util: Add a helper to get the users shell entry

util/shell.go

@@ -0,0 +1,120 @@
+// Mgmt
+// Copyright (C) James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package util
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+)
+
+const (
+	// EtcPasswdFile is the location of the /etc/passwd file.
+	EtcPasswdFile = "/etc/passwd"
+
+	// ErrUsernameNotFound means we couldn't find that username.
+	ErrUsernameNotFound = Error("can't find username")
+)
+
+// UserShell returns shell of the user.
+// TODO: return a well-known error if the user is simply not found.
+func UserShell(ctx context.Context, username string) (string, error) {
+	// TODO: use getpwnam_r ?
+
+	// First see if we can use `getent` to get the passwd database.
+	if shell, err := getentShell(ctx, username); err == nil {
+		return shell, nil
+	}
+
+	// We can always look in /etc/passwd manually.
+	shell, err := passwdShell(ctx, username)
+	if err != nil {
+		return "", err
+	}
+	return shell, nil
+}
+
+// getentShell gets a user shell by running the `getent` command.
+func getentShell(ctx context.Context, username string) (string, error) {
+	if username == "" {
+		return "", fmt.Errorf("empty username")
+	}
+	getent, err := exec.LookPath("getent")
+	if err != nil {
+		return "", err
+	}
+	args := []string{
+		"passwd",
+		username,
+	}
+	cmd := exec.CommandContext(ctx, getent, args...)
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setpgid: true,
+		Pgid:    0,
+	}
+
+	// XXX: return ErrUsernameNotFound where appropriate
+	output, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	fields := strings.SplitN(strings.TrimSuffix(string(output), "\n"), ":", 7)
+
+	if len(fields) != 7 {
+		return "", err
+	}
+	return fields[6], nil
+}
+
+// passwdShell gets a user shell by looking through the `/etc/passwd` file.
+func passwdShell(ctx context.Context, username string) (string, error) {
+	f, err := os.Open(EtcPasswdFile)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	s := bufio.NewScanner(f)
+	for s.Scan() {
+		fields := strings.SplitN(strings.TrimSuffix(s.Text(), "\n"), ":", 7)
+		if len(fields) == 7 && fields[0] == username {
+			return fields[6], nil
+		}
+	}
+	if err := s.Err(); err != nil {
+		return "", err
+	}
+
+	return "", ErrUsernameNotFound
+}