# Execution Model
A Scrut [test document](/docs/reference/fundamentals/test-document/) can contain arbitrary amounts of [test cases](/docs/reference/fundamentals/test-case/). Scrut provides a shared execution environment for all executions from a single document, which results in certain behaviors and side-effects that should be known:
## Shared Shell Environment
Each subsequent [test case](/docs/reference/fundamentals/test-case/) in the same document inherits the shell environment of the previous [test case](/docs/reference/fundamentals/test-case/). This means: All environment variables, shell variables, aliases, functions, etc that have been set in one [test case](/docs/reference/fundamentals/test-case/) are available to the immediate following [test case](/docs/reference/fundamentals/test-case/).
- E.g. `export FOO=bar` in one [test case](/docs/reference/fundamentals/test-case/) will still be set in the following [test case](/docs/reference/fundamentals/test-case/).
- *Exception*: Environments set in [`detached`](/docs/reference/fundamentals/inline-configuration/) [test cases](/docs/reference/fundamentals/test-case/) are not inherited.
## Shared Ephemeral Directories
Each [test cases](/docs/reference/fundamentals/test-case/) in the same document executes in the the same [working directory](/docs/reference/behavior/working-directory/) and is provided with the same temporary directory ([`$TEMPDIR`](/docs/reference/fundamentals/environment-variables/)). Both directories will be removed (cleaned up) after test execution - independent of whether the test execution succeeds or fails.
- *Exception*: If the `--work-directory` command-line parameter is provided, then this directory will not be cleaned up (deleted) after execution. A temporary directory, that still will be removed after execution, will be created within the working directory.
## Process Isolation
Scrut starts individual `bash` processes for executing each [shell expression](/docs/reference/fundamentals/shell-expression/) of each [test case](/docs/reference/fundamentals/test-case/) in the same document. The environment of the previous execution is pulled in through a shared `state` file, that contains all environment variables, shell variables, aliases, functions and settings as they were set when the the previous [test case](/docs/reference/fundamentals/test-case/) execution ended.
:::warning Markdown vs Cram
[Markdown](/docs/reference/formats/markdown-format/) is the default Scrut [test document](/docs/reference/fundamentals/test-document/) format. [Cram](/docs/reference/formats/cram-format/) is supported for legacy reasons. Hence it's legacy mode of execution is also respected. The main difference in Cram from the above is:
> Each execution from the same [test document](/docs/reference/fundamentals/test-document/) is executed *in the same shell process*.
This is less flexible (e.g. Scrut cannot constraint max execution time per [test case](/docs/reference/fundamentals/test-case/)) and more prone to unintended side-effects (e.g. `set -e` terminating all test executions, not only a single test case or detached processes interfering with output association to specific tests). **We recommend to use Markdown**.
:::