powershell 是如何懒洋洋地评价这个说法的？

Question

I was searching for a way to to read only the first few lines of a csv file and came across this answer . The accepted answer suggests using

Get-Content "C:\start.csv" | select -First 10 | Out-File "C:\stop.csv"

Another answers suggests using

Get-Content C:\Temp\Test.csv -TotalCount 3

Because my csv is fairly large I went with the second option. It worked fine. Out of curiosity I decided to try the first option assuming I could ctrl+c if it took forever. I was surprised to see that it returned just as quickly.

Is it safe to use the first approach when working with large files? How does powershell achieve this?

Answer 1

Yes, Select-Object -First n is "safe" for large files (provided you want to read only a small number of lines, so pipeline overhead will be insignificant, else Get-Content -TotalCount n will be more efficient).

It works like break in a loop, by exiting the pipeline early, when the given number of items have been processed. Internally it throws a special exception that the PowerShell pipeline machinery recognizes.

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Here is a demonstration that "abuses" Select-Object to break from a ForEach-Object "loop", which is not possible using normal break statement.

1..10 | ForEach-Object {
   Write-Host $_             # goes directly to console, so is ignored by Select-Object
   if( $_ -ge 3 ) { $true }  # "break" by outputting one item
} | Select-Object -First 1 | Out-Null

Output:

1
2
3

As you can see, Select-Object -First n actually breaks the pipeline instead of first reading all input and then selecting only the specified number of items.

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Another, more common use case is when you want to find only a single item in the output of a pipeline. Then it makes sense to exit from the pipeline as soon as you have found that item:

Get-ChildItem -Recurse | Where-Object { SomeCondition } | Select-Object -First 1

Answer 2

According to Microsoft the Get-Content cmdlet has a parameter called -ReadCount. Their documentation states

Specifies how many lines of content are sent through the pipeline at a time. The default value is 1. A value of 0 (zero) sends all of the content at one time.

This parameter does not change the content displayed, but it does affect the time it takes to display the content. As the value of ReadCount increases, the time it takes to return the first line increases, but the total time for the operation decreases. This can make a perceptible difference in large items.

Since -ReadCount defaults to 1 Get-Content effectively acts as a generator for reading a file line-by-line.