From 6b319db6e453f802f77e28bb09a14593e34f19df Mon Sep 17 00:00:00 2001
From: dinesh <dineshpy07@gmail.com>
Date: Sun, 1 Jul 2018 15:29:47 +0530
Subject: [PATCH] Add an example to read an environment variable

---
 src/basics.md | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 src/intro.md  |  2 ++
 2 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/src/basics.md b/src/basics.md
index 770e5008..acbaa92b 100644
--- a/src/basics.md
+++ b/src/basics.md
@@ -38,7 +38,8 @@
 | [Parse string into DateTime struct][ex-parse-datetime] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
 | [Perform checked date and time calculations][ex-datetime-arithmetic] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
 | [Examine the date and time][ex-examine-date-and-time] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
-| [File names that have been modified in the last 24 hours for the working directory][ex-file-24-hours-modified] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] [![cat-os-badge]][cat-os] |
+| [File names that have been modified in the last 24 hours for the working directory][ex-file-24-hours-modified] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] [![cat-os-badge]][cat-os] | 
+| [Read environment variable][ex-read-environment-variable] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] |
 
 [ex-std-read-lines]: #ex-std-read-lines
 <a name="ex-std-read-lines"></a>
@@ -1770,6 +1771,48 @@ fn run() -> Result<()> {
 # quick_main!(run);
 ```
 
+[ex-read-environment-variable]: #ex-read-environment-variable
+<a name="ex-read-environment-variable"></a>
+## Read environment variable
+[![std-badge]][std] [![cat-filesystem-badge]][cat-filesystem]
+
+Set an environment variable and read it back. If the variable is not found, then fallback
+to a default value.
+
+```rust
+# #[macro_use]
+# extern crate error_chain;
+use std::env;
+use std::path::PathBuf;
+
+# error_chain! {
+#     foreign_links {
+#         Utf8(std::str::Utf8Error);
+#         AddrParse(std::net::AddrParseError);
+#     }
+# }
+
+fn set_env_variable() {
+  env::set_var("CONFIG_FILE", "config.toml");
+}
+
+fn run() -> Result<()> {
+    set_env_variable();
+
+    let default_config_file = "config.toml";
+
+    let config_file_path = match env::var("CONFIG_FILE") {
+        Ok(val) => PathBuf::from(val),
+        Err(_) => PathBuf::from(default_config_file)
+    };
+
+    println!("Env value is {}", config_file_path.display());
+    Ok(())
+}
+#
+# quick_main!(run);
+```
+
 {{#include links.md}}
 
 <!-- API Reference -->
diff --git a/src/intro.md b/src/intro.md
index 26af488b..8d2efc4f 100644
--- a/src/intro.md
+++ b/src/intro.md
@@ -57,6 +57,7 @@ community. It needs and welcomes help. For details see
 | [Perform checked date and time calculations][ex-datetime-arithmetic] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
 | [Examine the date and time][ex-examine-date-and-time] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
 | [File names that have been modified in the last 24 hours for the working directory][ex-file-24-hours-modified] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] [![cat-os-badge]][cat-os] |
+| [Read environment variable][ex-read-environment-variable] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] |
 
 ## [Encoding](encoding.html)
 
@@ -193,6 +194,7 @@ community. It needs and welcomes help. For details see
 [ex-extract-links-webpage]: net.html#ex-extract-links-webpage
 [ex-extract-mediawiki-links]: net.html#ex-extract-mediawiki-links
 [ex-file-24-hours-modified]: basics.html#ex-file-24-hours-modified
+[ex-read-environment-variable]: basics.html#ex-read-environment-variable
 [ex-file-post]: net.html#ex-file-post
 [ex-file-predicate]: app.html#ex-file-predicate
 [ex-file-sizes]: app.html#ex-file-sizes