create the shell loop

src/include/loop.h

@@ -0,0 +1,2 @@
+void print_input_line();
+void loop();

src/loop.c

@@ -0,0 +1,18 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+
+void print_input_line() {
+    printf("$ ");
+}
+
+void loop() {
+    size_t buffer_size = 0;
+    char *line = NULL;
+
+    while (1) {
+        print_input_line();
+        getline(&line, &buffer_size, stdin);
+    }
+}

src/myshellin.c

@@ -1,5 +1,6 @@
-int main(int argc, char **argv)
-{
+#include "loop.h"
 
+int main(int argc, char **argv) {
+    loop();
     return 0;
 }