Disable styling for --color auto if the output is being piped

Cargo.toml

@@ -17,6 +17,7 @@ path = "src/main.rs"
 [dependencies]
 ansi_term = "0.12.0"
 app_dirs = { version = "2", package = "app_dirs2" }
+atty = "0.2"
 docopt = "1"
 env_logger = { version = "0.7", optional = true }
 flate2 = "1"

src/main.rs

@@ -25,6 +25,7 @@ use std::process;
 
 use ansi_term::{Color, Style};
 use app_dirs::AppInfo;
+use atty::Stream;
 use docopt::Docopt;
 #[cfg(not(target_os = "windows"))]
 use pager::Pager;
@@ -323,9 +324,13 @@ fn main() {
     let enable_styles = match args.flag_color {
         // Always use styling as long as there is `ansi_support`
         ColorOptions::Always => ansi_support,
-        // Disable if:
-        // * NO_COLOR env var is set to anything: https://no-color.org/
-        ColorOptions::Auto => ansi_support && env::var_os("NO_COLOR").is_none(),
+        // Enable styling if:
+        // * There is `ansi_support`
+        // * NO_COLOR env var isn't set: https://no-color.org/
+        // * The output stream is stdout (not being piped)
+        ColorOptions::Auto => {
+            ansi_support && env::var_os("NO_COLOR").is_none() && atty::is(Stream::Stdout)
+        }
         // Disable styling
         ColorOptions::Never => false,
     };

tests/lib.rs

@@ -253,7 +253,7 @@ fn _test_correct_rendering(input_file: &str, filename: &str) {
     let expected = include_str!("inkscape-default.expected");
     testenv
         .command()
-        .args(&["-f", &file_path.to_str().unwrap()])
+        .args(&["--color", "always", "-f", &file_path.to_str().unwrap()])
         .assert()
         .success()
         .stdout(similar(expected));
@@ -299,7 +299,7 @@ fn test_correct_rendering_with_config() {
 
     testenv
         .command()
-        .args(&["-f", &file_path.to_str().unwrap()])
+        .args(&["--color", "always", "-f", &file_path.to_str().unwrap()])
         .assert()
         .success()
         .stdout(similar(expected));