FreeFileSync Open Source File Synchronization

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FreeFileSync User Manual:

#Quick Start Command Line Comparison Settings Daylight Saving Time Exclude Files Expert Settings External Applications Macros Performance RealTimeSync RTS: Run as Service Schedule Batch Jobs Scripting Synchronization Settings (S)FTP Setup Tips and Tricks Variable Drive Letters Versioning Volume Shadow Copy

Scripting

When FreeFileSync is run via the command line, the synchronization result is written to standard output in JSON format. This output can be evaluated by scripts to customize functionality.

Windows: To view FreeFileSync's standard output, the pipe (|) operator is required because the operating system classifies FreeFileSync as a GUI application rather than a console application:

FreeFileSync.exe "D:\Backup Projects.ffs_batch" | more

The command prints JSON data such as:

{
  "syncResult": "success",     → can be: success, warning, error, cancelled
  "startTime": "2026-01-05T12:35:42+00:00",
  "totalTimeSec": 12,
  "errors": 0,
  "warnings": 0,
  "totalItems": 1000,
  "totalBytes": 1024,
  "processedItems": 1000,
  "processedBytes": 1024,
  "logFile": "D:\\Logs\\Backup Projects 2026-01-05 123542.123.html"
}

Parsing sync result via PowerShell

Windows batch scripts (.cmd/.bat) cannot parse JSON, so PowerShell can be used instead:

Example: Delete unnecessary log files

#run an .ffs_batch config and parse the JSON output
$data = & "C:\Program Files\FreeFileSync\FreeFileSync.exe" "D:\Backup Projects.ffs_batch" | ConvertFrom-Json

#delete the log file if no items were synced
if ($data.syncResult -eq "success" -and $data.totalItems -eq 0)
Remove-Item $data.logFile

Parsing sync result via Python

On Linux, the same task can be implemented using Python:

#!/usr/bin/env python3

import json, subprocess, os

#run an .ffs_batch config and parse the JSON output
result = subprocess.run(
["/opt/FreeFileSync/FreeFileSync", "/home/zenju/Backup Projects.ffs_batch"], stdout=subprocess.PIPE, text=True)

data = json.loads(result.stdout)

#delete the log file if no items were synced
if data["syncResult"] == "success" and data["totalItems"] == 0:
os.remove(data["logFile"])