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Add section about pre and post commands in README.md file.

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Alexandre Menif 2018-05-30 21:53:12 +02:00
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@ -275,3 +275,40 @@ docker run -d \
-e WATCHTOWER_NOTIFICATION_MSTEAMS_USE_LOG_DATA=true \
v2tec/watchtower
```
## Executing commands before and after updating
For every container that could be updated by watchtower, it is possible to
specify a command that will be executed before stopping the container (a
`pre-update` command), and a command that will be executed after restarting the
container (a `post-update` command).
These commands are specified using the *com.centurylinklabs.watchtower.pre-update-command*
and the *com.centurylinklabs.watchtower.post-update-command* labels. The values
of these labels are JSON formatted strings that describes the commands.
These labels can be declared as instructions in a Dockerfile:
```docker
LABEL com.centurylinklabs.watchtower.pre-update-command='{"Cmd": ["sh", "-c", "/dump-data.sh"]}'
LABEL com.centurylinklabs.watchtower.post-update-command='{"Cmd": ["sh", "-c", "/restore-data.sh"]}'
```
Or be specified as part of the `docker run` command line:
```bash
docker run -d \
--label=com.centurylinklabs.watchtower.pre-update-command='{"Cmd": ["sh", "-c", "/dump-data.sh"]}' \
--label=com.centurylinklabs.watchtower.post-update-command='{"Cmd": ["sh", "-c", "/restore-data.sh"]}' \
someimage
```
The JSON object is made of the following fields:
* `Cmd`: the command to execute, as an array of strings (required)
* `User`: a string representing a username or a UID. It is of the form `user`,
`user:group`, `uid`, or `uid:gid` (optional)
* `Privileged`: `true` if the command should be given extended privileges,
`false` otherwise (optional, default to `false`)
* `Env`: a list of instructions to set enviroment variables, for example
`["FOO=bar", BAZ=quux]` (optional)