devshell enter: use shell.nix if it exists

Allow to fallback on shell.nix to allow user overrides.

Author: zimbatm

go/devshell/cmd_enter.go

@@ -4,21 +4,16 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/urfave/cli/v2"
 )
 
-const enterNix = `
+const shellNix = `
 let
 	pkgs = import <nixpkgs> {};
-	mkDevShell =
-		if pkgs ? mkDevShell then
-		pkgs.mkDevShell
-		else
-		pkgs.callPackage (fetchTarball
-		"https://github.com/numtide/devshell/archive/master.tar.gz") {}
-		;
+	mkDevShell = pkgs.callPackage <devshell> {};
 in
 mkDevShell.fromTOML ./devshell.toml
 `
@@ -27,22 +22,48 @@ var cmdEnter = &cli.Command{
 	Name:    "enter",
 	Aliases: []string{"e"},
 	Usage:   "builds and enters the shell",
+	Flags: []cli.Flag{
+		&cli.StringFlag{
+			Name:  "path",
+			Usage: "path to the project root",
+			Value: ".",
+		},
+		&cli.StringSliceFlag{
+			Name:  "I",
+			Usage: "Add a path to the NIX_PATH",
+			Value: cli.NewStringSlice("devshell=https://github.com/numtide/devshell/archive/master.tar.gz"),
+		},
+	},
 	Action: func(c *cli.Context) error {
-		// instantiate eval
-		out, err := exec.Command("nix-instantiate", "--expr", enterNix).Output()
+		path := c.String("path")
+		paths := c.StringSlice("I")
+		shellFile := filepath.Join(path, "shell.nix")
+
+		args := []string{"-v"}
+		exists, err := fileExists(shellFile)
 		if err != nil {
 			return err
 		}
+		if !exists {
+			args = append(args, "--expr", shellNix)
+		}
+		for _, p := range paths {
+			args = append(args, "-I", p)
+		}
+
+		// instantiate eval
+		out, err := exec.Command("nix-instantiate", args...).Output()
+		if err != nil {
+			return fmt.Errorf("nix-instantiate: %w", err)
+		}
 		drvPath := strings.TrimSpace(string(out))
 		drvPath = strings.Trim(drvPath, "\"")
-		fmt.Println("drvPath", drvPath)
 		// realize
 		out, err = exec.Command("nix-store", "--realize", drvPath).Output()
 		if err != nil {
-			return err
+			return fmt.Errorf("nix-store: %w", err)
 		}
 		outPath := strings.TrimSpace(string(out))
-		fmt.Println("outPath", outPath)
 		// execute
 		cmd := exec.Command(outPath, c.Args().Slice()...)
 		cmd.Stdin = os.Stdin

go/devshell/path_utils.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"os"
+)
+
+func fileExists(path string) (bool, error) {
+	_, err := os.Stat(path)
+	if err == nil {
+		return true, nil
+	}
+	if os.IsNotExist(err) {
+		return false, nil
+	}
+	return false, err
+}