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watchtower/docs/lifecycle-hooks.md

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## Executing commands before and after updating
!!! note
These are shell commands executed with `sh`, and therefore require the container to provide the `sh`
executable.
> **DO NOTE**: If the container is not running then lifecycle hooks can not run and therefore
> the update is executed without running any lifecycle hooks.
It is possible to execute _pre/post\-check_ and _pre/post\-update_ commands
**inside** every container updated by watchtower.
- The _pre-check_ command is executed for each container prior to every update cycle.
- The _pre-update_ command is executed before stopping the container when an update is about to start.
- The _post-update_ command is executed after restarting the updated container
- The _post-check_ command is executed for each container post every update cycle.
This feature is disabled by default. To enable it, you need to set the option
`--enable-lifecycle-hooks` on the command line, or set the environment variable
`WATCHTOWER_LIFECYCLE_HOOKS` to `true`.
### Specifying update commands
The commands are specified using docker container labels, the following are currently available:
| Type | Docker Container Label |
| ----------- | ------------------------------------------------------ |
| Pre Check | `com.centurylinklabs.watchtower.lifecycle.pre-check` |
| Pre Update | `com.centurylinklabs.watchtower.lifecycle.pre-update` |
| Post Update | `com.centurylinklabs.watchtower.lifecycle.post-update` |
| Post Check | `com.centurylinklabs.watchtower.lifecycle.post-check` |
These labels can be declared as instructions in a Dockerfile (with some example .sh files) or be specified as part of
the `docker run` command line:
=== "Dockerfile"
```docker
LABEL com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh"
LABEL com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh"
```
=== "docker run"
```bash
docker run -d \
--label=com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh" \
--label=com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh" \
--label=com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh" \
someimage --label=com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh" \
```
### Timeouts
The timeout for all lifecycle commands is 60 seconds. After that, a timeout will
occur, forcing Watchtower to continue the update loop.
#### Pre-update timeouts
For the `pre-update` lifecycle command, it is possible to override this timeout to
allow the script to finish before forcefully killing it. This is done by adding the
label `com.centurylinklabs.watchtower.lifecycle.pre-update-timeout` followed by
the timeout expressed in minutes.
If the label value is explicitly set to `0`, the timeout will be disabled.
### Execution failure
The failure of a command to execute, identified by an exit code different than
0 or 75 (EX_TEMPFAIL), will not prevent watchtower from updating the container. Only an error
log statement containing the exit code will be reported.