Add basic docs for variables expressions feature

doc/ci/variables/README.md

@@ -449,6 +449,67 @@ export CI_REGISTRY_USER="gitlab-ci-token"
 export CI_REGISTRY_PASSWORD="longalfanumstring"
 ```
 
+## Variables expressions
+
+> Variables expressions were added in GitLab 10.7.
+
+It is possible to use variables expressions with only / except policies in
+`.gitlab-ci.yml`. By using this approach you can limit what builds are going to
+be created within a pipeline after pushing code to GitLab.
+
+This is particularly useful in combination with secret variables and triggered
+pipeline variables.
+
+```yaml
+deploy:
+  script: cap staging deploy
+  environment: staging
+  only:
+    variables:
+      - $RELEASE == "staging"
+      - $STAGING
+```
+
+Each provided variables expression is going to be evaluated before creating
+a pipeline.
+
+If any of the conditions in `variables` evaluates to truth when using `only`,
+new build is going to be created. If any of the expressions evaluates to truth
+when `except` is being used, a build is not going to be created.
+
+This follows usual rules for [`only` / `except` policies][build policies].
+
+### Supported syntax
+
+Below you can find currently supported syntax reference:
+
+1. Equality matching using a string.
+
+    Example: `$VARIABLE == "some value"`
+
+    You can use equality operator `==` to compare a variable content to a
+    string. We support both, double quotes and single quotes to define a string
+    value, so both `$VARIABLE == "some value"` and `$VARIABLE == 'some value'`
+    are supported. `"some value" == $VARIABLE` is correct too.
+
+1. Checking for an undefined value.
+
+    It sometimes happens that you want to check whether variable is defined or
+    not. To do that, you can compare variable to `null` value, like
+    `$VARIABLE == null`. This expression is going to evaluate to truth if
+    variable is not set.
+
+1. Comparing two variables.
+
+    It is possible to compare two variables. `$VARIABLE_1 == $VARIABLE_2`.
+
+1. Variable presence check.
+
+    If you only want to create a job when there is some variable present,
+    which means that it is defined and non-empty, you can simply use
+    variable name as an expression, like `$STAGING`. If `$STAGING` variable
+    is defined, and is non empty, expression will evaluate to truth.
+
 [ce-13784]: https://gitlab.com/gitlab-org/gitlab-ce/issues/13784 "Simple protection of CI secret variables"
 [eep]: https://about.gitlab.com/products/ "Available only in GitLab Premium"
 [envs]: ../environments.md
@@ -459,3 +520,4 @@ export CI_REGISTRY_PASSWORD="longalfanumstring"
 [triggered]: ../triggers/README.md
 [triggers]: ../triggers/README.md#pass-job-variables-to-a-trigger
 [subgroups]: ../../user/group/subgroups/index.md
+[build policies]: ../yaml/#only-and-except-complex

doc/ci/yaml/README.md

@@ -315,9 +315,14 @@ policy configuration.
 GitLab now supports both, simple and complex strategies, so it is possible to
 use an array and a hash configuration scheme.
 
-Two keys are now available: `refs` and `kubernetes`. Refs strategy equals to
-simplified only/except configuration, whereas kubernetes strategy accepts only
-`active` keyword.
+Three keys are now available: `refs`, `kubernetes` and `variables`.
+Refs strategy equals to simplified only/except configuration, whereas
+kubernetes strategy accepts only `active` keyword.
+
+`variables` keyword is used to define variables expressions. In other words
+you can use predefined variables / secret variables / project / group or
+environment-scoped variables to define an expression GitLab is going to
+evaluate in order to decide whether a job should be created or not.
 
 See the example below. Job is going to be created only when pipeline has been
 scheduled or runs for a `master` branch, and only if kubernetes service is
@@ -332,6 +337,20 @@ job:
     kubernetes: active
 ```
 
+Example of using variables expressions:
+
+```yaml
+deploy:
+  only:
+    refs:
+      - branches
+    variables:
+      - $RELEASE == "staging"
+      - $STAGING
+```
+
+Learn more about variables expressions on [separate page][variables-expressions].
+
 ## `tags`
 
 `tags` is used to select specific Runners from the list of all Runners that are
@@ -1549,3 +1568,4 @@ CI with various languages.
 [ce-7447]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7447
 [ce-12909]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12909
 [schedules]: ../../user/project/pipelines/schedules.md
+[variables expressions]: ../variables#variables-expressions