id.c

   1 /* `id.c` - return user identity
   2    Copyright (c) 2022, Alan Potteiger
   3    See `LICENSE` for copyright and license details */
   4
   5 #define _POSIX_C_SOURCE 200809L
   6
   7 #include <stdio.h>
   8 #include <unistd.h>
   9 #include <errno.h>
  10 #include <pwd.h>
  11 #include <grp.h>
  12
  13 static const char *usage = {
  14         "usage: id [user]\n"
  15         "id -G [-n] [user]\n"
  16         "id -g [-nr] [user]\n"
  17         "id -u [-nr] [user]\n"
  18 };
  19
  20 int
  21 main(int argc, char *argv[])
  22 {
  23         char ch;
  24         int all, Gflag, gflag, nflag, rflag, uflag;
  25         char *user;
  26         struct passwd *pwd;
  27         struct group *grp;
  28         uid_t uid;
  29         gid_t gid;
  30         gid_t *groups;
  31         int ngroups;
  32
  33
  34         all = Gflag = gflag = uflag = nflag = rflag = 0;
  35         user = NULL;
  36         pwd = NULL;
  37         ngroups = 0;
  38         groups = NULL;
  39
  40         while ((ch = getopt(argc, argv, "Ggunr")) != -1) {
  41                 switch (ch) {
  42                 case 'G':
  43                         Gflag = 1;
  44                         break;
  45                 case 'g':
  46                         gflag = 1;
  47                         break;
  48                 case 'n':
  49                         nflag = 1;
  50                         break;
  51                 case 'r':
  52                         rflag = 1;
  53                         break;
  54                 case 'u':
  55                         uflag = 1;
  56                         break;
  57                 case '?':
  58                 default:
  59                         fputs(usage, stderr);
  60                         return 1;
  61                 }
  62         }
  63
  64         /* some options should be mutually exclusive */
  65         if ((Gflag && gflag) || (Gflag && uflag) || (gflag && uflag)) {
  66                 fputs(usage, stderr);
  67                 return 1;
  68         }
  69         /* -n and -r are useless without any of the main flags */
  70         if (Gflag == 0 && gflag == 0 && uflag == 0) {
  71                 if (nflag == 1 || rflag == 1) {
  72                         fputs(usage, stderr);
  73                         return 1;
  74                 }
  75                 if (nflag == 0 && rflag == 0)
  76                         all = 1;
  77         }
  78
  79         argc -= optind;
  80         argv += optind;
  81
  82         /* obtain passwd struct if user name provided */
  83         if (argc > 0) {
  84                 user = argv[0];
  85                 errno = 0;
  86                 pwd = getpwnam(user);
  87                 if (pwd == NULL) {
  88                         if (errno != 0)
  89                                 perror("id");
  90                         else
  91                                 fprintf(stderr, "id: user %s does not exist\n",
  92                                                         user);
  93                         return 1;
  94                 }
  95         } else {
  96                 user = getlogin();
  97         }
  98
  99         if (user == NULL) {
 100                 fputs("id: cannot obtain login information\n", stderr);
 101                 return 1;
 102         }
 103
 104         /* TODO: no flag functionality */
 105
 106         if (Gflag) {
 107                 if (pwd != NULL)
 108                         gid = pwd->pw_gid;
 109                 else {
 110                         gid = getgid();
 111                 }
 112
 113                 /* TODO: Gflag functionality */
 114         }
 115
 116         if (gflag) {
 117                 if (pwd != NULL) {
 118                         gid = pwd->pw_gid;
 119                 } else {
 120                         if (rflag)
 121                                 gid = getgid();
 122                         else
 123                                 gid = geteuid();
 124                 }
 125
 126                 if (nflag) {
 127                         grp = getgrgid(gid);
 128                         printf("%s\n", grp->gr_name);
 129                         return 0;
 130                 }
 131
 132                 printf("%u\n", gid);
 133                 return 0;
 134         }
 135
 136         if (uflag) {
 137                 if (nflag) {
 138                         printf("%s\n", user);
 139                         return 0;
 140                 }
 141
 142                 if (pwd != NULL) {
 143                         uid = pwd->pw_uid;
 144                 } else {
 145                         if (rflag)
 146                                 uid = getuid();
 147                         else
 148                                 uid = geteuid();
 149                 }
 150
 151                 printf("%u\n", uid);
 152                 return 0;
 153         }
 154
 155         return 0;
 156 }
 157