Initial Commit

loginshell/loginshell.go

@@ -0,0 +1,69 @@
+package loginshell
+
+import (
+	"bufio"
+	"errors"
+	"io/fs"
+	"os"
+	"strconv"
+	"strings"
+)
+
+var (
+	// ErrInvalidPasswd is returned when the passwd file is invalid and can't be parsed
+	ErrInvalidPasswd = errors.New("loginshell: invalid passwd file")
+
+	// ErrNoSuchUser is returned when a given user isn't in the passwd file
+	ErrNoSuchUser = errors.New("loginshell: provided uid not found in passwd file")
+
+	// ErrUnsupported is returned on unsupported platforms, such as Windows
+	ErrUnsupported = errors.New("loginshell: unsupported platform")
+)
+
+// Get returns the login shell belonging to the provided uid by parsing the passwd file.
+// If uid is less than zero, the current uid will be used instead.
+func Get(uid int) (string, error) {
+	if uid < 0 {
+		uid = os.Getuid()
+	}
+
+	// os.Getuid returns -1 on unsupported platforms
+	if uid == -1 {
+		return "", ErrUnsupported
+	}
+
+	fl, err := os.Open("/etc/passwd")
+	if errors.Is(err, fs.ErrNotExist) {
+		return "", ErrUnsupported
+	} else if err != nil {
+		return "", err
+	}
+	defer fl.Close()
+
+	s := bufio.NewScanner(fl)
+	for s.Scan() {
+		luid, shell, err := parsePasswdLine(s.Text())
+		if err != nil {
+			return "", err
+		}
+		if luid == uid {
+			return shell, nil
+		}
+	}
+	if err := s.Err(); err != nil {
+		return "", err
+	}
+	return "", ErrNoSuchUser
+}
+
+func parsePasswdLine(line string) (int, string, error) {
+	sline := strings.Split(line, ":")
+	if len(sline) < 7 {
+		return 0, "", ErrInvalidPasswd
+	}
+	uid, err := strconv.Atoi(sline[2])
+	if err != nil {
+		return 0, "", err
+	}
+	return uid, sline[6], nil
+}