【PowerShell】ログファイルをリアルタイム監視する方法｜ローテーション・再接続対応

$LogPath = "C:\Logs\application.log"

Get-Content -LiteralPath $LogPath -Tail 0 -Wait -Encoding utf8

$LogPath = "C:\Logs\application.log"
$message = "{0} INFO test message" -f (Get-Date -Format "yyyy-MM-dd HH:mm:ss")

Add-Content -LiteralPath $LogPath -Value $message -Encoding utf8

$LogPath = "C:\Logs\application.log"

Get-Content -LiteralPath $LogPath -Tail 20 -Wait -Encoding utf8

param(
    [string]$LogPath = "C:\Logs\application.log",
    [int]$RetrySeconds = 3,
    [string]$Encoding = "utf8"
)

while (-not (Test-Path -LiteralPath $LogPath -PathType Leaf)) {
    Write-Host "ログファイルの作成を待っています: $LogPath"
    Start-Sleep -Seconds $RetrySeconds
}

Write-Host "ログ監視を開始します: $LogPath"
Get-Content -LiteralPath $LogPath -Tail 0 -Wait -Encoding $Encoding

param(
    [string]$LogPath = "C:\Logs\application.log",
    [string]$Encoding = "utf8",
    [int]$CheckIntervalSeconds = 1,
    [int]$ReconnectDelaySeconds = 2
)

$ErrorActionPreference = "Stop"

function Wait-LogFile {
    while (-not (Test-Path -LiteralPath $LogPath -PathType Leaf)) {
        Write-Host "$(Get-Date -Format s) WAIT file=$LogPath"
        Start-Sleep -Seconds $ReconnectDelaySeconds
    }
}

while ($true) {
    $job = $null

    try {
        Wait-LogFile

        $file = Get-Item -LiteralPath $LogPath
        $generation = $file.CreationTimeUtc.Ticks

        Write-Host "$(Get-Date -Format s) CONNECT file=$LogPath"

        $job = Start-Job -ArgumentList $LogPath, $Encoding -ScriptBlock {
            param($Path, $TextEncoding)

            Get-Content -LiteralPath $Path `
                -Tail 0 `
                -Wait `
                -Encoding $TextEncoding `
                -ErrorAction Stop
        }

        while ($job.State -eq "Running") {
            Receive-Job -Job $job

            if (-not (Test-Path -LiteralPath $LogPath -PathType Leaf)) {
                Write-Warning "ログファイルが削除されました。再作成を待ちます。"
                break
            }

            $current = Get-Item -LiteralPath $LogPath
            if ($current.CreationTimeUtc.Ticks -ne $generation) {
                Write-Warning "ログファイルの世代変更を検知しました。再接続します。"
                break
            }

            Start-Sleep -Seconds $CheckIntervalSeconds
        }

        Receive-Job -Job $job

        if ($job.State -eq "Failed") {
            $reason = $job.ChildJobs[0].JobStateInfo.Reason
            Write-Warning "ログ監視が停止しました: $reason"
        }
    }
    catch {
        Write-Warning "監視処理でエラーが発生しました: $($_.Exception.Message)"
    }
    finally {
        if ($job) {
            Stop-Job -Job $job -ErrorAction SilentlyContinue
            Remove-Job -Job $job -Force -ErrorAction SilentlyContinue
        }
    }

    Start-Sleep -Seconds $ReconnectDelaySeconds
}

$LogDirectory = "C:\Logs"
$currentPath = $null

while ($true) {
    $todayPath = Join-Path $LogDirectory (
        "application-{0}.log" -f (Get-Date -Format "yyyyMMdd")
    )

    if ($todayPath -ne $currentPath) {
        Write-Host "監視対象を切り替えます: $todayPath"
        $currentPath = $todayPath
    }

    if (Test-Path -LiteralPath $currentPath -PathType Leaf) {
        Get-Content -LiteralPath $currentPath -Tail 20 -Encoding utf8
    }
    else {
        Write-Host "ログファイルの作成待ち: $currentPath"
    }

    Start-Sleep -Seconds 60
}

$LogPaths = @(
    "C:\Logs\web.log",
    "C:\Logs\batch.log"
)

$jobs = foreach ($path in $LogPaths) {
    Start-Job -ArgumentList $path -ScriptBlock {
        param($LogPath)

        Get-Content -LiteralPath $LogPath -Tail 0 -Wait -Encoding utf8 |
            ForEach-Object {
                "[{0}] {1}" -f (Split-Path $LogPath -Leaf), $_
            }
    }
}

try {
    while ($true) {
        $jobs | Receive-Job
        Start-Sleep -Seconds 1
    }
}
finally {
    $jobs | Stop-Job
    $jobs | Remove-Job -Force
}

プログラム:
powershell.exe

引数:
-NoProfile -ExecutionPolicy Bypass -File "C:\Scripts\watch-log-with-reconnect.ps1" -LogPath "C:\Logs\application.log" -Encoding utf8

開始:
C:\Scripts

$LogPath = "C:\Logs\application.log"

New-Item -ItemType Directory -Path (Split-Path $LogPath) -Force | Out-Null
Set-Content -LiteralPath $LogPath -Value "initial line" -Encoding utf8

1..3 | ForEach-Object {
    Start-Sleep -Seconds 2
    Add-Content -LiteralPath $LogPath -Value "before rotation $_" -Encoding utf8
}

Start-Sleep -Seconds 2
Move-Item -LiteralPath $LogPath -Destination "$LogPath.1" -Force
Set-Content -LiteralPath $LogPath -Value "new generation" -Encoding utf8

1..3 | ForEach-Object {
    Start-Sleep -Seconds 2
    Add-Content -LiteralPath $LogPath -Value "after rotation $_" -Encoding utf8
}