02Credits Blog

Day3 - Okeydokey Cont.

Rust Command Line Tools

2019-02-09

Project Page

Continuing on from yesterday, I built a feature equivalent version of my lost directory profile tool in rust. What follows were the steps and tools I used in the process.

Previous Version Structure

The decompiled source we got from yesterday gave us the basic structure of the original version. The tool has two main executable paths. Either it is run without any arguments, or it is passed the prefix of a command to run.

In the first case, it searches for the .ok profile and lists the command names to the console.

  > ScriptProfileManager
use
build
clean

In the second case it finds the longest match in the profile commands for the passed in prefix and prints the associated command to the console.

  > ScriptProfileManager b
cargo build

The original version of the tool assumed that the output list of commands would be post-processed by some form of pretty printer. I think this puts too much burden on the script side of things, so my new version will print the command list in one line.

Similarly the old version assumed that the script environment contained a pushd and popd command. In the new version I am relaxing that requirement by adding prefix and suffix arguments. They take a string which can optionally have a {} hole which will be filled with the directory path to the .ok file. This lets the user of okeydokey specify how they would like to wrap the command.

Clap

I decided to use the popular Clap crate for command line argument parsing. The library allows to declaritively describe the command line interface and then query the parsed arguments in a simple way. Clap gives you many options for ways to describe the interface, but he one I picked was a yaml file which gets parsed at compile time. This lets me separate the interface out of the rust code and move on with the actual command logic.

The yaml file was pretty simple, but I did have to do some guesswork in order to get everything right. For example it wasn't clear what value_name did. Eventually I landed on this structure:

  name: Okeydokey
version: "0.1"
author: Keith Simmons
about: .ok file manager
args:
  - COMMAND:
      help: The command in the profile to run
  - prefix:
      short: p
      long: prefix
      value_name: PREFIX
      help: Prepends argument to the returned command replacing {} with the full path to the found .ok file.
  - suffix:
      short: s
      long: suffix
      value_name: SUFFIX
      help: Appends argument to the returned command replacing {} with the full path to the found .ok file.

Then in the main function I was able to build up the matches and query them like so:

  let yaml = load_yaml!("cli.yml");
let matches = App::from_yaml(yaml).get_matches();
println!("{}", matches.value_of("COMMAND"));

Walking Up the Directory Tree

Both of the execution paths of the tool require a parsed .ok file. To find it the tool needs to walk up the directory tree searching for a parent directory which contains the .ok file. Rust has safe and complete file system apis, but as with most things in the Rust standard library, it does some gymnastics to make sure everything is above board with regards to memory safety. Similar to the relationship between &str and String, Rust has Path and PathBuf where Path is an immutable filesystem path and PathBuf is an owned mutable variant. The confusing part is that PathBuf implements the trait Deref to Path which as far as I can understand it means that the compiler is allowed to dereference the PathBuf implicitly. So any place a function can take a Path you can also pass in a PathBuf and things should work out. For example, although PathBuf does not contain a parent function directly, you can still call parent on it since it gets dereferenced into a Path which does.

On top of being immutable, str and Path are unsized types meaning that you can't store variables to them without a pointer or similar without the compiler yelling at you. Given my C# background, I'm still a little fuzzy about the idiomatic way to use these types, but in practice I have found that using the PathBuf and String versions of the types is maybe not the most efficient method, but gets the job done, allows you the most flexibility, and keeps our friend the compiler happy.

Frustratingly, there is a fair amount of syntactic overhead for ensuring that you are using the correct type. The previously mentioned parent function does not return a PathBuf but instead the more frustrating Path, so whenever I use the function, I needed to call to_path_buf just adding to the visual noise. I get that it is for my own good, but its an example of the Rust making easy things harder.

After stumbling through understanding the above, the actual task of finding the .ok file and parsing it into command name, command pairs was pretty trivial.

  fn find_profile(current_path: PathBuf) -> Option<Profile> {
    let possible_profile = current_path.join(".ok");
    if possible_profile.exists() {
        Some(read_profile(possible_profile)?)
    } else {
        Some(find_profile(current_path.parent()?.to_path_buf())?)
    }
}
fn read_profile(profile_path: PathBuf) -> Option<Profile> {
    match File::open(profile_path.clone()) {
        Ok(ref mut file) => {
            let mut commands = HashMap::new();
            for line in BufReader::new(file).lines() {
                let (name, command) = split_on_colon(line.unwrap())?;
                commands.insert(name, command);
            }
            Some(Profile { commands, path: profile_path })
        },
        Err(_) => None
    }
}
fn split_on_colon(line: String) -> Option<(String, String)> {
    let mut splitter = line.splitn(2, ':');
    let name = splitter.next()?;
    let command = splitter.next()?;
    Some((name.to_string(), command.to_string()))
}

String Manipulation

The argumentless version of okeydokey which prints the commands to the console was deceptively difficult to get right. The problem stems from the fact that I store the commands in a HashMap<String, String> where the key is the command name and the value is the command. The naive solution would be to pull the keys out into a collection, and use the String utilities to join them into a single String. In practice though, we run into ownership problems. HashMap.keys returns an iterator of &String, not String. This prevents us from using join which must operate on values not references. Eventually I landed on a call to fold passing in String accumulator but it wasn't my first choice.

The query execution path went relatively smoothly after the above. The query function finds the best match in the commands list using filter_map and max_by_key and then passes the found command as well as the passed in prefix and suffix options on to get formatted into the final output.

  fn list(profile: Profile) {
    let list = profile.commands
        .keys()
        .fold(String::new(), |acc, next| {
            acc + " " + next
        });
    println!("{}", list.trim());
}
fn query(profile: Profile, command: &str, prefix: Option<&str>, suffix: Option<&str>) {
    let best_option = profile
        .commands
        .keys()
        .filter_map(|possible_command| shared_prefix(possible_command, command))
        .max_by_key(|&(shared_chars, _)| shared_chars);
    match best_option {
        Some((_, actual_command)) => print_decorated_command(profile, actual_command, prefix, suffix),
        None => ()
    }
}
fn shared_prefix(possible_command: &str, command: &str) -> Option<(usize, String)> {
    match possible_command.starts_with(command) {
        true => Some((command.len(), possible_command.to_string())),
        false => None
    }
}
fn print_decorated_command(profile: Profile, command_name: String, prefix: Option<&str>, suffix: Option<&str>) {
    let prefix = fill_in_profile_directory(&profile, prefix);
    let suffix = fill_in_profile_directory(&profile, suffix);
    let command = profile.commands.get(&command_name).unwrap();
    println!("{}", vec![prefix, command.to_string(), suffix].concat())
}
fn fill_in_profile_directory(profile: &Profile, pattern: Option<&str>) -> String {
    let profile_directory = profile.path.parent().unwrap().to_str().unwrap();
    pattern.unwrap_or_default().replace("{}", profile_directory)
}

The Script Wrapper

At this stage, the tool is feature complete for the first version, but it requires a couple of modifications to my script profile to work properly. I have a function defined in my PowerShell profile called ok which gets called in my prompt function so that ever time I change directories the command list is printed if it exists. If the function is given any arguments, they are fed into the profile utility and the output is executed directly.

Previously, I did special formatting of the command list in PowerShell, which has since been pushed to the tool side. Similarly the command execution assumed that the directory management was handled in the tool, but I have pushed that to the command arguments instead to be more flexible. So the execution of okeydokey needed to be modified as well. I landed on this:

  function ok
{
  Param($command = $null)
  if ($command -eq $null) {
    $fore = $host.UI.RawUI.ForegroundColor
    $host.UI.RawUI.ForegroundColor = 'Blue'
    okeydokey
    $host.UI.RawUI.ForegroundColor = $fore
  } else {
    $script = okeydokey $command -p "pushd {};" -s "; popd"
    if ($script -ne $null) {
      iex $script
    }
  }
}

Simple and straight forward.

Summary

I have pushed the current version of the tool to github so anyone can use it if they like. Depending on how much I've got left in me (this took many hours again... I need to dial this back if I want to do it every day), I may build a simple home page for the tool describing its usage shortly. I've used a version of this tool pretty much daily for the past 6 months, and I have some ideas for how to make it even better. Among them is argument support, profile references, and a better UI. For now though, I will probably do some site styling and call it a day.

Till tomorrow,
Keith