id(1) can handle uid arguments

id.1

@@ -3,10 +3,10 @@
 id \- print real and effective user and group IDs
 .SH SYNOPSIS
 .B id
-.RB [ user ]
+.RB [ user | uid ]
 .SH DESCRIPTION
 Print user and group information of the calling process to standard output.
-If a login name is specified, the user and group information of that user
+If a login name or uid is specified, the user and group information of that
-is displayed.
+user is displayed.
 .SH SEE ALSO
 .IR who(1)

id.c

@@ -7,21 +7,22 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <limits.h>
+#include <ctype.h>
 #include "util.h"
 
 static void user(struct passwd *pw);
+static void userid(uid_t id);
+static void usernam(const char *nam);
 
 static void
 usage(void)
 {
-	eprintf("usage: %s [user]\n", argv0);
+	eprintf("usage: %s [user | uid]\n", argv0);
 }
 
 int
 main(int argc, char *argv[])
 {
-	struct passwd *pw;
-
 	ARGBEGIN {
 	default:
 		usage();
@@ -30,20 +31,14 @@ main(int argc, char *argv[])
 	errno = 0;
 	switch (argc) {
 	case 0:
-		pw = getpwuid(getuid());
+		userid(getuid());
-		if (errno != 0)
-			eprintf("getpwuid %d:", getuid());
-		else if (!pw)
-			eprintf("getpwuid %d: no such user\n", getuid());
-		user(pw);
 		break;
 	case 1:
-		pw = getpwnam(argv[0]);
+		/* user names can't begin [0-9] */
-		if (errno != 0)
+		if (isdigit(argv[0][0]))
-			eprintf("getpwnam %s:", argv[0]);
+			userid(estrtol(argv[0], 0));
-		else if (!pw)
+		else
-			eprintf("getpwnam %s: no such user\n", argv[0]);
+			usernam(argv[0]);
-		user(pw);
 		break;
 	default:
 		usage();
@@ -52,6 +47,32 @@ main(int argc, char *argv[])
 	return EXIT_SUCCESS;
 }
 
+static void usernam(const char *nam)
+{
+	struct passwd *pw;
+
+	errno = 0;
+	pw = getpwnam(nam);
+	if (errno != 0)
+		eprintf("getpwnam %s:", nam);
+	else if (!pw)
+		eprintf("getpwnam %s: no such user\n", nam);
+	user(pw);
+}
+
+static void userid(uid_t id)
+{
+	struct passwd *pw;
+
+	errno = 0;
+	pw = getpwuid(id);
+	if (errno != 0)
+		eprintf("getpwuid %d:", id);
+	else if (!pw)
+		eprintf("getpwuid %d: no such user\n", id);
+	user(pw);
+}
+
 static void
 user(struct passwd *pw)
 {