nix-env-set-flag - Man Page

nix-env --set-flag name value drvnames

The --set-flag operation allows meta attributes of installed packages to be modified. There are several attributes that can be usefully modified, because they affect the behaviour of nix-env or the user environment build script:

• priority can be changed to resolve filename clashes. The user environment build script uses the meta.priority attribute of derivations to resolve filename collisions between packages. Lower priority values denote a higher priority. For instance, the GCC wrapper package and the Binutils package in Nixpkgs both have a file bin/ld, so previously if you tried to install both you would get a collision. Now, on the other hand, the GCC wrapper declares a higher priority than Binutils, so the former’s bin/ld is symlinked in the user environment.
• keep can be set to true to prevent the package from being upgraded or replaced. This is useful if you want to hang on to an older version of a package.
• active can be set to false to “disable” the package. That is, no symlinks will be generated to the files of the package, but it remains part of the profile (so it won’t be garbage-collected). It can be set back to true to re-enable the package.

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To prevent the currently installed Firefox from being upgraded:

$ nix-env --set-flag keep true firefox

After this, nix-env --upgrade will ignore Firefox.

To disable the currently installed Firefox, then install a new Firefox while the old remains part of the profile:

$ nix-env --query
firefox-2.0.0.9 (the current one)

$ nix-env --preserve-installed --install firefox-2.0.0.11
installing `firefox-2.0.0.11'
building path(s) `/nix/store/myy0y59q3ig70dgq37jqwg1j0rsapzsl-user-environment'
collision between `/nix/store/...-firefox-2.0.0.11/bin/firefox'
  and `/nix/store/...-firefox-2.0.0.9/bin/firefox'.
(i.e., can’t have two active at the same time)

$ nix-env --set-flag active false firefox
setting flag on `firefox-2.0.0.9'

$ nix-env --preserve-installed --install firefox-2.0.0.11
installing `firefox-2.0.0.11'

$ nix-env --query
firefox-2.0.0.11 (the enabled one)
firefox-2.0.0.9 (the disabled one)

To make files from binutils take precedence over files from gcc:

$ nix-env --set-flag priority 5 binutils
$ nix-env --set-flag priority 10 gcc