diff --git a/src/lib/source.sh b/src/lib/source.sh
new file mode 100644
index 0000000..d1c39ea
--- /dev/null
+++ b/src/lib/source.sh
@@ -0,0 +1,114 @@
+#! /usr/bin/env bash
+
+# script.sh - utilitary functions.
+# Copyright (c) 2022 ge <https://nixhacks.net/>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+validate_sources() {
+    # Check sources array.
+    #
+    # Usage: validate_sources ARRAY
+
+    # Allowed URI schemes: file, mysql postgres, sqlite
+    # No required schemes.
+
+    local array=("$@")
+    local scheme=
+
+    for uri in "${array[@]}"; do
+        scheme="${uri%%:*}"
+        case "$scheme" in
+            file|mysql|postgres|sqlite) : ;; # do nothing, this is OK
+            *) echo "Error: Unsupported URI scheme: $scheme" >&2; exit 1;;
+        esac
+    done
+}
+
+validate_targets() {
+    # Check targets array and set local_target variable.
+    #
+    # Usage: validate_targets ARRAY
+
+    # Allowed URI schemes: file, ftp, sftp, rsync, s3, swift, sj, dav, davs
+    # Required schemes (one or more times): file
+
+    local array=("$@")
+    local scheme=
+    local file_targets=()
+
+    for uri in "${array[@]}"; do
+        scheme="${uri%%:*}"
+        case "$scheme" in
+            file|ftp|sftp|rsync|s3|swift|sj|dav|davs)
+                if [[ "$scheme" == file ]]; then
+                    file_targets+=("$uri")
+                fi
+            ;;
+            *) echo "Error: Unsupported URI scheme: $scheme" >&2; exit 1;;
+        esac
+    done
+
+    if [ "${#file_targets[@]}" -eq 0 ]; then
+        echo "Error: 'file' scheme is not set in targets." \
+            "You must provide one or more targets with 'file' scheme." >&2
+        exit 1
+    else
+        # Set local_target. This variable contains path to save local backups.
+        # Files to additional targets will be coped from this directory.
+        local_target="${file_targets[0]}"
+    fi
+}
+
+source_script() {
+    # Safely as possible source backup script.
+    #
+    # Usage: source_script SCRIPT
+
+    local script="$1"
+
+    if ! test -f "$script"; then
+        echo "Error: No such file: $script" >&2; exit 1
+    fi
+
+    # Dry run script, check syntax. See set(1p)
+    if ! bash -n "$script"; then
+        echo Error: Please check your syntax >&2; exit 1
+    fi
+
+    # Source script
+    . "$@"
+
+    # Check required variables
+    if [[ "$sources" ]]; then
+        validate_sources "${sources[@]}"
+    else
+        echo Error: sources array is not set >&2; exit 1
+    fi
+
+    if [[ "$targets" ]]; then
+        validate_targets "${targets[@]}"
+    else
+        echo Error: 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
+}
diff --git a/tests/source_script.bats b/tests/source_script.bats
index ae2ced2..0ff95de 100644
--- a/tests/source_script.bats
+++ b/tests/source_script.bats
@@ -1,6 +1,6 @@
 #! /usr/bin/env bats
 
-# source_script() from lib/script.sh tests.
+# source_script() from lib/source.sh tests.
 # See: https://bats-core.readthedocs.io/en/latest/index.html
 
 setup() {
@@ -16,55 +16,55 @@ setup() {
 # ------------------------------ #
 
 @test "Bad script syntax" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/bad_syntax.plan
     assert_output --partial 'Error: Please check your syntax'
 }
 
 @test "Empty script" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/empty_script.plan
     assert_output --partial 'Error: sources array is not set'
 }
 
 @test "Empty sources array" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/empty_sources.plan
     assert_output --partial 'Error: sources array is not set'
 }
 
 @test "Empty targets array" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/empty_targets.plan
     assert_output --partial 'Error: targets array is not set'
 }
 
 @test "No targets with 'file' URI scheme" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/no_file_target.plan
     assert_output --partial "Error: 'file' scheme is not set in targets. You must provide one or more targets with 'file' scheme."
 }
 
 @test "Unsuported source scheme" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/unsupported_source_scheme.plan
     assert_output --partial 'Error: Unsupported URI scheme: mongo'
 }
 
 @test "Unsuported target scheme" {
-    . script.sh
+    . source.sh
     run source_script $DIR/files/unsupported_target_scheme.plan
     assert_output --partial 'Error: Unsupported URI scheme: scp'
 }
 
 @test "Set local target" {
-    . script.sh
+    . source.sh
     source_script $DIR/files/basic.plan
     [ "$local_target" == 'file:/var/backup' ]
 }
 
 @test "Set local target from multiple 'file' targets" {
-    . script.sh
+    . source.sh
     source_script $DIR/files/multiple_file_targets.plan
     [ "$local_target" == 'file:///home/backups' ]
 }