devbox shell -- cmd (#158)

## Summary
Simple implementation of `devbox shell -- <cmd>`. I struggled
integrating this with all the other stuff that is happening with `devbox
shell`. Would like thoughts on whether this is a good enough for an MVP
of the feature, and later we improve it, or whether we think integrating
with the other `devbox shell` logic is a must have.

## How was it tested?
Ran a few commands with `devbox shell --`

Author: loreto

boxcli/shell.go

@@ -15,17 +15,17 @@ import (
 
 func ShellCmd() *cobra.Command {
 	command := &cobra.Command{
-		Use:               "shell [<dir>]",
+		Use:               "shell [<dir>] -- [<cmd>]",
 		Short:             "Start a new shell with access to your packages",
-		Args:              cobra.MaximumNArgs(1),
+		Args:              validateShellArgs,
 		PersistentPreRunE: nixShellPersistentPreRunE,
 		RunE:              runShellCmd,
 	}
 	return command
 }
 
 func runShellCmd(cmd *cobra.Command, args []string) error {
-	path := pathArg(args)
+	path, cmds := parseShellArgs(cmd, args)
 
 	// Check the directory exists.
 	box, err := devbox.Open(path)
@@ -40,12 +40,12 @@ func runShellCmd(cmd *cobra.Command, args []string) error {
 
 	fmt.Println("Installing nix packages. This may take a while...")
 
-	err = box.Shell()
-	var exitErr *exec.ExitError
-	if errors.As(err, &exitErr) {
-		cmd.SilenceErrors = true
-		cmd.SilenceUsage = true
+	if len(cmds) > 0 {
+		err = box.Exec(cmds...)
+	} else {
+		err = box.Shell()
 	}
+
 	return err
 }
 
@@ -56,3 +56,22 @@ func nixShellPersistentPreRunE(cmd *cobra.Command, args []string) error {
 	}
 	return nil
 }
+
+func validateShellArgs(cmd *cobra.Command, args []string) error {
+	lenAtDash := cmd.ArgsLenAtDash()
+	if lenAtDash > 1 {
+		return fmt.Errorf("accepts at most 1 directory, received %d", lenAtDash)
+	}
+	return nil
+}
+
+func parseShellArgs(cmd *cobra.Command, args []string) (string, []string) {
+	index := cmd.ArgsLenAtDash()
+	if index < 0 {
+		index = 0
+	}
+
+	path := pathArg(args[:index])
+	cmds := args[index:]
+	return path, cmds
+}

devbox.go

@@ -149,6 +149,22 @@ func (d *Devbox) Shell() error {
 	return sh.Run(nixDir)
 }
 
+func (d *Devbox) Exec(cmds ...string) error {
+	plan, err := d.Plan()
+	if err != nil {
+		return errors.WithStack(err)
+	}
+	if plan.Invalid() {
+		return plan.Error()
+	}
+	err = generate(d.srcDir, plan, shellFiles)
+	if err != nil {
+		return errors.WithStack(err)
+	}
+	nixDir := filepath.Join(d.srcDir, ".devbox/gen/shell.nix")
+	return nix.Exec(nixDir, cmds)
+}
+
 // saveCfg writes the config file to the devbox directory.
 func (d *Devbox) saveCfg() error {
 	cfgPath := filepath.Join(d.srcDir, configFilename)

nix/nix.go

@@ -7,7 +7,11 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"os"
 	"os/exec"
+	"strings"
+
+	"github.com/pkg/errors"
 )
 
 func PkgExists(pkg string) bool {
@@ -22,6 +26,15 @@ type Info struct {
 	System  string
 }
 
+func Exec(path string, command []string) error {
+	runCmd := strings.Join(command, " ")
+	cmd := exec.Command("nix-shell", path, "--run", runCmd)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return errors.WithStack(cmd.Run())
+}
+
 func PkgInfo(pkg string) (*Info, bool) {
 	buf := new(bytes.Buffer)
 	attr := fmt.Sprintf("nixpkgs.%s", pkg)