From 08cd1a61d34106e36862b64d478c997775c2e4f1 Mon Sep 17 00:00:00 2001
From: Daniel Lewan <vision360.daniel@gmail.com>
Date: Fri, 12 Nov 2021 12:54:01 +0100
Subject: [PATCH] add git like subcommand deferring mechanism

This commit implements a simple mechanism of deferring unknown
subcommands to other executables in $PATH.

When users executes `kks foo` kks will try to run `kks-foo` command.
---
 README.md       |  6 ++++++
 cmd/external.go | 27 +++++++++++++++++++++++++++
 cmd/root.go     |  5 ++++-
 main.go         |  8 +++++++-
 4 files changed, 44 insertions(+), 2 deletions(-)
 create mode 100644 cmd/external.go

diff --git a/README.md b/README.md
index ff2dea3..a43a1a3 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,12 @@ ENVIRONMENT VARIABLES
 Use "kks <command> -h" for command usage.
 ```
 
+### Unknown command
+
+When unknown command is run, `kks` will try to find an executable named
+`kks-<command>` in `$PATH`. If the executable is found, `kks` will run it
+with all arguments that were provided to the unknown command.
+ 
 ## Configuration
 
 `kks` can be configured through environment variables.
diff --git a/cmd/external.go b/cmd/external.go
new file mode 100644
index 0000000..237a2e1
--- /dev/null
+++ b/cmd/external.go
@@ -0,0 +1,27 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"syscall"
+)
+
+func External(args []string, original error) error {
+	if len(args) == 0 {
+		return original
+	}
+
+	thisExecutable := filepath.Base(os.Args[0])
+	path, err := exec.LookPath(fmt.Sprintf("%s-%s", thisExecutable, args[0]))
+	if err != nil {
+		// no such executable - return original error
+		return original
+	}
+	if len(args) < 1 {
+		args = args[1:]
+	}
+
+	return syscall.Exec(path, args, os.Environ())
+}
diff --git a/cmd/root.go b/cmd/root.go
index b04146a..2c77bf4 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -2,6 +2,7 @@ package cmd
 
 import (
 	_ "embed"
+	"errors"
 	"fmt"
 	"os"
 )
@@ -9,6 +10,8 @@ import (
 //go:embed embed/help
 var helpTxt string
 
+var UnknownSubcommand = errors.New("unknown subcommand")
+
 func Root(args []string) error {
 	if len(args) < 1 || args[0] == "-h" || args[0] == "--help" {
 		printHelp()
@@ -39,7 +42,7 @@ func Root(args []string) error {
 		}
 	}
 
-	return fmt.Errorf("unknown subcommand: %s", subcommand)
+	return fmt.Errorf("can't run %s: %w", subcommand, UnknownSubcommand)
 }
 
 func containsString(s []string, e string) bool {
diff --git a/main.go b/main.go
index 55df9b7..a7e26c9 100644
--- a/main.go
+++ b/main.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -18,7 +19,12 @@ func main() {
 		os.Exit(0)
 	}
 
-	if err := cmd.Root(os.Args[1:]); err != nil {
+	err := cmd.Root(os.Args[1:])
+
+	if err != nil && errors.Is(err, cmd.UnknownSubcommand) {
+		err = cmd.External(os.Args[1:], err)
+	}
+	if err != nil {
 		log.Fatal(err)
 	}
 }