# Backups That Work: \`robocopy\` robocopy, the copying program that comes with Windows, copies a folder and after that only what changed, mirrors it with /MIR (which deletes from the backup too), rehearses with /L, and ends with an exit code where anything below 8 means success. Journeyman · PowerShell · Operating System · Updated November 10, 2026 ## After this lesson you can - Copy a folder with robocopy, then only what changed, mirroring with /MIR only after rehearsing it with /L - Read robocopy's exit code from $LASTEXITCODE and turn it into success or failure, where anything below 8 is success - Choose between a scheduled task, a log, and robocopy with /E, /MIR or /L for each part of a backup that runs by itself ## Key takeaways - robocopy SOURCE DESTINATION /E copies a folder and every folder inside it; run again, it copies only new and changed files. - /MIR makes the backup a mirror: a file gone from the source is deleted from the backup. An empty or wrong source empties the backup. - /L only lists what robocopy would copy and delete, and changes nothing. Rehearse every new /MIR line with it. - robocopy's exit code: 0 nothing to copy, 1 files copied, 2 extra files, 8 or more something failed. Below 8 is success. - In a script, /R:2 /W:5 replace robocopy's default of a million retries, 30 seconds apart. Every Friday, Sam drags the school-work folder onto a USB stick. Windows asks "Replace or skip?" about files that haven't changed, copies the whole folder again to update three files, and a draft Sam deleted in September is still on the stick in November. `Copy-Item` has the same habits. Windows comes with a copier built for this job: `robocopy`, read "robo-copy", short for *robust copy*. IT techs use it to move whole file servers, and it sits inside a great many backup scripts. It's a program, not a cmdlet, so its options start with `/`, and it works the same in Windows PowerShell 5.1 and Command Prompt. ## The first copy Make the school work, and an empty `backup-drive` folder to play the USB stick: ```powershell New-Item -ItemType Directory -Force -Path "$HOME\terminal-practice\school-work\science", "$HOME\terminal-practice\school-work\math", "$HOME\terminal-practice\backup-drive" | Out-Null Set-Content -Path "$HOME\terminal-practice\school-work\history-essay.txt" -Value 'Why the Town Library Matters', 'Second draft, with the quotes fixed.' Set-Content -Path "$HOME\terminal-practice\school-work\reading-list.txt" -Value 'Chapter 4 of the science book', 'Two poems for English class' Set-Content -Path "$HOME\terminal-practice\school-work\science\lab-report.txt" -Value 'Lab 4: how fast does ice melt in salt water?' Set-Content -Path "$HOME\terminal-practice\school-work\science\volcano.jpg" -Value '(a practice photo: the science fair volcano)' Set-Content -Path "$HOME\terminal-practice\school-work\math\homework-week-10.txt" -Value 'Problems 1 to 20, page 112' ``` Then copy, source first and destination second, as with `Copy-Item`: ```powershell Set-Location "$HOME\terminal-practice" robocopy school-work backup-drive\school-work /E /NP ``` ```text ------------------------------------------------------------------------------- ROBOCOPY :: Robust File Copy for Windows ------------------------------------------------------------------------------- Started : Monday, November 9, 2026 8:14:15 AM Source : C:\Users\Sam\terminal-practice\school-work\ Dest : C:\Users\Sam\terminal-practice\backup-drive\school-work\ Files : *.* Options : *.* /S /E /DCOPY:DA /COPY:DAT /NP /R:1000000 /W:30 ------------------------------------------------------------------------------ New Dir 2 C:\Users\Sam\terminal-practice\school-work\ New File 68 history-essay.txt New File 60 reading-list.txt New Dir 1 C:\Users\Sam\terminal-practice\school-work\math\ New File 28 homework-week-10.txt New Dir 2 C:\Users\Sam\terminal-practice\school-work\science\ New File 46 lab-report.txt New File 46 volcano.jpg ------------------------------------------------------------------------------ Total Copied Skipped Mismatch FAILED Extras Dirs : 3 3 0 0 0 0 Files : 5 5 0 0 0 0 Bytes : 248 248 0 0 0 0 Times : 0:00:00 0:00:00 0:00:00 0:00:00 Speed : 62,000 Bytes/sec. Speed : 3.548 MegaBytes/min. Ended : Monday, November 9, 2026 8:14:15 AM ``` `/E` takes every folder inside, empty ones too, and `/NP` (no progress) leaves out a percentage for each file, which keeps a log readable. The report is a header, a line for each folder (with how many files it holds) and file (with its size in bytes), and a summary table. Your dates and speeds will be your own. ## Only what changed Change one file, add one, and run again. `/NJH` (no job header) leaves out the header you've already read: ```powershell Add-Content -Path school-work\science\lab-report.txt -Value 'Result: the salt water melted it twice as fast.' Set-Content -Path school-work\math\homework-week-11.txt -Value 'Problems 1 to 15, page 120' robocopy school-work backup-drive\school-work /E /NP /NJH $LASTEXITCODE ``` ```text 2 C:\Users\Sam\terminal-practice\school-work\ 2 C:\Users\Sam\terminal-practice\school-work\math\ New File 28 homework-week-11.txt 2 C:\Users\Sam\terminal-practice\school-work\science\ Newer 95 lab-report.txt ------------------------------------------------------------------------------ Total Copied Skipped Mismatch FAILED Extras Dirs : 3 0 3 0 0 0 Files : 6 2 4 0 0 0 Bytes : 325 123 202 0 0 0 Times : 0:00:00 0:00:00 0:00:00 0:00:00 Speed : 61,500 Bytes/sec. Speed : 3.519 MegaBytes/min. Ended : Monday, November 9, 2026 8:14:15 AM 1 ``` Two copied, four skipped: robocopy compares each file's size and date with the backup's copy and leaves matching files alone. Run it once more and it copies nothing, in a moment, whether the folder holds six files or sixty thousand. The `1` is its exit code; more on that below. ## /MIR with care, and /L first `/E` never deletes, so a file you delete from school-work stays in the backup forever. `/MIR` (mirror) is `/E` plus one more job: it deletes from the backup whatever is no longer in the source, so the backup becomes a **mirror**. Deleting is exactly why to rehearse first, with `/L` (list only), which lists what robocopy would do and changes nothing. Delete one practice file and rehearse: ```powershell Remove-Item school-work\reading-list.txt robocopy school-work backup-drive\school-work /MIR /NP /NJH /L $LASTEXITCODE Test-Path backup-drive\school-work\reading-list.txt ``` ```text 1 C:\Users\Sam\terminal-practice\school-work\ *EXTRA File 60 reading-list.txt 2 C:\Users\Sam\terminal-practice\school-work\math\ 2 C:\Users\Sam\terminal-practice\school-work\science\ ------------------------------------------------------------------------------ Total Copied Skipped Mismatch FAILED Extras Dirs : 3 0 3 0 0 0 Files : 5 0 5 0 0 1 Bytes : 265 0 265 0 0 60 Times : 0:00:00 0:00:00 0:00:00 0:00:00 Ended : Monday, November 9, 2026 8:14:15 AM 2 True ``` `*EXTRA File` means "in the backup, not in the source": with `/MIR`, it gets deleted. The file is still there, because `/L` only looked. The same line without `/L` deletes it. Now see why the rehearsal matters. A USB stick that comes up empty, or a typo that names the wrong folder, and `/MIR` makes the backup match nothing: ```powershell New-Item -ItemType Directory -Force -Path empty-stick | Out-Null robocopy empty-stick backup-drive\school-work /MIR /NP /NJH /L ``` ```text 0 C:\Users\Sam\terminal-practice\empty-stick\ *EXTRA Dir -1 C:\Users\Sam\terminal-practice\backup-drive\school-work\math\ *EXTRA File 28 homework-week-10.txt *EXTRA File 28 homework-week-11.txt *EXTRA Dir -1 C:\Users\Sam\terminal-practice\backup-drive\school-work\science\ *EXTRA File 95 lab-report.txt *EXTRA File 46 volcano.jpg *EXTRA File 68 history-essay.txt ------------------------------------------------------------------------------ Total Copied Skipped Mismatch FAILED Extras Dirs : 1 0 1 0 0 2 Files : 0 0 0 0 0 5 Bytes : 0 0 0 0 0 265 Times : 0:00:00 0:00:00 0:00:00 0:00:00 Ended : Monday, November 9, 2026 8:14:15 AM ``` Every file in the backup, listed for deleting. So: rehearse each new `/MIR` line with `/L`, keep the source first, and in a script check that the source is there and not empty before mirroring. ![Two rows. Top row: a folder named school-work holding history-essay.txt and lab-report.txt, an arrow labeled /MIR, and a folder named backup-drive holding the same two files and reading-list.txt crossed out, because it is no longer in school-work. Bottom row: an empty folder named empty-stick, an arrow labeled /MIR, and backup-drive with all three files crossed out](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/11/7bc1f24691e04833b3e766e2fdcb728e.webp) ## Exit codes: below 8 is success Most programs end with 0 for "it worked" (*Did It Work? `$?`, `$LASTEXITCODE`, `&&` and `||`*). robocopy's number says *what* it did, and only 8 and up mean trouble: | Exit code | Means | |---|---| | 0 | Nothing needed copying | | 1 | Files were copied | | 2 | Extra files in the destination (deleted by `/MIR`, or listed by `/L`) | | 3 | 1 and 2 together; 4 to 7 add mismatched files to the mix | | 8 | Some files or folders couldn't be copied | | 16 | A serious error: nothing was copied | A typo in the source is a 16: ```powershell robocopy school-wrok backup-drive\school-work /MIR /NP /NJH $LASTEXITCODE ``` ```text 2026/11/09 08:14:15 ERROR 2 (0x00000002) Accessing Source Directory C:\Users\Sam\terminal-practice\school-wrok\ The system cannot find the file specified. 16 ``` A scheduled task treats any number but 0 as a failure, so a script turns robocopy's answer into its own, right after robocopy runs: ```powershell robocopy $Source $Destination /MIR /R:2 /W:5 /NP if ($LASTEXITCODE -ge 8) { exit 1 } else { exit 0 } ``` `/R:2 /W:5` (read "retry twice, wait five seconds") matter in a script too. The first report's Options line showed robocopy's defaults, `/R:1000000 /W:30`: a file another program holds open would be tried a million times, 30 seconds apart, which is almost a year. A Mac or Linux has no robocopy; backups there use other tools. ## Check yourself 2 questions to try on the lesson page. ## So what Copy only what changed, mirror only after a rehearsal, and read the exit code the way robocopy means it: that's a backup you can schedule and stop thinking about. ## Key terms - [robocopy](https://www.ask-codey.com/glossary/robocopy) - [Backup](https://www.ask-codey.com/glossary/backup) - [Mirror Backup](https://www.ask-codey.com/glossary/mirror-backup) - [Dry run](https://www.ask-codey.com/glossary/dry-run) - [Exit Code](https://www.ask-codey.com/glossary/exit-code-operating-system) ## Cheat sheets - [Backups That Work: \`robocopy\`](https://www.ask-codey.com/cheat-sheets/powershell-robocopy-cheat-sheet) ## Related - [Did It Work? \`$?\`, \`$LASTEXITCODE\`, \`&&\` and \`||\`](https://www.ask-codey.com/lessons/powershell-did-it-work) - [Scheduled Tasks](https://www.ask-codey.com/lessons/powershell-scheduled-tasks) - [Scripts That Run Without You](https://www.ask-codey.com/lessons/powershell-unattended-scripts) Source: https://www.ask-codey.com/lessons/powershell-robocopy