Revise command line interface

Author: ngrash

README.md

@@ -1,27 +1,41 @@
 # twshim
 
-twshim is a transparent Go wrapper around the standalone TailwindCSS CLI. 
-The goal of this project is to unify how developers add TailwindCSS to their Go project by taking care of downloading the executable for the current architecture.
+twshim is a transparent Go wrapper around the standalone Tailwind CSS CLI. 
+The goal of this project is to unify how developers add Tailwind CSS to their Go project by taking care of downloading the executable for the current architecture.
+
+## Example
+
+```
+twshim -release v3.2.4 -downloads $HOME/.twshim -- -i in.css -o out.css --minify
+```
 
 ## Configuration
 
-Because all parameters are passed to the TailwindCSS executable, twshim itself is configured through environment variables.
+twshim can be configured through environment variables or command line arguments:
+
+```
+  -downloads string
+        Target directory for executables (override TWSHIM_DOWNLOADS)
+  -release string
+        Tag of the desired release (overrides TWSHIM_RELEASE)
+```
+
+A double dash (--) is required before the Tailwind CSS parameters to distinguish twshim configuration from Tailwind arguments.
 
-* `TWTAG` is the tag of the desired TailwindCSS release, e.g. `v3.2.4`.
-* `TWROOT` is the directory for downloaded TailwindCSS executables, e.g. `$HOME/.twshim/downloads`.
- 
 twshim uses `runtime.GOOS` and `runtime.GOARCH` to decide which executable to download.
 
+See https://github.com/tailwindlabs/tailwindcss/releases for a list of Tailwind releases.
+
 ## Usage
 
-You can use `go run` to invoke twshim if you want to quickly execute a specific version of TailwindCSS CLI.
+You can use `go run` to invoke twshim if you want to quickly execute a specific version of Tailwind CSS CLI.
 
 ```shell
-TWTAG=v3.2.4 TWROOT=$HOME/.twshim/downloads go run github.com/ngrash/twshim/cmd/twshim@v0.2.0
+go run github.com/ngrash/twshim/cmd/twshim@v0.3.0 -release v3.2.4 -downloads $HOME/.twshim/downloads -- --help
 ```
 
 You can also use `go get` to add twshim to your application and use the `twshim` package from your code.
 
 ```shell
-go get github.com/ngrash/twshim@v0.2.0
+go get github.com/ngrash/twshim@v0.3.0
 ```

cmd/twshim/main.go

@@ -1,11 +1,28 @@
 package main
 
 import (
+	"flag"
+	"fmt"
 	"github.com/ngrash/twshim"
 	"log"
 	"os"
 )
 
+var (
+	releaseFlag   = flag.String("release", "", "Tag of the desired release (overrides TWSHIM_RELEASE)")
+	downloadsFlag = flag.String("downloads", "", "Target directory for executables (override TWSHIM_DOWNLOADS)")
+)
+
+func valueOrEnv(value, env string) (string, bool) {
+	if value != "" {
+		return value, true
+	}
+	if v, ok := os.LookupEnv(env); ok {
+		return v, true
+	}
+	return "", false
+}
+
 func main() {
 	l := log.New(os.Stderr, "twshim: ", 0)
 	twshim.Log = l
@@ -16,21 +33,37 @@ func main() {
 		l.Fatal(err)
 	}
 
-	tag, found := os.LookupEnv("TWTAG")
-	if !found {
-		l.Fatal("Set TWTAG to select a release.\n\nSee https://github.com/tailwindlabs/tailwindcss/releases for available releases.")
+	u := flag.Usage
+	//goland:noinspection GoUnhandledErrorResult
+	flag.Usage = func() {
+		u()
+		w := flag.CommandLine.Output()
+		fmt.Fprintln(w, "See https://github.com/tailwindlabs/tailwindcss/releases for a list of releases.")
+		fmt.Fprintln(w, "Use a double dash (--) to pass parameters to Tailwind CSS CLI.")
+		fmt.Fprintln(w, "Example:")
+		fmt.Fprintln(w, "  twshim -release v3.2.4 -downloads /tmp/tw -- -i in.css -o out.css --minify")
+		fmt.Fprintln(w)
+	}
+	flag.Parse()
+
+	release, ok := valueOrEnv(*releaseFlag, "TWSHIM_RELEASE")
+	if !ok {
+		flag.CommandLine.SetOutput(os.Stdout) // write usage to stdout to highlight error
+		flag.Usage()
+		l.Fatal("Insufficient parameters: Please specify release.")
 	}
 
-	root, found := os.LookupEnv("TWROOT")
-	if !found {
-		l.Fatal("Set TWROOT to configure a download directory.")
+	downloads, ok := valueOrEnv(*downloadsFlag, "TWSHIM_DOWNLOADS")
+	if !ok {
+		flag.CommandLine.SetOutput(os.Stdout) // write usage to stdout to highlight error
+		flag.Usage()
+		l.Fatal("Insufficient parameters: Please specify downloads directory.")
 	}
 
-	cmd, err := twshim.Command(root, tag, assetName, os.Args[1:]...)
+	cmd, err := twshim.Command(downloads, release, assetName, flag.Args()...)
 	if err != nil {
 		l.Fatal(err)
 	}
-
 	if err := cmd.Run(); err != nil {
 		l.Fatal(err)
 	}