Shell: Build prompt based on the PROMPT environment variable if present

Shell/main.cpp

@@ -55,6 +55,8 @@ static int run_command(const String&);
 
 static String prompt()
 {
+    auto* ps1 = getenv("PROMPT");
+    if (!ps1) {
         if (g.uid == 0)
             return "# ";
 
@@ -64,6 +66,42 @@ static String prompt()
         return builder.to_string();
     }
 
+    StringBuilder builder;
+    for (char* ptr = ps1; *ptr; ++ptr) {
+        if (*ptr == '\\') {
+            ++ptr;
+            if (!*ptr)
+                break;
+            switch (*ptr) {
+            case 'X':
+                builder.append("\033]0;");
+                break;
+            case 'a':
+                builder.append(0x07);
+                break;
+            case 'e':
+                builder.append(0x1b);
+                break;
+            case 'u':
+                builder.append(g.username);
+                break;
+            case 'h':
+                builder.append(g.hostname);
+                break;
+            case 'w':
+                builder.append(g.cwd);
+                break;
+            case 'p':
+                builder.append(g.uid == 0 ? '#' : '$');
+                break;
+            }
+            continue;
+        }
+        builder.append(*ptr);
+    }
+    return builder.to_string();
+}
+
 static int sh_pwd(int, char**)
 {
     printf("%s\n", g.cwd.characters());