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Using Update Alternatives ​

The update-alternatives command is useful when you want to support more than one version of a shared command.

It is typical for a Linux machine to use symbolic links to place programs in the PATH without having to create copies of the program.

Example

sh
$ which ftp
/usr/bin/ftp
$ ls -l /usr/bin/ftp
rwxrwxrwx  1  root  root  21 B  ..date..  ftp  ⇒ /etc/alternatives/ftp
$ ls -l /etc/alternatives/ftp
rwxrwxrwx  1  root  root  19 B  ..date..  ftp  ⇒ /usr/bin/netkit-ftp
$ which ftp
/usr/bin/ftp
$ ls -l /usr/bin/ftp
rwxrwxrwx  1  root  root  21 B  ..date..  ftp  ⇒ /etc/alternatives/ftp
$ ls -l /etc/alternatives/ftp
rwxrwxrwx  1  root  root  19 B  ..date..  ftp  ⇒ /usr/bin/netkit-ftp

Here we see that when I call ftp I'm actually calling a double symbolic link through /etc/alternatives/ftp (not in my PATH), back to the /usr/bin/netkit-ftp (which is in my PATH). This redirection using symbolic links is what update-alternatives uses to manage multiple versions of the same command.

Essential Usage ​

The essential usage of update-alternatives relies on three commands

update-alternatives --install <link> <name> <path> <priority>

update-alternatives --list <name>

update-alternatives --config <name>

  • <link> is the location where you are placing the symbolic link, the program. This link should point to somewhere in your PATH, preferably in /usr/local/bin or /home/$USER/.local/bin.
  • <name> is the name given to the program. This should be the command line usage you're familiar with (e.g. python, gcc, etc.).
  • <path> is the location of the alternative you're specifying.
  • <priority> is an arbitrary priority that determines which alternative is the default.

Example

Say you need to manage multiple versions of GCC and G++, in this case update-alternatives is a good solution.

First install all of the alternatives you may need.

sh
$ sudo apt -y install gcc-7 g++-7 gcc-8 g++-8 gcc-9 g++-9
$ sudo apt -y install gcc-7 g++-7 gcc-8 g++-8 gcc-9 g++-9

Next add these alternatives to the update-alternatives interface.

sh
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 7
$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 7
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 8
$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 8
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 9
$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 9
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 7
$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 7
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 8
$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 8
$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 9
$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 9

Next, list the alternatives with --list or switch between them with --config.

sh
$ sudo update-alternatives --config gcc
There are 3 choices for the alternative gcc (providing /usr/bin/gcc).

  Selection    Path            Priority   Status
------------------------------------------------------------
  0            /usr/bin/gcc-9   9         auto mode
  1            /usr/bin/gcc-7   7         manual mode
* 2            /usr/bin/gcc-8   8         manual mode
  3            /usr/bin/gcc-9   9         manual mode
Press  to keep the current choice[*], or type selection number:
$ sudo update-alternatives --config gcc
There are 3 choices for the alternative gcc (providing /usr/bin/gcc).

  Selection    Path            Priority   Status
------------------------------------------------------------
  0            /usr/bin/gcc-9   9         auto mode
  1            /usr/bin/gcc-7   7         manual mode
* 2            /usr/bin/gcc-8   8         manual mode
  3            /usr/bin/gcc-9   9         manual mode
Press  to keep the current choice[*], or type selection number: