diff --git a/README.md b/README.md
index 9b7be86..8479a33 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,8 @@ This a collection of scripts that could be a Ghist each.
 
 During my _Unix_ course, this is one of the exercice I ask my students in
 in order to start playing with `grep`: 
-"Using only grep, write a script that will extract from a given shell script,
-after checking the file argument, all comments that start at firsh column."
+"Write a script that will extract comment from a shell script. But make it
+fail when the script is called with no argument, and also when the script
+file cannot be found or read. Try to use only grep and keep your script the
+smallest possible."
 
diff --git a/get_comments b/get_comments
index 16736be..02a1f11 100755
--- a/get_comments
+++ b/get_comments
@@ -3,6 +3,9 @@
 # readable file requiered
 grep -q "" "$1" || exit 1
 
+# check first line
+head -n 1 "$1" | grep -q '^#!/s\?bin/[a-z]*sh$' || exit 2
+
 # extract comments 
-grep '^#' "$1" | cut -c 2- 
+grep -o '#.*$' "$1" | cut -c 2-