Go code to get the shell of the current user

tools/utils/passwd.go

@@ -0,0 +1,171 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"os/user"
+	"runtime"
+	"strconv"
+	"strings"
+
+	"howett.net/plist"
+)
+
+var _ = fmt.Print
+
+type PasswdEntry struct {
+	Username, Pass, Uid, Gid, Gecos, Home, Shell string
+}
+
+func ParsePasswdLine(line string) (PasswdEntry, error) {
+	parts := strings.Split(line, ":")
+	if len(parts) == 7 {
+		return PasswdEntry{parts[0], parts[1], parts[2], parts[3], parts[4], parts[5], parts[6]}, nil
+	}
+	return PasswdEntry{}, fmt.Errorf("passwd line has %d colon delimited fields instead of 7", len(parts))
+}
+
+func ParsePasswdDatabase(raw string) (ans map[string]PasswdEntry) {
+	scanner := NewLineScanner(raw)
+	ans = make(map[string]PasswdEntry)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if entry, e := ParsePasswdLine(line); e == nil {
+			ans[entry.Uid] = entry
+		}
+	}
+	return ans
+}
+
+func ParsePasswdFile(path string) (ans map[string]PasswdEntry, err error) {
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return ParsePasswdDatabase(UnsafeBytesToString(raw)), nil
+}
+
+var passwd_err error
+var passwd_database = Once(func() (ans map[string]PasswdEntry) {
+	ans, passwd_err = ParsePasswdFile("/etc/passwd")
+	return
+})
+
+func PwdEntryForUid(uid string) (ans PasswdEntry, err error) {
+	pwd := passwd_database()
+	if passwd_err != nil {
+		return ans, passwd_err
+	}
+	ans, found := pwd[uid]
+	if !found {
+		return ans, fmt.Errorf("No user matching the UID: %#v found", uid)
+	}
+	return ans, nil
+}
+
+func parse_dscl_data(raw []byte) (ans map[string]PasswdEntry, err error) {
+	var pd []any
+	_, err = plist.Unmarshal(raw, &pd)
+	if err != nil {
+		return
+	}
+	ans = make(map[string]PasswdEntry, 256)
+	for _, entry := range pd {
+		if e, ok := entry.(map[string]any); ok {
+			item := PasswdEntry{}
+			for key, a := range e {
+				array, ok := a.([]any)
+				if !ok || len(array) == 0 || !strings.HasPrefix(key, "dsAttrTypeNative:") {
+					continue
+				}
+				_, key, _ = strings.Cut(key, ":")
+				if val, ok := array[0].(string); ok {
+					switch key {
+					case "uid":
+						item.Uid = val
+					case "gid":
+						item.Gid = val
+					case "home":
+						item.Home = val
+					case "name":
+						item.Username = val
+					case "realname":
+						item.Gecos = val
+					case "shell":
+						item.Shell = val
+					}
+				}
+			}
+			ans[item.Uid] = item
+		}
+	}
+	return
+}
+
+var dscl_error error
+
+var dscl_user_database = Once(func() map[string]PasswdEntry {
+	c := exec.Command("/usr/bin/dscl", "-plist", ".", "-readall", "/Users", "uid", "gid", "name", "realname", "home", "shell")
+	raw, err := c.Output()
+	if err != nil {
+		dscl_error = err
+		return nil
+	}
+	ans, err := parse_dscl_data(raw)
+	if err != nil {
+		dscl_error = err
+		return nil
+	}
+	return ans
+
+})
+
+func LoginShellForUser(u *user.User) (ans string, err error) {
+	var db map[string]PasswdEntry
+	switch runtime.GOOS {
+	case "darwin":
+		db = dscl_user_database()
+		err = dscl_error
+	default:
+		db = passwd_database()
+		err = passwd_err
+	}
+	if err != nil {
+		return
+	}
+	if rec, found := db[u.Uid]; found {
+		return rec.Shell, nil
+	}
+	return ans, fmt.Errorf("No user record available for user with UID: %#v", u.Uid)
+}
+
+func CurrentUser() (ans *user.User, err error) {
+	ans, err = user.Current()
+	if err != nil && runtime.GOOS == "darwin" {
+		uid := strconv.Itoa(os.Geteuid())
+		db := dscl_user_database()
+		if dscl_error != nil {
+			err = dscl_error
+			return
+		}
+		if rec, found := db[uid]; found {
+			u := user.User{Uid: uid, Gid: rec.Gid, Username: rec.Username, Name: rec.Gecos, HomeDir: rec.Home}
+			ans = &u
+			err = nil
+		} else {
+			err = fmt.Errorf("Could not find the current uid: %d in the DSCL user database", os.Geteuid())
+		}
+	}
+	return
+}
+
+func LoginShellForCurrentUser() (ans string, err error) {
+	u, err := CurrentUser()
+	if err != nil {
+		return ans, err
+	}
+	return LoginShellForUser(u)
+}