diff --git a/shell-functions b/shell-functions
index 962fdaa..a44eba1 100644
--- a/shell-functions
+++ b/shell-functions
@@ -21,4 +21,22 @@ function print_info_sub_noisy { [ ${NOISY:-0} -ge $1 ] && print_info_sub $@[2,-1
 function print_error_sub_noisy { [ ${NOISY:-0} -ge $1 ] && print_error_sub $@[2,-1] }
 
 # Return 1 if the binary exists (according to hash); 0 otherwise.
-function binary_exists { return $(hash $1 1>/dev/null 2>&1) }
+function binary_exists {
+    hash $1 &>/dev/null
+    if [[ $? ]]; then
+        return 1
+    else
+        return 0
+    fi
+}
+
+# Return 1 if the binary doesn't exist (according to hash); 0 otherwise. See
+# binary_exists.
+function binary_not_exists {
+    binary_exists $1 &>/dev/null
+    if [[ $? ]]; then
+        return 0
+    else
+        return 1
+    fi
+}