# Built-in hooks You can use hooks to execute actions at specific stages of stack deploy and undeploy operations. Takomo has two built-in hooks, and you can also implement your own. In a stack configuration, you choose which type of hook to use by providing value to the `type` property. You also need to give a name for each hook. In addition to these two mandatory properties, each hook type may have its own set of additional properties. ## Command hook The command hook executes the specified shell command. ### Properties Here are the properties of the Command hook: | Key | Required | Type | Description | | ---------------------- | -------- | ------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------- | | type | yes | string | Type of the hook, this must be **cmd**. | | name | yes | string | Name of the hook. | | command | yes | string | Shell command to execute. | | cwd | no | string | Path to the working directory from where the shell command is executed. Relative to the current project directory. | | exposeStackCredentials | no | boolean | Make the current stack's AWS credentials available for the shell command. Defaults to false. | | exposeStackRegion | no | boolean | Make the current stack's region available for the shell command. Defaults to false. | | capture | no | string | Controls how to capture the output of the executed shell command. By default, all output is captured. To capture only the last line, set this to **last-line.** | ### Environment variables available in the shell command The following environment variables are available in the shell command: | Name | Description | | ------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | TKM\_COMMAND\_STAGE | The current stack operation stage. | | TKM\_COMMAND\_OPERATION | The current stack operation. | | TKM\_COMMAND\_STATUS | The current stack operation status, not present in before stage | | AWS\_ACCESS\_KEY\_ID | If `exposeStackCredentials` is **true**, this will hold the access key id of credentials of the current stack. | | AWS\_SECRET\_ACCESS\_KEY | If `exposeStackCredentials` is **true**, this will hold the secret access key of credentials of the current stack. | | AWS\_SESSION\_TOKEN | If `exposeStackCredentials` is **true**, this will hold the session token of credentials of the current stack. | | AWS\_SECURITY\_TOKEN | If `exposeStackCredentials` is **true**, this will hold the session token of credentials of the current stack. | | AWS\_DEFAULT\_REGION | If `exposeStackRegion` is **true**, this will hold the region of the current stack. | | TKM\_HOOK\_{hook-name} | Values returned from previous hooks are exposed in environment variables where the {hook-name} placeholder is replaced with the hook's name. Hooks whose name has unsafe characters not compatible with a pattern /^\[a-zA-Z\_]+\[a-zA-Z0-9*\_*]\*$/ are not exposed and a warning is logged instead. | Any output the hook prints to the stdout is captured and exposed to other hooks. ### Examples A command hook that runs a simple shell command: ```yaml - name: my-hook type: cmd command: echo 'hello world' ``` A command hook that exposes the current stack's AWS credentials to the shell command: ```yaml - name: my-another-hook type: cmd exposeStackCredentials: true command: aws sts get-caller-identity ``` ## Checksum hook The checksum hook calculates a checksum from a specified directory. The checksum is calculated recursively, i.e. all files and directories under the specified directory are included in the checksum. ### Properties Here are the properties of the Checksum hook: | Key | Required | Type | Description | | -------- | -------- | ------ | ---------------------------------------------------------------------------------------------------------------------------------- | | type | yes | string | Type of the hook, this must be **checksum** | | name | yes | string | Name of the hook | | dir | yes | string | Path to the directory from where the checksum should be calculated. The path can be absolute or relative to the project directory. | | encoding | no | string | Encoding used to encode the calculated checksum. Supported values are **base64** (default) and **hex**. | ### Examples Calculate a checksum from a directory lambda/scripts located in the project directory. ```yaml - name: my-checksum type: checksum dir: lambda/scripts ``` The same as above but using the hex encoding. ```yaml - name: my-hex-checksum type: checksum dir: lambda/scripts encoding: hex ``` ## See also * [Configuring hooks in stack configuration](https://docs.takomo.io/stack-properties/hooks)