Differential backup with 7zip and Powershell

backup.ps1

# CONFIGURATION
$dirToBackup = "C:\Users\John" # path to directory we back up (no following backslash)
$outputDir = "E:\bak" # path directory we store our backups (no following backslash)
$params = '-t7z', '-r', '-ms=off', '-mx1'

# THE SCRIPT
$fullBackup = $outputDir + "\full.7z"

if (Test-Path  ($fullBackup)) { # Let's check whether full backup exists
  Write-Host "Full backup already exists"
  $args = ,'u' + $params
  $args += '-u-', "-up0q3r2x2y2z0w2!`"$($outputDir)\diff-$(Get-Date -format "yyyyMMdd-HHmmss").7z`""
  $args += $fullBackup, $dirToBackup
} else {
  $args = , ('a') + $params + $fullBackup + $dirToBackup
}

& 7z $args

Thanks for the inspiration!

Note: Having $args in an array is essential to having the !difffile passed to 7z with quotes around it. Otherwise the parser will strip them off and 7z will throw an error.

<#
https://sevenzip.osdn.jp/chm/cmdline/switches/method.htm

-t7z - use a .7z archive
-r - recurse subdirectories
-ms=off - disable solid mode for quicker compression
-mx1 - level 1 compression LZMA2	64 KB	32	HC4	BCJ	Fastest compressing

#>

<#
-u update options
    p0 - if exists in archive, ignore
    q3 - if exists in archive BUT NOT DISK, item that will note deletion
    r2 - if NOT IN ARCHIVE but on disk, compress
    x2 - if NEWER in archive, compress
    y2 - if NEWER in disk, compress
    z0 - if same, ignore
    w2 - if cannot decide which is newer, compress
    !

#> 

Alternative way: https://github.com/mitoteam/mtsaver