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Improve nix-collect-garbage docs

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Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
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John Ericson 2023-05-16 10:32:38 -04:00
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doc/manual/src/command-ref/nix-collect-garbage.md
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# Name
`nix-collect-garbage` - delete unreachable store paths
`nix-collect-garbage` - delete unreachable [store objects]
# Synopsis
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# Description
The command `nix-collect-garbage` is mostly an alias of [`nix-store
--gc`](@docroot@/command-ref/nix-store/gc.md), that is, it deletes all
unreachable paths in the Nix store to clean up your system. However,
it provides two additional options: `-d` (`--delete-old`), which
deletes all old generations of all profiles in `/nix/var/nix/profiles`
by invoking `nix-env --delete-generations old` on all profiles (of
course, this makes rollbacks to previous configurations impossible);
and `--delete-older-than` *period*, where period is a value such as
`30d`, which deletes all generations older than the specified number
of days in all profiles in `/nix/var/nix/profiles` (except for the
generations that were active at that point in time).
The command `nix-collect-garbage` is mostly an alias of [`nix-store --gc`](@docroot@/command-ref/nix-store/gc.md).
That is, it deletes all unreachable [store objects] in the Nix store to clean up your system.
However, it provides two additional options,
[`--delete-old`](#opt-delete-old) and [`--delete-older-than`](#opt-delete-older-than),
which also delete old [profiles], allowing potentially more [store objects] to be deleted because profiles are also garbage collection roots.
These options are the equivalent of running
[`nix-env --delete-generations`](@docroot@/command-ref/nix-env/delete-generations.md)
with various augments on multiple profiles,
prior to running `nix-collect-garbage` (or just `nix-store --gc`) without any flags.
> **Note**
>
> Deleting previous configurations makes rollbacks to them impossible.
These flags should be used with care, because they potentially delete generations of profiles used by other users on the system.
## Locations searched for profiles
`nix-collect-garbage` cannot know about all profiles; that information doesn't exist.
Instead, it looks in a few locations, and acts on all profiles it finds there:
1. The default profile locations as specified in the [profiles] section of the manual.
2. > **NOTE**
>
> Not stable; subject to change
>
> Do not rely on this functionality; it just exists for migration purposes and is may change in the future.
> These deprecated paths remain a private implementation detail of Nix.
`$NIX_STATE_DIR/profiles` and `$NIX_STATE_DIR/profiles/per-user`.
With the exception of `$NIX_STATE_DIR/profiles/per-user/root` and `$NIX_STATE_DIR/profiles/default`, these directories are no longer used by other commands.
`nix-collect-garbage` looks there anyways in order to clean up profiles from older versions of Nix.
# Options
These options are for deleting old [profiles] prior to deleting unreachable [store objects].
- <span id="opt-delete-old">[`--delete-old`](#opt-delete-old)</span> / `-d`\
Delete all old generations of profiles.
This is the equivalent of invoking `nix-env --delete-generations old` on each found profile.
- <span id="opt-delete-older-than">[`--delete-older-than`](#opt-delete-older-than)</span> *period*\
Delete all generations of profiles older than the specified amount (except for the generations that were active at that point in time).
*period* is a value such as `30d`, which would mean 30 days.
This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-days) on each found profile.
See the documentation of that command for additional information about the *period* argument.
{{#include ./opt-common.md}}
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```console
$ nix-collect-garbage -d
```
[profiles]: @docroot@/command-ref/files/profiles.md
[store objects]: @docroot@/glossary.md#gloss-store-object