diff --git a/21-shell/libc/string.h b/21-shell/libc/string.h
index 75f2d1b..584183b 100644
--- a/21-shell/libc/string.h
+++ b/21-shell/libc/string.h
@@ -7,5 +7,6 @@ int strlen(char s[]);
 void backspace(char s[]);
 void append(char s[], char n);
 int strcmp(char s1[], char s2[]);
+char chrupper(char chr);
 
 #endif