diff --git a/cmd/tools/vfmt.v b/cmd/tools/vfmt.v
index 7d92350c5e..ea68a86da3 100644
--- a/cmd/tools/vfmt.v
+++ b/cmd/tools/vfmt.v
@@ -181,11 +181,11 @@ fn (foptions &FormatOptions) post_process_file(file, formatted_file_path string)
 		return
 	}
 	if foptions.is_diff {
-		diff_cmd := find_working_diff_command() or {
+		diff_cmd := util.find_working_diff_command() or {
 			eprintln('No working "diff" CLI command found.')
 			return
 		}
-		os.system('$diff_cmd --minimal  --text   --unified=2 --show-function-line="fn " "$file" "$formatted_file_path" ')
+		println(util.color_compare_files(diff_cmd, file, formatted_file_path))
 		return
 	}
 	fc := os.read_file(file) or {
@@ -224,17 +224,6 @@ fn (foptions &FormatOptions) post_process_file(file, formatted_file_path string)
 	print(formatted_fc)
 }
 
-fn find_working_diff_command() ?string {
-	for diffcmd in ['colordiff', 'diff', 'colordiff.exe', 'diff.exe'] {
-		p := os.exec('$diffcmd --version') or {
-			continue
-		}
-		if p.exit_code == 0 {
-			return diffcmd
-		}
-	}
-	return error('no working diff command found')
-}
 
 fn (f FormatOptions) str() string {
 	return 'FormatOptions{ is_l: $f.is_l' + ' is_w: $f.is_w' + ' is_diff: $f.is_diff' + ' is_verbose: $f.is_verbose' +
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index bdc0aecaf5..6d660d90ab 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -73,7 +73,7 @@ pub fn (mut f Fmt) write(s string) {
 			f.out.write(tabs[f.indent])
 		} else {
 			// too many indents, do it the slow way:
-			for i in 0 .. f.indent {
+			for _ in 0 .. f.indent {
 				f.out.write('\t')
 			}
 		}
diff --git a/vlib/v/fmt/fmt_keep_test.v b/vlib/v/fmt/fmt_keep_test.v
index 9da65183ad..2ae0b3d445 100644
--- a/vlib/v/fmt/fmt_keep_test.v
+++ b/vlib/v/fmt/fmt_keep_test.v
@@ -6,6 +6,7 @@ import v.fmt
 import v.parser
 import v.table
 import v.pref
+import v.util
 
 const (
 	error_missing_vexe = 1
@@ -22,9 +23,7 @@ fn test_fmt() {
 	}
 	vroot := os.dir(vexe)
 	tmpfolder := os.temp_dir()
-	diff_cmd := find_working_diff_command() or {
-		''
-	}
+	diff_cmd := util.find_working_diff_command() or { '' }
 	mut fmt_bench := benchmark.new_benchmark()
 	keep_input_files := os.walk_ext('$vroot/vlib/v/fmt/tests', '_keep.vv')
 	expected_input_files := os.walk_ext('$vroot/vlib/v/fmt/tests', '_expected.vv')
@@ -56,7 +55,7 @@ fn test_fmt() {
 			}
 			vfmt_result_file := os.join_path(tmpfolder, 'vfmt_run_over_${ifilename}')
 			os.write_file(vfmt_result_file, result_ocontent)
-			os.system('$diff_cmd --minimal  --text   --unified=2 --show-function-line="fn " "$opath" "$vfmt_result_file"')
+            eprintln(util.color_compare_files(diff_cmd, opath, vfmt_result_file))
 			continue
 		}
 		fmt_bench.ok()
@@ -69,15 +68,3 @@ fn test_fmt() {
 		exit(error_failed_tests)
 	}
 }
-
-fn find_working_diff_command() ?string {
-	for diffcmd in ['colordiff', 'diff', 'colordiff.exe', 'diff.exe'] {
-		p := os.exec('$diffcmd --version') or {
-			continue
-		}
-		if p.exit_code == 0 {
-			return diffcmd
-		}
-	}
-	return error('no working diff command found')
-}
diff --git a/vlib/v/fmt/fmt_test.v b/vlib/v/fmt/fmt_test.v
index 196e46dd53..c34c1f4fa9 100644
--- a/vlib/v/fmt/fmt_test.v
+++ b/vlib/v/fmt/fmt_test.v
@@ -6,6 +6,7 @@ import v.fmt
 import v.parser
 import v.table
 import v.pref
+import v.util
 
 const (
 	error_missing_vexe = 1
@@ -22,9 +23,7 @@ fn test_fmt() {
 	}
 	vroot := os.dir(vexe)
 	tmpfolder := os.temp_dir()
-	diff_cmd := find_working_diff_command() or {
-		''
-	}
+	diff_cmd := util.find_working_diff_command() or { '' }
 	mut fmt_bench := benchmark.new_benchmark()
 	// Lookup the existing test _input.vv files:
 	input_files := os.walk_ext('$vroot/vlib/v/fmt/tests', '_input.vv')
@@ -58,7 +57,7 @@ fn test_fmt() {
 			}
 			vfmt_result_file := os.join_path(tmpfolder, 'vfmt_run_over_${ifilename}')
 			os.write_file(vfmt_result_file, result_ocontent)
-			os.system('$diff_cmd --minimal  --text   --unified=2 --show-function-line="fn " "$opath" "$vfmt_result_file"')
+            eprintln(util.color_compare_files(diff_cmd, opath, vfmt_result_file))
 			continue
 		}
 		fmt_bench.ok()
@@ -71,15 +70,3 @@ fn test_fmt() {
 		exit(error_failed_tests)
 	}
 }
-
-fn find_working_diff_command() ?string {
-	for diffcmd in ['colordiff', 'diff', 'colordiff.exe', 'diff.exe'] {
-		p := os.exec('$diffcmd --version') or {
-			continue
-		}
-		if p.exit_code == 0 {
-			return diffcmd
-		}
-	}
-	return error('no working diff command found')
-}
diff --git a/vlib/v/tests/repl/runner/runner.v b/vlib/v/tests/repl/runner/runner.v
index 01ae3e5f44..70499840e3 100644
--- a/vlib/v/tests/repl/runner/runner.v
+++ b/vlib/v/tests/repl/runner/runner.v
@@ -1,6 +1,7 @@
 module runner
 
 import os
+import v.util
 
 pub struct RunnerOptions {
 pub:
@@ -33,18 +34,9 @@ pub fn full_path_to_v(dirs_in int) string {
 	return vexec
 }
 
-fn find_working_diff_command() ?string {
-	for diffcmd in ['colordiff', 'diff', 'colordiff.exe', 'diff.exe'] {
-		p := os.exec('$diffcmd --version') or { continue }
-		if p.exit_code == 0 { return diffcmd }
-	}
-	return error('no working diff command found')
-}
-
 fn diff_files( file_result, file_expected string ) string {
-	diffcmd := find_working_diff_command() or { return err }
-	diff := os.exec('$diffcmd   --minimal  --text   --unified=2  ${file_result}  ${file_expected}') or { return 'found diff command "$diffcmd" does not work' }
-	return diff.output
+	diffcmd := util.find_working_diff_command() or { return err }
+	return util.color_compare_files(diffcmd, file_result, file_expected)
 }
 
 pub fn run_repl_file(wd string, vexec string, file string) ?string {
diff --git a/vlib/v/util/errors.v b/vlib/v/util/errors.v
index f27086526b..3f436997b5 100644
--- a/vlib/v/util/errors.v
+++ b/vlib/v/util/errors.v
@@ -156,3 +156,27 @@ pub fn verror(kind, s string) {
 	}
 	exit(1)
 }
+
+pub fn find_working_diff_command() ?string {
+	for diffcmd in ['colordiff', 'diff', 'colordiff.exe', 'diff.exe'] {
+		p := os.exec('$diffcmd --version') or {
+			continue
+		}
+		if p.exit_code == 0 {
+			return diffcmd
+		}
+	}
+	return error('no working diff command found')
+}
+
+pub fn color_compare_files(diff_cmd, file1, file2 string) string {
+	if diff_cmd != '' {
+		full_cmd := '$diff_cmd --minimal --text --unified=2 ' +
+		        ' --show-function-line="fn " "$file1" "$file2" '
+		x := os.exec(full_cmd) or {
+			return 'comparison command: `${full_cmd}` failed'
+        }
+        return x.output
+    }
+    return ''
+}