diff --git a/download/download.go b/download/download.go
index 3d7edbf..d9d9c09 100644
--- a/download/download.go
+++ b/download/download.go
@@ -227,6 +227,11 @@ func extractFile(ctx context.Context, input io.Reader, hash hash.Hash, format ar
 				return err
 			}
 			defer fr.Close()
+			fi, err := f.Stat()
+			if err != nil {
+				return err
+			}
+			fm := fi.Mode()
 
 			path := filepath.Join(opts.Destination, f.NameInArchive)
 
@@ -241,7 +246,7 @@ func extractFile(ctx context.Context, input io.Reader, hash hash.Hash, format ar
 					return err
 				}
 			} else {
-				outFl, err := os.Create(path)
+				outFl, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, fm.Perm())
 				if err != nil {
 					return err
 				}
diff --git a/helpers.go b/helpers.go
index 02a6f6a..13c8f7d 100644
--- a/helpers.go
+++ b/helpers.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -10,6 +11,8 @@ import (
 	"mvdan.cc/sh/v3/interp"
 )
 
+var ErrNoPipe = errors.New("command requires data to be piped in")
+
 var helpers = shutils.ExecFuncs{
 	"install-binary":       installHelperCmd("/usr/bin", 0o755),
 	"install-systemd-user": installHelperCmd("/usr/lib/systemd/user", 0o644),
@@ -18,6 +21,7 @@ var helpers = shutils.ExecFuncs{
 	"install-license":      installHelperCmd("/usr/share/licenses", 0o644),
 	"install-manual":       installHelperCmd("/usr/share/man/man1", 0o644),
 	"install-desktop":      installHelperCmd("/usr/share/applications", 0o644),
+	"install-completion":   installCompletionCmd,
 }
 
 func installHelperCmd(prefix string, perms os.FileMode) shutils.ExecFunc {
@@ -42,6 +46,49 @@ func installHelperCmd(prefix string, perms os.FileMode) shutils.ExecFunc {
 	}
 }
 
+func installCompletionCmd(hc interp.HandlerContext, cmd string, args []string) error {
+	// If the command's stdin is the same as the system's,
+	// that means nothing was piped in. In this case, return an error.
+	if hc.Stdin == os.Stdin {
+		return fmt.Errorf("install-completion: %w", ErrNoPipe)
+	}
+
+	if len(args) < 2 {
+		return shutils.InsufficientArgsError(cmd, 2, len(args))
+	}
+
+	shell := args[0]
+	name := args[1]
+
+	var prefix string
+	switch shell {
+	case "bash":
+		prefix = "/usr/share/bash-completion/completion"
+	case "zsh":
+		prefix = "/usr/share/zsh/site-functions"
+		name = "_" + name
+	case "fish":
+		prefix = "/usr/share/fish/vendor_completions.d"
+		name += ".fish"
+	}
+
+	path := filepath.Join(hc.Env.Get("pkgdir").Str, prefix, name)
+
+	err := os.MkdirAll(filepath.Dir(path), 0o755)
+	if err != nil {
+		return err
+	}
+
+	dst, err := os.OpenFile(path, os.O_TRUNC|os.O_CREATE|os.O_RDWR, 0o644)
+	if err != nil {
+		return err
+	}
+	defer dst.Close()
+
+	_, err = io.Copy(dst, hc.Stdin)
+	return err
+}
+
 func helperInstall(from, to string, perms os.FileMode) error {
 	err := os.MkdirAll(filepath.Dir(to), 0o755)
 	if err != nil {