.envrc

@@ -1 +0,0 @@
-use flake

.gitignore

@@ -1,7 +1,11 @@
+# build
 /target
 /result
 
-.direnv
+# environments
 .env*
 !.env.example
 
+# direnv
+.envrc
+.direnv

Cargo.lock

@@ -196,7 +196,9 @@ checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
 name = "vnetod"
 version = "0.3.3"
 dependencies = [
+ "atty",
  "clap",
+ "termcolor",
 ]
 
 [[package]]

Cargo.toml

@@ -13,6 +13,8 @@ categories = ["command-line-interface", "config", "development-tools"]
 
 [dependencies]
 clap = { version = "3.2.15", default-features = false, features = ["std", "env", "derive"] }
+atty = { version = "0.2.14", optional = true }
+termcolor = { version = "1.1.3", optional = true }
 
 [features]
-color = ["clap/color"]
+color = ["clap/color", "dep:atty", "dep:termcolor"]

flake.nix

@@ -11,22 +11,28 @@
       cargoToml = fromTOML (readFile ./Cargo.toml);
       version = "${cargoToml.package.version}+${substring 0 8 self.lastModifiedDate}.${self.shortRev or "dirty"}";
 
-      mkVnetod = { lib, rustPlatform, ... }:
+      mkVnetod = { lib, rustPlatform, vnetodFeatures ? [ ], ... }:
         rustPlatform.buildRustPackage {
           name = "vnetod-${version}";
 
           src = lib.cleanSource ./.;
           cargoLock.lockFile = ./Cargo.lock;
 
+          buildFeatures = vnetodFeatures;
+
           doCheck = true;
         };
     in
     {
-      overlays = rec {
-        vnetod = final: prev: {
+      overlays = {
+        default = final: prev: {
           vnetod = final.callPackage mkVnetod { };
         };
-        default = vnetod;
+        colored = final: prev: {
+          vnetod = final.callPackage mkVnetod {
+            vnetodFeatures = [ "color" ];
+          };
+        };
       };
     }
     // flake-utils.lib.eachDefaultSystem (system:
@@ -34,6 +40,7 @@
         pkgs = import nixpkgs { inherit system; };
 
         vnetod = pkgs.callPackage mkVnetod { };
+        coloredVnetod = pkgs.callPackage mkVnetod { vnetodFeatures = [ "color" ]; };
 
         docker = pkgs.dockerTools.buildLayeredImage {
           name = "pleshevskiy/vnetod";
@@ -41,19 +48,25 @@
           config = {
             Volumes."/data" = { };
             WorkingDir = "/data";
-            Entrypoint = [ "${vnetod}/bin/vnetod" ];
+            Entrypoint = [ "${coloredVnetod}/bin/vnetod" ];
           };
         };
+
+        mkApp = prog: {
+          type = "app";
+          program = "${vnetod}/bin/vnetod";
+        };
       in
       {
-        apps.default = {
-          type = "app";
-          program = "${vnetod}/bin/vnetod";
+        apps = {
+          default = mkApp vnetod;
+          colored = mkApp coloredVnetod;
         };
 
         packages = {
           inherit docker vnetod;
           default = vnetod;
+          colored = coloredVnetod;
         };
 
         devShell = pkgs.mkShell {

src/cli.rs

@@ -20,7 +20,7 @@ use std::path::PathBuf;
 
 use clap::Parser;
 
-#[derive(Parser)]
+#[derive(Parser, Debug)]
 #[clap(
     author,
     version,
@@ -62,4 +62,63 @@ pub struct Args {
         help = "Environment varible sections that will be enabled"
     )]
     pub sections: Vec<String>,
+
+    #[cfg(feature = "color")]
+    #[clap(
+        long,
+        value_enum,
+        default_value = "auto",
+        long_help = "
+This flag controls when to use colors. The default setting is 'auto', which
+means vnetod will try to guess when to use colors. For example, if vnetod is
+printing to a terminal, then it will use colors, but if it is redirected to a
+file or a pipe, then it will suppress color output. vnetod will suppress color
+output in some other circumstances as well. For example, if the TERM
+environment variable is not set or set to 'dumb', then vnetod will not use
+colors.
+
+The possible values for this flag are:
+
+never    Colors will never be used.
+auto     The default. vnetod tries to be smart.
+always   Colors will always be used regardless of where output is sent.
+ansi     Like 'always', but emits ANSI escapes (even in a Windows console).
+
+"
+    )]
+    pub color: ColorVariant,
+}
+
+#[cfg(feature = "color")]
+#[derive(clap::ValueEnum, Clone, Debug)]
+pub enum ColorVariant {
+    Auto,
+    Always,
+    Ansi,
+    Never,
+}
+
+#[cfg(feature = "color")]
+impl From<ColorVariant> for termcolor::ColorChoice {
+    fn from(col: ColorVariant) -> Self {
+        match col {
+            ColorVariant::Never => Self::Never,
+            ColorVariant::Always => Self::Always,
+            ColorVariant::Ansi => Self::AlwaysAnsi,
+            ColorVariant::Auto => {
+                if atty::is(atty::Stream::Stdout) {
+                    // Otherwise let's `termcolor` decide by inspecting the environment. From the [doc]:
+                    // - If `NO_COLOR` is set to any value, then colors will be suppressed.
+                    // - If `TERM` is set to dumb, then colors will be suppressed.
+                    // - In non-Windows environments, if `TERM` is not set, then colors will be suppressed.
+                    //
+                    // [doc]: https://github.com/BurntSushi/termcolor#automatic-color-selection
+                    Self::Auto
+                } else {
+                    // Colors should be deactivated if the terminal is not a tty.
+                    Self::Never
+                }
+            }
+        }
+    }
 }

src/cli/switch.rs

@@ -14,21 +14,24 @@
 //! along with vnetod.  If not, see <https://www.gnu.org/licenses/>.
 //!
 
+#[cfg(feature = "color")]
+use termcolor::{Color, ColorSpec, StandardStream, WriteColor};
+
 use crate::{cli::Args, domain};
 use std::fs::File;
-use std::io::{stdout, Write};
+use std::io::Write;
 
 #[derive(Debug)]
 pub enum Error {
     OpenFile,
-    Switch(domain::switch::Error),
+    WriteFile,
 }
 
 impl std::fmt::Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Error::OpenFile => f.write_str("Cannot open file"),
-            Error::Switch(inner) => write!(f, "Cannot switch between states: {}", inner),
+            Error::WriteFile => f.write_str("Cannot write file"),
         }
     }
 }
@@ -38,19 +41,51 @@ impl std::error::Error for Error {}
 pub fn execute(args: &Args) -> Result<(), Error> {
     let content = std::fs::read_to_string(&args.file).map_err(|_| Error::OpenFile)?;
 
-    let writer: Box<dyn Write> = if args.dry_run {
-        Box::new(stdout())
-    } else {
-        Box::new(
-            File::create(args.output.as_ref().unwrap_or(&args.file))
-                .map_err(|_| Error::OpenFile)?,
-        )
-    };
-
-    domain::switch::execute(domain::switch::Request {
-        content: &content,
-        writer,
-        sections: &args.sections,
-    })
-    .map_err(Error::Switch)
+    if args.dry_run {
+        println!("Your file will be changed to the following")
+    }
+
+    let fs_writer = (!args.dry_run)
+        .then(|| {
+            File::create(args.output.as_ref().unwrap_or(&args.file)).map_err(|_| Error::OpenFile)
+        })
+        .transpose()?;
+
+    #[cfg(feature = "color")]
+    let color = args.color.clone();
+
+    println!();
+
+    let new_content = domain::switch::execute(domain::switch::Request {
+        content: &content.trim(),
+        sections: &args.sections,
+        on_line: Some(Box::new(move |line| {
+            #[cfg(feature = "color")]
+            print_line(line, color.clone());
+            #[cfg(not(feature = "color"))]
+            print!("{}", line)
+        })),
+    });
+
+    println!();
+
+    if let Some(mut fs_writer) = fs_writer {
+        fs_writer
+            .write_all(new_content.as_bytes())
+            .map_err(|_| Error::WriteFile)?;
+    }
+
+    Ok(())
+}
+
+#[cfg(feature = "color")]
+fn print_line(line: &String, color: crate::cli::ColorVariant) {
+    let mut stdout = StandardStream::stdout(color.into());
+    let color = line
+        .starts_with("###")
+        .then_some(Color::Yellow)
+        .or_else(|| (!line.starts_with("#")).then_some(Color::Green));
+    stdout.set_color(ColorSpec::new().set_fg(color)).ok();
+    write!(&mut stdout, "{}", line).unwrap();
+    stdout.reset().ok();
 }

src/domain.rs

@@ -44,7 +44,7 @@ impl Section {
     }
 }
 
-struct SectionInfo {
+pub struct SectionInfo {
     enable_variable: bool,
     disable_variable: bool,
 }

src/domain/switch.rs

@@ -14,54 +14,32 @@
 //! along with vnetod.  If not, see <https://www.gnu.org/licenses/>.
 //!
 
-use std::io::{BufWriter, Write};
-
 use super::{Section, SectionInfo};
 
-#[derive(Debug)]
-pub enum Error {
-    WriteData,
-}
+pub type OnLineFn = Box<dyn Fn(&String)>;
 
-impl std::fmt::Display for Error {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            Error::WriteData => f.write_str("Cannot write data"),
-        }
-    }
-}
-
-impl std::error::Error for Error {}
-
-pub struct Request<'args, W>
-where
-    W: Write,
-{
+pub struct Request<'args> {
     pub content: &'args str,
-    pub writer: W,
     pub sections: &'args [String],
+    pub on_line: Option<OnLineFn>,
 }
 
-pub fn execute<W>(req: Request<W>) -> Result<(), Error>
-where
-    W: Write,
-{
-    let mut writer = BufWriter::new(req.writer);
-
+pub fn execute(req: Request) -> String {
     let choose_sections = req
         .sections
         .iter()
         .map(|s| Section::parse(s.as_str()))
         .collect::<Vec<_>>();
 
-    let mut current_sections: Option<SectionInfo> = None;
+    let mut current_section: Option<SectionInfo> = None;
+    let mut new_content = String::new();
 
     for line in req.content.split_inclusive('\n') {
         let new_line = if is_section_end(line) {
-            current_sections = None;
+            current_section = None;
             line.to_string()
         } else if let Some(section_info) = line.strip_prefix("### ") {
-            current_sections = section_info.split_whitespace().next().map(|r| {
+            current_section = section_info.split_whitespace().next().map(|r| {
                 let current_sections = r.split(',').map(Section::parse).collect::<Vec<_>>();
                 SectionInfo {
                     enable_variable: should_enable_variable(&choose_sections, &current_sections),
@@ -69,7 +47,7 @@ where
                 }
             });
             line.to_string()
-        } else if let Some(section_info) = current_sections.as_ref() {
+        } else if let Some(section_info) = current_section.as_ref() {
             let trimmed_line = line.trim_start_matches(['#', ' ']);
             let is_var = is_variable(trimmed_line);
             if is_var && section_info.enable_variable {
@@ -83,12 +61,13 @@ where
             line.to_string()
         };
 
-        writer
-            .write_all(new_line.as_bytes())
-            .map_err(|_| Error::WriteData)?;
+        new_content.push_str(&new_line);
+        if let Some(on_line) = req.on_line.as_ref() {
+            on_line(&new_line);
+        }
     }
 
-    writer.flush().map_err(|_| Error::WriteData)
+    new_content
 }
 
 fn is_variable(trimmed_line: &str) -> bool {
@@ -144,29 +123,20 @@ fn should_disable_variable(choose_sections: &[Section], current_sections: &[Sect
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::io::Cursor;
 
     const BASE_ENV: &str = include_str!("../../test_data/base_env");
 
     fn make_test(input: &str, expected_output: &str, sections: Vec<&str>) {
-        let mut output_data = vec![];
-        let writer = Cursor::new(&mut output_data);
-
-        match execute(Request {
+        let output_data = execute(Request {
             content: input,
-            writer,
             sections: &sections.into_iter().map(String::from).collect::<Vec<_>>(),
-        }) {
-            Ok(()) => {
-                let output = String::from_utf8(output_data).unwrap();
+            on_line: None,
+        });
         assert_eq!(
-                    output.lines().collect::<Vec<_>>(),
+            output_data.lines().collect::<Vec<_>>(),
             expected_output.lines().collect::<Vec<_>>()
         );
     }
-            _ => unreachable!(),
-        }
-    }
 
     #[test]
     fn should_disable_all_sections() {