在 Windows 中获取大文件的最后 n 行或字节（如 Unix 的尾部）。避免耗时的选项
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Get last n lines or bytes of a huge file in Windows (like Unix's tail). Avoid time consuming options
提问by sancho.s ReinstateMonicaCellio
I need to retrieve the last n lines of huge files (1-4 Gb), in Windows 7. Due to corporate restrictions, I cannot run any command that is not built-in. The problem is that all solutions I found appear to read the whole file, so they are extremely slow.
我需要在 Windows 7 中检索最后 n 行大文件（1-4 Gb）。由于公司限制，我无法运行任何非内置命令。问题是我找到的所有解决方案似乎都读取了整个文件，因此它们非常慢。
Can this be accomplished, fast?
这能很快完成吗？
Notes:
笔记：
- I managed to get the first n lines, fast.
- It is ok if I get the last n bytes. (I used this https://stackoverflow.com/a/18936628/2707864for the first n bytes).
- 我设法快速获得前 n 行。
- 如果我得到最后 n 个字节就可以了。（我使用这个https://stackoverflow.com/a/18936628/2707864作为前 n 个字节）。
Solutions here Unix tail equivalent command in Windows Powershelldid not work.
Using -waitdoes not make it fast. I do not have -tail(and I do not know if it will work fast).
此处的解决方案Windows Powershell 中的 Unix tail 等效命令不起作用。使用-wait并不能使它变快。我没有-tail（我不知道它是否会很快工作）。
PS: There are quite a few related questions for headand tail, but not focused on the issue of speed. Therefore, useful or accepted answers there may not be useful here. E.g.,
PS:head和相关的问题比较多tail，但没有集中在速度问题上。因此，有用的或被接受的答案在这里可能没有用。例如，
Windows equivalent of the 'tail' command
CMD.EXE batch script to display last 10 lines from a txt file
CMD.EXE 批处理脚本显示 txt 文件的最后 10 行
Extract N lines from file using single windows command
powershell to get the first x MB of a file
https://superuser.com/questions/859870/windows-equivalent-of-the-head-c-command
https://superuser.com/questions/859870/windows-equivalent-of-the-head-c-command
采纳答案by Aziz Kabyshev
How about this (reads last 8 bytes for demo):
这个怎么样（读取演示的最后 8 个字节）：
$fpath = "C:GBfile.dat"
$fs = [IO.File]::OpenRead($fpath)
$fs.Seek(-8, 'End') | Out-Null
for ($i = 0; $i -lt 8; $i++)
{
$fs.ReadByte()
}
UPDATE. To interpret bytes as string (but be sure to select correct encoding - here UTF8 is used):
更新。要将字节解释为字符串（但请确保选择正确的编码 - 此处使用 UTF8）：
$N = 8
$fpath = "C:GBfile.dat"
$fs = [IO.File]::OpenRead($fpath)
$fs.Seek(-$N, [System.IO.SeekOrigin]::End) | Out-Null
$buffer = new-object Byte[] $N
$fs.Read($buffer, 0, $N) | Out-Null
$fs.Close()
[System.Text.Encoding]::UTF8.GetString($buffer)
UPDATE 2. To read last M lines, we'll be reading the file by portions until there are more than Mnewline char sequences in the result:
更新 2。要读取最后 M 行，我们将按部分读取文件，直到结果中有超过 M 个换行符序列：
$M = 3
$fpath = "C:GBfile.dat"
$result = ""
$seq = "`r`n"
$buffer_size = 10
$buffer = new-object Byte[] $buffer_size
$fs = [IO.File]::OpenRead($fpath)
while (([regex]::Matches($result, $seq)).Count -lt $M)
{
$fs.Seek(-($result.Length + $buffer_size), [System.IO.SeekOrigin]::End) | Out-Null
$fs.Read($buffer, 0, $buffer_size) | Out-Null
$result = [System.Text.Encoding]::UTF8.GetString($buffer) + $result
}
$fs.Close()
($result -split $seq) | Select -Last $M
Try playing with bigger $buffer_size- this ideally is equal to expected average line length to make fewer disk operations. Also pay attention to $seq - this could be \r\nor just \n.
This is very dirty code without any error handling and optimizations.
尝试使用更大的$buffer_size- 理想情况下，这等于预期的平均行长度，以减少磁盘操作。还要注意 $seq - 这可能是\r\n或只是\n. 这是非常脏的代码，没有任何错误处理和优化。
回答by alroc
If you have PowerShell 3 or higher, you can use the -Tailparameter for Get-Contentto get the last nlines.
如果你有 PowerShell 3 或更高版本，你可以使用-Tail参数 forGet-Content来获取最后n几行。
Get-content -tail 5 PATH_TO_FILE;
On a 34MB text file on my local SSD, this returned in 1 millisecond vs. 8.5 seconds for get-content |select -last 5
在我本地 SSD 上的 34MB 文本文件中，返回时间为 1 毫秒，而不是 8.5 秒 get-content |select -last 5
回答by sancho.s ReinstateMonicaCellio
With the awesome answer by Aziz Kabyshev, which solves the issue of speed, and with some googling, I ended up using this script
随着阿齐兹Kabyshev的真棒答案，解决了速度的问题，并与一些google搜索，我结束了使用这个脚本
$fpath = $Args[1]
$fs = [IO.File]::OpenRead($fpath)
$fs.Seek(-$Args[0], 'End') | Out-Null
$mystr = ''
for ($i = 0; $i -lt $Args[0]; $i++)
{
$mystr = ($mystr) + ([char[]]($fs.ReadByte()))
}
$fs.Close()
Write-Host $mystr
which I call from a batch file containing
我从包含的批处理文件中调用
@PowerShell -NoProfile -ExecutionPolicy Bypass -Command "& '.\myscript.ps1' %1 %2"
(thanks to How to run a PowerShell script from a batch file).
回答by Aacini
This is not an answer, but a large comment as reply to sancho.s' answer.
这不是答案，而是作为对 sancho.s 答案的回复的大评论。
When you want to use small PowerShell scripts from a Batch file, I suggest you to use the method below, that is simpler and allows to keep all the code in the same Batch file:
当您想使用批处理文件中的小型 PowerShell 脚本时，我建议您使用以下方法，该方法更简单，并且允许将所有代码保存在同一个批处理文件中：
@PowerShell ^
$fpath = %2; ^
$fs = [IO.File]::OpenRead($fpath); ^
$fs.Seek(-%1, 'End') ^| Out-Null; ^
$mystr = ''; ^
for ($i = 0; $i -lt %1; $i++) ^
{ ^
$mystr = ($mystr) + ([char[]]($fs.ReadByte())); ^
} ^
Write-Host $mystr
%End PowerShell%
