[WIP] Add "who" utilitiy.

src/who.c

@@ -0,0 +1,91 @@
+/**
+ * NAME
+ * ====
+ *     who - display who is on the system
+ *
+ * SYNOPSIS
+ * ========
+ *     who [-mTu]
+ *
+ * DESCRIPTION
+ * ===========
+ *     Print information about users that are on the system.
+ *
+ *     -m           Print information about only the current terminal.
+ *     -T           Show the state of each terminal.
+ *     -u           Include the "idle time" for each displayed user.
+ *
+ *     --help       Print help text and exit.
+ *     --version    Print version information and exit.
+ */
+
+
+#include <stdio.h>
+#include "boreutils.h"
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/who.html
+
+int main(int argc, char **argv) {
+    if (has_arg(argc, argv, "-h") || has_arg(argc, argv, "--help")) {
+        puts("Usage: who [-mTu]");
+        puts("Print information about users that are on the system.");
+        return 1;
+    }
+
+    int dash_m = 0;
+    int dash_T = 0;
+    int dash_u = 0;
+
+    for (int i = 1; i < argc; i++) {
+        char *arg = argv[i];
+        if (arg[0] != '-') {
+            bu_invalid_argument(argv[0], argv[i]);
+            return 1;
+        }
+
+        arg++;
+        for (; *arg; arg++) {
+            char flag = *arg;
+
+            if (flag == 'm') { dash_m = 1; }
+            else if (flag == 'T') { dash_T = 1; }
+            else if (flag == 'u') { dash_u = 1; }
+            else {
+                char tmp[3] = "-X";
+                tmp[1] = *arg;
+                bu_invalid_argument(argv[0], tmp);
+                return 1;
+            }
+        }
+    }
+
+
+    char *user_name = "USERNAME";
+    char terminal_state = '?';
+    char *terminal_name = "TERMNAME";
+    char *time_of_login = "TIME";
+    char *idle_time;
+
+    if (dash_T) {
+        // Actual terminal state.
+        // +        allows write access to other users
+        // -        denies write access to other users
+        // ?        cannot be determined
+        // <space>  not associated with a terminal
+        // TODO: Actually set terminal_state.
+    }
+
+    if (dash_u) {
+        idle_time = " IDLE";
+    } else {
+        idle_time = "";
+    }
+
+    if (dash_T) {
+        printf("%s %c %s %s%s\n", user_name, terminal_state, terminal_name, time_of_login, idle_time);
+    } else {
+        printf("%s %s %s%s\n", user_name, terminal_name, time_of_login, idle_time);
+    }
+
+    return 0;
+}