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Improve nix-env --delete-generations docs

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Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
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John Ericson 2023-05-16 12:41:39 -04:00
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doc/manual/src/command-ref/nix-env/delete-generations.md
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# Description
This operation deletes the specified generations of the current profile.
The generations can be a list of generation numbers, the special value
`old` to delete all non-current generations, a value such as `30d` to
delete all generations older than the specified number of days (except
for the generation that was active at that point in time), or a value
such as `+5` to keep the last `5` generations ignoring any newer than
current, e.g., if `30` is the current generation `+5` will delete
generation `25` and all older generations. Periodically deleting old
generations is important to make garbage collection effective.
*generations* can be a one of the following:
- <span id="generations-list">`<number>...`</span>:\
A list of generation numbers, each one a separate command-line argument.
Delete exactly the profile generations given by their generation number.
Deleting the current generation is not allowed.
- The special value <span id="generations-old">`old`</span>
Delete all generations older than the current one.
- <span id="generations-days">`<days>d`</span>:\
The last *days* days
*Example*: `30d`
Delete all generations older than *days* days.
The generation that was active at that point in time is excluded, and will not be deleted.
- <span id="generations-count">`+<count>`</span>:\
The last *count* generations up to the present
*Example*: `+5`
Keep the last *count* generations, along with any newer than current.
Periodically deleting old generations is important to make garbage collection
effective.
The is because profiles are also garbage collection roots — any [store object] reachable from a profile is "alive" and ineligible for deletion.
[store object]: @docroot@/glossary.md#gloss-store-object
{{#include ./opt-common.md}}
@ -28,19 +53,35 @@ generations is important to make garbage collection effective.
# Examples
## Delete explicit generation numbers
```console
$ nix-env --delete-generations 3 4 8
```
Delete the generations numbered 3, 4, and 8, so long as the current active generation is not any of those.
## Keep most-recent by count count
```console
$ nix-env --delete-generations +5
```
Suppose `30` is the current generation, and we currently have generations numbered `20` through `32`.
Then this command will delete generations `20` through `25` (`<= 30 - 5`),
and keep generations `26` through `31` (`> 30 - 5`).
## Keep most-recent in days
```console
$ nix-env --delete-generations 30d
```
This command will delete all generations older than 30 days, except for the generation that was active 30 days ago (if it currently exists).
## Delete all older
```console
$ nix-env --profile other_profile --delete-generations old
```