diff --git a/src/lib/common.sh b/src/lib/common.sh
index 0ffc66b..9111fc2 100644
--- a/src/lib/common.sh
+++ b/src/lib/common.sh
@@ -54,7 +54,7 @@ log() {
 }
 
 err() {
-    # Handle error. Print errors to STDERR, log, process
+    # Handle errors. Print errors to STDERR, log, process
     # and exit from script.
     #
     # Usage: err [-eao] MESSAGE
@@ -95,6 +95,7 @@ err() {
     # Run user defined function on_error()
     if [ "$onerr" ]; then
         if is_function_set on_error; then
+            echo -e "Run \e[1mon_error()\e[0m ..." | log -p
             on_error
         fi
     fi
@@ -118,3 +119,26 @@ try() {
         err -aeo "Check $__log_file log for details."
     fi
 }
+
+is_installed() {
+    # Check if the program is installed.
+    # See good answer: https://stackoverflow.com/a/677212
+    #
+    # Usage: is_installed COMMAND
+
+    if ! command -v "$1" >/dev/null 2>&1; then
+        echo "Error: Command $1 not found." \
+            "Please install $1 or check your PATH if it's actually installed." >&2
+        exit 1
+    fi
+}
+
+is_function_set() {
+    # Test function is set or not. Return exit code.
+    # Useful with 'if' statement.
+    #
+    # Usage: is_function_set FUNCTION
+
+    declare -F -- "$1" > /dev/null
+    return "$?"
+}
diff --git a/src/lib/source.sh b/src/lib/source.sh
index 9ee6cd3..ea783d9 100644
--- a/src/lib/source.sh
+++ b/src/lib/source.sh
@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-# script.sh - utilitary functions.
+# source.sh - source and validate scripts.
 # Copyright (c) 2022 ge <https://nixhacks.net/>
 #
 # This program is free software: you can redistribute it and/or modify
@@ -120,13 +120,3 @@ source_script() {
         echo Error: $__user_script: targets array is not set >&2; exit 1
     fi
 }
-
-is_function_set() {
-    # Test function is set or not. Return exit code.
-    # Useful with 'if' statement.
-    #
-    # Usage: is_function_set FUNCTION
-
-    declare -F -- "$1" > /dev/null
-    return "$?"
-}