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## Executing commands before and after updating
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> **DO NOTE**: These are shell commands executed with `sh`, and therefore require the
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> container to provide the `sh` executable.
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2019-08-26 16:44:10 +02:00
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It is possible to execute _pre/post\-check_ and _pre/post\-update_ commands
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**inside** every container updated by watchtower.
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- The _pre-check_ command is executed before checking the container for updates.
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- The _pre-update_ command is executed before stopping the container when an update is about to start.
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- The _post-update_ command is executed after restarting the updated container
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- The _post-check_ command is executed last after a updated container is started or no update was needed.
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This feature is disabled by default. To enable it, you need to set the option
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`--enable-lifecycle-hooks` on the command line, or set the environment variable
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`WATCHTOWER_LIFECYCLE_HOOKS` to `true`.
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### Specifying update commands
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2019-08-26 16:44:10 +02:00
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The commands are specified using docker container labels, the following are currently available:
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- `com.centurylinklabs.watchtower.lifecycle.pre-check` - _pre-check_
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- `com.centurylinklabs.watchtower.lifecycle.pre-update-command` - _pre-update_
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- `com.centurylinklabs.watchtower.lifecycle.post-update` - _post-update_
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- `com.centurylinklabs.watchtower.lifecycle.post-check` - _post-check_
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2019-08-26 16:44:10 +02:00
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These labels can be declared as instructions in a Dockerfile (with some example .sh files):
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```docker
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LABEL com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh"
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LABEL com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh"
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LABEL com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh"
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LABEL com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh"
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```
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Or be specified as part of the `docker run` command line:
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```bash
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docker run -d \
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--label=com.centurylinklabs.watchtower.lifecycle.pre-check="/sync.sh" \
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--label=com.centurylinklabs.watchtower.lifecycle.pre-update="/dump-data.sh" \
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--label=com.centurylinklabs.watchtower.lifecycle.post-update="/restore-data.sh" \
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someimage
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--label=com.centurylinklabs.watchtower.lifecycle.post-check="/send-heartbeat.sh" \
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```
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### Execution failure
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The failure of a command to execute, identified by an exit code different than
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0, will not prevent watchtower from updating the container. Only an error
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log statement containing the exit code will be reported.
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