read environment variable (#604)

* read environment variable

* set default config path

src/os.md

@@ -7,11 +7,15 @@
 | [Run piped external commands][ex-run-piped-external-commands] | [![std-badge]][std] | [![cat-os-badge]][cat-os] |
 | [Redirect both stdout and stderr of child process to the same file][ex-redirect-stdout-stderr-same-file] | [![std-badge]][std] | [![cat-os-badge]][cat-os] |
 | [Continuously process child process' outputs][ex-continuous-process-output] | [![std-badge]][std] | [![cat-os-badge]][cat-os][![cat-text-processing-badge]][cat-text-processing] |
+| [Read environment variable][ex-read-env-variable] | [![std-badge]][std] | [![cat-os-badge]][cat-os] |
+
 
 [ex-parse-subprocess-output]: os/external.html#run-an-external-command-and-process-stdout
 [ex-parse-subprocess-input]: os/external.html#run-an-external-command-passing-it-stdin-and-check-for-an-error-code
 [ex-run-piped-external-commands]: os/external.html#run-piped-external-commands
 [ex-redirect-stdout-stderr-same-file]: os/external.html#redirect-both-stdout-and-stderr-of-child-process-to-the-same-file
 [ex-continuous-process-output]: os/external.html#continuously-process-child-process-outputs
+[ex-read-env-variable]: os/external.html#read-environment-variable
+
 
 {{#include links.md}}

src/os/external.md

@@ -10,4 +10,6 @@
 
 {{#include external/continuous.md}}
 
+{{#include external/read-env-variable.md}}
+
 {{#include ../links.md}}

src/os/external/read-env-variable.md

@@ -0,0 +1,25 @@
+## Read Environment Variable 
+
+[![std-badge]][std] [![cat-os-badge]][cat-os]
+
+Reads an environment variable via [std::env::var].
+
+```rust,edition2018,no_run
+use std::env;
+use std::fs;
+use std::io::Error;
+
+fn main() -> Result<(), Error> {
+    // read `config_path` from the environment variable `CONFIG`.
+    // If `CONFIG` isn't set, fall back to a default config path.
+    let config_path = env::var("CONFIG")
+        .unwrap_or("/etc/myapp/config".to_string());
+
+    let config: String = fs::read_to_string(config_path)?;
+    println!("Config: {}", config);
+
+    Ok(())
+}
+```
+
+[std::env::var]: https://doc.rust-lang.org/std/env/fn.var.html