minor versions of these packages can get updated from time to time, e.g. 8.3.0 to 8.4.0.

• P: ppa:ubuntu-toolchain-r/test, which is owned by Ubuntu people and therefore can be trusted to not be a virus, although it is possibly unstable:

  sudo add-apt-repository ppa:ubuntu-toolchain-r/test
  sudo apt-get update
  sudo apt-get install gcc-X g++-X
  gcc-X --version
  

  Full list: https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test

  The minor versions of these packages can get updated from time to time, e.g. 8.3.0 to 8.4.0.

All the questions:

• install gcc-9 on Ubuntu 18.04?
• How to install gcc-7 or clang 4.0?
• install gcc-9 on Ubuntu 18.04?
• What are the GCC and clang versions available in Ubuntu 18.04?

How to set a non-default GCC as the default?

E.g., you installed /usr/bin/gcc-7 but you want to use that instead of /usr/bin/gcc when you run gcc main.c.

Use sudo update-alternatives as mentioned in other answers: https://askubuntu.com/a/581497/52975 It creates the required symlinks for you.

See also: What exactly does `update-alternatives` do?

How to build your own toolchain from source

If even the PPA is not old/new enough for you, see this:

• https://stackoverflow.com/questions/847179/multiple-glibc-libraries-on-a-single-host/52454603#52454603
• https://stackoverflow.com/questions/26305738/can-i-build-gcc-for-arm-with-an-x64-one/26306591#26306591

Older GCC version questions

• Ubuntu 20.04 - gcc version lower than gcc-7
• How to use an older version of GCC