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Git Patches

A patch can carry a textual change for inspection or transfer. git apply changes files, while git am applies a mailbox-format patch and records commit metadata.

How it works

The desk creates a small committed reminder, exports it with format-patch and applies it on a new branch based on the initial commit. The patch stays inside the disposable lab. We verify content and the commit subject. No email is sent and no external contributor identity is verified by this exercise.

Run the disposable lab

Use Windows PowerShell 5.1 or later and Git 2.28 or later. The reviewed CLI is Git for Windows 2.53.0.windows.2. These are PowerShell examples, not Git Bash commands. Save the download as patches.ps1 in a separate empty learning folder and run it in a separate process: powershell -NoProfile -ExecutionPolicy Bypass -File .\patches.ps1. The execution-policy option applies to that process, not your machine policy.

The script creates bhylu-git-patches and refuses an existing folder. It supplies two fictional seed files and repository-local settings, then runs the lesson commands. The literal separator is quoted as '--' so PowerShell passes it through the guard. A guard function named git calls the real CLI and stops on unexpected failures; Expect-GitFailure checks deliberately refused operations. Child-process configuration isolates your personal Git settings. File transports are allowed only for owned local remotes; internet transports are disabled in this lab. No hosting account, Apache or MySQL service is required. There is no Git playground. Leave the generated folder for inspection; choose another empty parent to run again.

Download the complete patches.ps1 lab

Example

patches.ps1

# Bhylu Git lab: Patches
# Run in a separate PowerShell process, in an empty learning folder.
# Existing lab folders are refused; nothing is pushed to an internet host.
$ErrorActionPreference = 'Stop'
$script:GitExe = (Get-Command git -CommandType Application | Select-Object -First 1).Source
$script:LabRoot = Join-Path (Get-Location) 'bhylu-git-patches'
if (Test-Path -LiteralPath $script:LabRoot) { throw 'Lab folder already exists. Choose another empty parent folder.' }
New-Item -ItemType Directory -Path $script:LabRoot | Out-Null
$env:GIT_CONFIG_NOSYSTEM = '1'
$env:GIT_CONFIG_GLOBAL = Join-Path $script:LabRoot 'empty-global-config'
[IO.File]::WriteAllText($env:GIT_CONFIG_GLOBAL, '')
$env:GIT_TERMINAL_PROMPT = '0'
$env:GIT_EDITOR = 'true'
$env:GIT_SEQUENCE_EDITOR = 'true'
$env:GIT_AUTHOR_DATE = '2026-01-10T10:00:00+00:00'
$env:GIT_COMMITTER_DATE = $env:GIT_AUTHOR_DATE
function git {
    & $script:GitExe -c protocol.allow=never -c protocol.file.allow=always @args
    if ($LASTEXITCODE -ne 0) { throw "Git exited with code $LASTEXITCODE" }
}
function Expect-GitFailure {
    & $script:GitExe -c protocol.allow=never -c protocol.file.allow=always @args
    if ($LASTEXITCODE -eq 0) { throw 'Expected Git to refuse this operation' }
}
function Assert-Equal($Actual, $Expected) {
    if ((($Actual | Out-String).Trim()) -cne (($Expected | Out-String).Trim())) { throw "Expected [$Expected], got [$Actual]" }
}
function Write-LabFile([string]$Name, [string]$Text) {
    $target = [IO.Path]::GetFullPath((Join-Path (Get-Location) $Name))
    if (!$target.StartsWith($script:LabRoot + [IO.Path]::DirectorySeparatorChar, [StringComparison]::OrdinalIgnoreCase)) { throw 'Path outside lab' }
    [IO.Directory]::CreateDirectory([IO.Path]::GetDirectoryName($target)) | Out-Null
    [IO.File]::WriteAllText($target, ($Text.Replace('\n', "`n") + "`n"), (New-Object Text.UTF8Encoding($false)))
}
function New-LabRepo([string]$Name) {
    New-Item -ItemType Directory -Path $Name | Out-Null
    Push-Location $Name
    try {
        git init --initial-branch=main
        git config --local user.name 'Repair Lab'
        git config --local user.email 'repair@example.invalid'
        git config --local core.autocrlf false
        git config --local commit.gpgSign false
        git config --local tag.gpgSign false
        git config --local core.hooksPath (Join-Path $script:LabRoot 'empty-hooks')
        Write-LabFile 'notes.md' 'Inspect hinge'
        Write-LabFile 'tasks.txt' 'desk=open'
        git add '--' notes.md tasks.txt
        git commit -m 'Record initial repair plan'
    } finally { Pop-Location }
}
Push-Location $script:LabRoot
try {
    New-LabRepo 'desk'
    Set-Location desk
    git switch -c reminder
    Write-LabFile 'reminder.txt' 'Bring the repair receipt'
    git add '--' 'reminder.txt'
    git commit -m 'Add collection reminder'
    git format-patch -1 --output-directory ../patches
    $patch = (Get-ChildItem ../patches/*.patch).FullName
    git switch -c applied main
    git am $patch
    Assert-Equal (git show HEAD:reminder.txt) @('Bring the repair receipt')
    Assert-Equal (git log -1 --format=%s) @('Add collection reminder')
    Write-Output 'Verified: patches'
} finally { Pop-Location }
Focus on the lesson commands

This excerpt omits the seed repository and guard helpers. Use the complete download to run it.

Lesson excerpt

git switch -c reminder
Write-LabFile 'reminder.txt' 'Bring the repair receipt'
git add '--' 'reminder.txt'
git commit -m 'Add collection reminder'
git format-patch -1 --output-directory ../patches
$patch = (Get-ChildItem ../patches/*.patch).FullName
git switch -c applied main
git am $patch
Assert-Equal (git show HEAD:reminder.txt) @('Bring the repair receipt')
Assert-Equal (git log -1 --format=%s) @('Add collection reminder')

Expected result

The applied branch gains the reminder with its original subject through git am.

The complete script ends with Verified: patches only after its checks pass. Object IDs, temporary paths and progress output can differ.

For an uncommitted patch, apply --check helps detect whether the patch can apply before changing files. Applying a patch does not make its content trustworthy; review executable scripts, dependencies and unexpected paths. Use am --abort if a mailbox application should be abandoned.

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