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# Name
`nix-env --set-flag` - modify meta attributes of installed packages
# Synopsis
`nix-env` `--set-flag` *name* *value* *drvnames*
# Description
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.
{{#include ./opt-common.md}}
{{#include ../opt-common.md}}
{{#include ../env-common.md}}
# Examples
To prevent the currently installed Firefox from being upgraded:
```console
$ nix-env --set-flag keep true firefox
```
After this, `nix-env -u` will ignore Firefox.
To disable the currently installed Firefox, then install a new Firefox
while the old remains part of the profile:
```console
$ nix-env -q
firefox-2.0.0.9 (the current one)
$ nix-env --preserve-installed -i 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 -i firefox-2.0.0.11
installing `firefox-2.0.0.11'
$ nix-env -q
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`:
```console
$ nix-env --set-flag priority 5 binutils
$ nix-env --set-flag priority 10 gcc
```
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