Add GitLab Unix shell scripting style guide to docs

doc/development/contributing/style_guides.md

@@ -23,6 +23,7 @@
 1. Code should be written in [US English][us-english]
 1. [Go](../go_guide/index.md)
 1. [Python](../python_guide/index.md)
+1. [Shell scripting](../shell_scripting_guide/index.md)
 
 This is also the style used by linting tools such as
 [RuboCop](https://github.com/rubocop-hq/rubocop) and [Hound CI](https://houndci.com).

doc/development/shell_scripting_guide/index.md

+# Shell scripting standards and style guidelines
+
+## Overview
+
+GitLab consists of many various services and sub-projects. The majority of
+their backend code is written in [Ruby](https://www.ruby-lang.org) and
+[Go](https://golang.org). However, some of them use shell scripts for
+automation of routine system administration tasks like deployment,
+installation, etc. It's being done either for historical reasons or as an effort
+to minimize the dependencies, for instance, for Docker images.
+
+This page aims to define and organize our shell scripting guidelines,
+based on our various experiences. All shell scripts across GitLab project
+should be eventually harmonized with this guide. If there are any per-project
+deviations from this guide, they should be described in the
+`README.md` or `PROCESS.md` file for such a project.
+
+### Avoid using shell scripts
+
+CAUTION: **Caution:**
+This is a must-read section.
+
+Having said all of the above, we recommend staying away from shell scripts
+as much as possible. A language like Ruby or Python (if required for
+consistency with codebases that we leverage) is almost always a better choice.
+The high-level interpreted languages have more readable syntax, offer much more
+mature capabilities for unit-testing, linting, and error reporting.  
+Use shell scripts only if there's a strong restriction on project's
+dependencies size or any other requirements that are more important
+in a particular case.
+
+## Scope of this guide
+
+According to the [GitLab installation requirements](../../install/requirements.md),
+this guide covers only those shells that are used by
+[supported Linux distributions](../../install/requirements.md#supported-linux-distributions),
+that is:
+
+- [POSIX Shell](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html)
+- [Bash](https://www.gnu.org/software/bash/)
+
+## Shell language choice
+
+- When you need to reduce the dependencies list, use what's provided by the environment. For example, for Docker images it's `sh` from `alpine` which is the base image for most of our tool images.
+- Everywhere else, use `bash` if possible. It's more powerful than `sh` but still a widespread shell.
+
+## Code style and format
+
+This section describes the tools that should be made a mandatory part of
+a project's CI pipeline if it contains shell scripts. These tools
+automate shell code formatting, checking for errors or vulnerabilities, etc.   
+
+### Linting
+
+We're using the [ShellCheck](https://www.shellcheck.net/) utility in its default configuration to lint our
+shell scripts. 
+
+All projects with shell scripts should use this GitLab CI/CD job:
+
+```yaml
+shell check:
+  image: koalaman/shellcheck-alpine
+  stage: test
+  before_script:
+    - shellcheck --version
+  script:
+    - shellcheck scripts/**/*.sh # path to your shell scripts
+```
+
+TIP: **Tip:**
+By default, ShellCheck will use the [shell detection](https://github.com/koalaman/shellcheck/wiki/SC2148#rationale)
+to determine the shell dialect in use. If the shell file is out of your control and ShellCheck cannot
+detect the dialect, use `-s` flag to specify it: `-s sh` or `-s bash`.
+
+### Formatting
+
+It's recommended to use the [shfmt](https://github.com/mvdan/sh#shfmt) tool to maintain consistent formatting.
+We format shell scripts according to the [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml),
+so the following `shfmt` invocation should be applied to the project's script files:
+
+```bash
+shfmt -i 2 -ci scripts/**/*.sh
+```
+
+TIP: **Tip:**
+By default, shfmt will use the [shell detection](https://github.com/mvdan/sh#shfmt) similar to one of ShellCheck
+and ignore files starting with a period. To override this, use `-ln` flag to specify the shell dialect:
+`-ln posix` or `-ln bash`.
+
+NOTE: **Note:**
+Currently, the `shfmt` tool [is not shipped](https://github.com/mvdan/sh/issues/68) as a Docker image containing
+a Linux shell. This makes it impossible to use the [official Docker image](https://hub.docker.com/r/mvdan/shfmt)
+in GitLab Runner. This [may change](https://github.com/mvdan/sh/issues/68#issuecomment-507721371) in future.
+
+## Testing
+
+NOTE: **Note:**
+This is a work in progress.
+
+It is an [ongoing effort](https://gitlab.com/gitlab-org/gitlab-ce/issues/64016) to evaluate different tools for the
+automated testing of shell scripts (like [BATS](https://github.com/sstephenson/bats)).  
+
+## Code Review
+
+The code review should be performed according to:
+
+- [ShellCheck Checks list](https://github.com/koalaman/shellcheck/wiki/Checks)
+- [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml)
+- [Shfmt formatting caveats](https://github.com/mvdan/sh#caveats)
+
+However, the recommended course of action is to use the aforementioned
+tools and address reported offenses. This should eliminate the need
+for code review.
+
+---
+
+[Return to Development documentation](../README.md).