additional usermod flag, '-a'

--- shadow-4.0.10/man/usermod.8.aflg	2005-06-16 18:08:37.000000000 +0200
+++ shadow-4.0.10/man/usermod.8	2005-07-06 14:36:35.000000000 +0200
@@ -24,7 +24,7 @@
 .ad l
 .hy 0
 .HP 8
-\fBusermod\fR [\-c\ \fIcomment\fR] [\-d\ \fIhome_dir\fR\ [\-m]] [\-e\ \fIexpire_date\fR] [\-f\ \fIinactive_days\fR] [\-g\ \fIinitial_group\fR] [\-G\ \fIgroup1\fR\ [\fI,group2,\fR...\ \fI,\fR\ [\fIgroupN\fR]]] [\-l\ \fIlogin_name\fR] [\-p\ \fIpasswd\fR] [\-s\ \fIshell\fR] [\-u\ \fIuid\fR\ [\-o]] [\fB\-L\fR | \fB\-U\fR] \fIlogin\fR
+\fBusermod\fR [\-c\ \fIcomment\fR] [\-d\ \fIhome_dir\fR\ [\-m]] [\-e\ \fIexpire_date\fR] [\-f\ \fIinactive_days\fR] [\-g\ \fIinitial_group\fR] [[-a] \-G\ \fIgroup1\fR\ [\fI,group2,\fR...\ \fI,\fR\ [\fIgroupN\fR]]] [\-l\ \fIlogin_name\fR] [\-p\ \fIpasswd\fR] [\-s\ \fIshell\fR] [\-u\ \fIuid\fR\ [\-o]] [\fB\-L\fR | \fB\-U\fR] \fIlogin\fR
 .ad
 .hy
 
@@ -60,7 +60,7 @@
 
 .TP
 \fB\-G\fR \fIgroup1\fR[\fI ,group2,\&.\&.\&., \fR [\fIgroupN\fR]]]
-A list of supplementary groups which the user is also a member of\&. Each group is separated from the next by a comma, with no intervening whitespace\&. The groups are subject to the same restrictions as the group given with the \fB\-g\fR option\&. If the user is currently a member of a group which is not listed, the user will be removed from the group\&.
+A list of supplementary groups which the user is also a member of\&. Each group is separated from the next by a comma, with no intervening whitespace\&. The groups are subject to the same restrictions as the group given with the \fB\-g\fR option\&. If the user is currently a member of a group which is not listed, the user will be removed from the group\&. This behaviour can be changed via \fB\-a\fR option, which appends user to the current supplementary group list\&.
 
 .TP
 \fB\-l\fR \fIlogin_name\fR
--- shadow-4.0.10/src/usermod.c.aflg	2005-06-20 12:17:10.000000000 +0200
+++ shadow-4.0.10/src/usermod.c	2005-07-06 14:37:25.000000000 +0200
@@ -94,6 +94,7 @@
  uflg = 0,			/* specify new user ID */
     oflg = 0,			/* permit non-unique user ID to be specified with -u */
     gflg = 0,			/* new primary group ID */
+    aflg = 0,                   /* append to existing secondary group set */
     Gflg = 0,			/* new secondary group set */
     dflg = 0,			/* new home directory */
     sflg = 0,			/* new shell program */
@@ -276,7 +277,7 @@
 {
 	fprintf (stderr,
 		 _
-		 ("Usage: %s\t[-u uid [-o]] [-g group] [-G group,...] \n"),
+		 ("Usage: %s\t[-u uid [-o]] [-g group] [[-a] [-G group,...]] \n"),
 		 Prog);
 	fprintf (stderr,
 		 _
@@ -473,7 +474,7 @@
 					 user_name, user_newname,
 					 ngrp->gr_name));
 			}
-		} else if (was_member && Gflg && !is_member) {
+		} else if (was_member && !aflg && Gflg && !is_member) {
 			ngrp->gr_mem = del_list (ngrp->gr_mem, user_name);
 			changed = 1;
 			SYSLOG ((LOG_INFO, "delete `%s' from group `%s'",
@@ -590,7 +591,7 @@
 					 user_name, user_newname,
 					 nsgrp->sg_name));
 			}
-		} else if (was_member && Gflg && !is_member) {
+		} else if (was_member && !aflg && Gflg && !is_member) {
 			nsgrp->sg_mem = del_list (nsgrp->sg_mem, user_name);
 			changed = 1;
 			SYSLOG ((LOG_INFO,
@@ -737,8 +738,11 @@
 		user_inactive = spwd->sp_inact;
 	}
 
-	while ((arg = getopt (argc, argv, "A:u:og:G:d:s:c:mf:e:l:p:LU")) != EOF) {
+	while ((arg = getopt (argc, argv, "aA:u:og:G:d:s:c:mf:e:l:p:LU")) != EOF) {
 		switch (arg) {
+                case 'a':
+                        aflg++;
+                        break;
 		case 'c':
 			if (!VALID (optarg)) {
 				fprintf (stderr,
@@ -878,6 +882,13 @@
 	if (optind != argc - 1)
 		usage ();
 
+        if (aflg && (!Gflg)) {
+                fprintf (stderr, "\n%s: -a flag is ONLY allowed with the -G flag\n\n",
+                Prog);
+                usage();
+                exit (E_USAGE);
+        }
+
 	if (dflg && strcmp (user_home, user_newhome) == 0)
 		dflg = mflg = 0;