diff --git a/tests/tool_ni.tcl b/tests/tool_ni.tcl
new file mode 100644
index 0000000..7282d35
--- /dev/null
+++ b/tests/tool_ni.tcl
@@ -0,0 +1,141 @@
+# @brief Common functions and variables for Tool Under Test (TUT).
+#
+# The script requires variables:
+# TUT_PATH - Assumed absolute path to the directory in which the TUT is located.
+# TUT_NAME - TUT name (without path).
+#
+# The script sets the variables:
+# TUT - The path (including the name) of the executable TUT.
+# error_prompt - Delimiter on error.
+# error_head - Header on error.
+
+# Complete the path for Tool Under Test (TUT). For example, on Windows, TUT can be located in the Debug or Release
+# subdirectory. Note that Release build takes precedence over Debug.
+set conftypes {{} Release Debug}
+foreach i $conftypes {
+    if { [file executable "$TUT_PATH/$i/$TUT_NAME"] || [file executable "$TUT_PATH/$i/$TUT_NAME.exe"] } {
+        set TUT "$TUT_PATH/$i/$TUT_NAME"
+        break
+    }
+}
+if {![info exists TUT]} {
+    error "$TUT_NAME executable not found"
+}
+
+# prompt of error message
+set error_prompt ">>>"
+# the beginning of error message
+set error_head "$error_prompt Check-failed"
+
+# Run commands from command line
+tcltest::loadTestedCommands
+
+# namespace of internal functions
+namespace eval ly::private {
+    namespace export *
+}
+
+# Run the process with arguments.
+# Parameter cmd is a string with arguments.
+# Parameter wrn is a flag. Set to 1 if stderr should be ignored.
+# Returns a pair where the first is the return code and the second is the output.
+proc ly::private::ly_exec {cmd {wrn ""}} {
+    global TUT
+    try {
+        set results [exec -- $TUT {*}$cmd]
+        set status 0
+    } trap CHILDSTATUS {results options} {
+        # return code is not 0
+        set status [lindex [dict get $options -errorcode] 2]
+    } trap NONE results {
+        if { $wrn == 1 } {
+            set status 0
+        } else {
+            error "return code is 0 but something was written to stderr:\n$results\n"
+        }
+    } trap CHILDKILLED {results options} {
+        set status [lindex [dict get $options -errorcode] 2]
+        error "process was killed: $status"
+    }
+    list $status $results
+}
+
+# Internal function.
+# Check the output with pattern.
+# Parameter pattern is a regex or an exact string to match.
+# Parameter msg is the output to check.
+# Parameter 'opt' is optional. If contains '-ex', then the 'pattern' parameter is
+# used as a simple string for exact matching of the output.
+proc ly::private::output_check {pattern msg {opt ""}} {
+    if { $opt eq "" } {
+        expr {![regexp -- $pattern $msg]}
+    } elseif { $opt eq "-ex" } {
+        expr {![string equal "$pattern" $msg]}
+    } else {
+        global error_head
+        error "$error_head unrecognized value of parameter 'opt'"
+    }
+}
+
+# Execute yanglint with arguments and expect success.
+# Parameter cmd is a string of arguments.
+# Parameter pattern is a regex or an exact string to match.
+# Parameter 'opt' is optional. If contains '-ex', then the 'pattern' parameter is
+# used as a simple string for exact matching of the output.
+proc ly_cmd {cmd {pattern ""} {opt ""}} {
+    namespace import ly::private::*
+    lassign [ly_exec $cmd] rc msg
+    if { $rc != 0 } {
+        error "unexpected return code $rc:\n$msg\n"
+    }
+    if { $pattern ne "" && [output_check $pattern $msg $opt] } {
+        error "unexpected output:\n$msg\n"
+    }
+    return
+}
+
+# Execute yanglint with arguments and expect error.
+# Parameter cmd is a string of arguments.
+# Parameter pattern is a regex.
+proc ly_cmd_err {cmd pattern} {
+    namespace import ly::private::*
+    lassign [ly_exec $cmd] rc msg
+    if { $rc == 0 } {
+        error "unexpected return code $rc"
+    }
+    if { [output_check $pattern $msg] } {
+        error "unexpected output:\n$msg\n"
+    }
+    return
+}
+
+# Execute yanglint with arguments, expect warning in stderr but success.
+# Parameter cmd is a string of arguments.
+# Parameter pattern is a regex.
+proc ly_cmd_wrn {cmd pattern} {
+    namespace import ly::private::*
+    lassign [ly_exec $cmd 1] rc msg
+    if { $rc != 0 } {
+        error "unexpected return code $rc:\n$msg\n"
+    }
+    if { [output_check $pattern $msg] } {
+        error "unexpected output:\n$msg\n"
+    }
+    return
+}
+
+# Check if yanglint supports the specified option.
+# Parameter opt is a option to be found.
+# Return true if option is found otherwise false.
+proc ly_opt_exists {opt} {
+    namespace import ly::private::*
+    lassign [ly_exec "--help"] rc msg
+    if { $rc != 0 } {
+        error "unexpected return code $rc:\n$msg\n"
+    }
+    if { [output_check $opt $msg] } {
+        return false
+    } else {
+        return true
+    }
+}