Split up docs into more pages

Closes #63

docs/src/SUMMARY.md

@@ -7,7 +7,11 @@
 # User Guide
 
 - [Key Concepts](./user_guide/key_concepts.md)
-- [Setting Environment Variables](./user_guide/setting_environment_variables.md)
+- [Setting Environment Variables](./user_guide/env/README.md)
+  - [Dynamic Values (Files & Shell Commands)](./user_guide/env/dynamic.md)
+  - [Multiple Values from a Single Source](./user_guide/env/multi.md)
+  - [Adding to the PATH Variable](./user_guide/env/path.md)
+  - [Load Values from Kubernetes](./user_guide/env/kubernetes.md)
 - [Inheritance & Cascading Configs](./user_guide/inheritance.md)
 - [Side Effects](./user_guide/side_effects.md)
 - [Troubleshooting](./user_guide/troubleshooting.md)

docs/src/user_guide/env/README.md

@@ -0,0 +1,13 @@
+# Setting Environment Variables
+
+The primary purpose of env-select is to configure environment variables. The most common way to do this is to provide static values:
+
+```toml
+[applications.server.profiles.dev]
+variables = {SERVICE1 = "dev", SERVICE2 = "also-dev"}
+
+[applications.server.profiles.prd]
+variables = {SERVICE1 = "prd", SERVICE2 = "also-prd"}
+```
+
+Beyond these simple values, there are several ways to customize how values are computed. Read on to learn more.

docs/src/user_guide/env/dynamic.md

@@ -0,0 +1,58 @@
+# Dynamic Values
+
+If your values are not statically known, there are several ways to load dynamic values. Fore more detailed information on each option, see [the API reference](../../api/value_source.md).
+
+## Shell Command
+
+You can specify a shell command to generate a value. This allows you to provide values that can change over time, or secrets that you don't want appearing in the file. For example:
+
+```toml
+[applications.db.profiles.dev.variables]
+DATABASE = "dev"
+DB_USER = "root"
+DB_PASSWORD = {type = "command", command = "curl https://www.random.org/strings/?format=plain&len=10&num=1&loweralpha=on", sensitive = true}
+```
+
+When the `dev` profile is selected for the `db` app, the `DB_PASSWORD` value will be randomly generated from a URL. The `sensitive` field is an _optional_ field that will mask the value in informational logging.
+
+The command is executed in the shell detected by env-select as your default (or the shell passed with `--shell`).
+
+## File
+
+You can also load values from a file:
+
+```toml
+[applications.db.profiles.dev.variables]
+DATABASE = {type = "file", path = "database.txt"}
+```
+
+`database.txt`:
+
+```
+dev
+```
+
+And now run it:
+
+```sh
+> es run db dev -- printenv
+DATABASE=dev
+```
+
+The file path will be relative to **the config file in which the path is defined**. For example, if you have two `.env-select.toml` files:
+
+```toml
+# ~/code/.env-select.toml
+[applications.server.profiles.dev.variables]
+SERVICE1 = {type = "file", path = "service1.txt"}
+```
+
+```toml
+# ~/.env-select.toml
+[applications.server.profiles.dev.variables]
+SERVICE2 = {type = "file", path = "service2.txt"}
+```
+
+In this scenario, `SERVICE1` will be loaded from `~/code/service1.txt` while `SERVICE2` will be loaded from `~/service2.txt`, **regardless of where env-select is invoked from**.
+
+This value source combines well with the `multiple` field to load `.env` files (see next section).

docs/src/user_guide/env/kubernetes.md

@@ -0,0 +1,15 @@
+# Load Values from Kubernetes
+
+If you want to load one or more values from a Kubernetes pod, you can do that with the `command` value source:
+
+```toml
+[applications.my-service.profile.dev]
+variables.DB_PASSWORD = {type = "command", sensitive = true, command = "kubectl exec -n development api -- printenv DB_PASSWORD"}
+```
+
+Loading multiple variables is easy too:
+
+```toml
+[applications.my-service.profile.dev]
+variables.db_creds = {type = "command", sensitive = true, multiple = ["DB_USERNAME", "DB_PASSWORD"], command = "kubectl exec -n development api -- printenv"}
+```

docs/src/user_guide/env/multi.md

@@ -0,0 +1,48 @@
+# Multiple Values from a Single Source
+
+If you want to load multiple values from a single source, you can use the `multiple = true` flag. This will tell env-select to expect a mapping of `VARIABLE=value` as output from the value source, with one entry per line. Whitespace lines and anything preceded by a `#` will be ignored (this is the standard `.env` file format).
+
+This means that **the key associated with this entry in the `variables` map will be ignored**.
+
+```toml
+[applications.db.profiles.dev.variables]
+DATABASE = "dev"
+creds = {type = "file", path = "creds.env", multiple = true}
+```
+
+`creds.env`:
+
+```sh
+DB_USER=root
+DB_PASSWORD=hunter2
+```
+
+`creds` will now be expanded into multiple variables:
+
+```sh
+> es run db dev -- printenv
+DATABASE=dev
+DB_USER=root
+DB_PASSWORD=hunter2
+```
+
+Notice the `creds` key never appears in the environment; this is just a placeholder. You can use any key you want here.
+
+## Filtering Loaded Values
+
+If you want to load only _some_ values from a source, you can filter which are loaded by passing a list of variables to `multiple`. This is useful in scenarios where you dump an entire environment. For example:
+
+```toml
+[applications.db.profiles.dev.variables]
+DATABASE = "dev"
+creds = {type = "command", command = "ssh me@remote printenv", multiple = ["DB_USER", "DB_PASSWORD"]}
+```
+
+This will only load the `DB_USER` and `DB_PASSWORD` variables:
+
+```sh
+> es run db dev -- printenv
+DATABASE=dev
+DB_USER=root
+DB_PASSWORD=hunter2
+```

docs/src/user_guide/env/path.md

@@ -0,0 +1,15 @@
+# Adding to the PATH Variable
+
+If you want to modify the `PATH` variable, typically you just want to add to it, rather than replace it. Because of this, env-select will treat the variable `PATH` specially. It will append to the beginning, using `:` as a separator:
+
+```toml
+[applications.server.profiles.dev.variables]
+PATH = "~/.bin"
+```
+
+```sh
+> printenv PATH
+/bin:/usr/bin
+> es run server dev -- printenv PATH
+~/.bin:/bin:/usr/bin
+```