$names | ForEach -Process { Write-Output $_}
$names | ForEach { Write-Output $_}
$names | % { Write-Output $_}
PowerShell’s three lookalike friends can confuse, especially for newcomers. Basically, you’ve got two entities:
- The ForEach-Object cmdlet, which has an alias ForEach (it also has the alias %). This is meant to operate in the pipeline, and it uses a? Process parameter that accepts a script block.
- The ForEach scripting construct. This has a specific syntax, is not intended to be used in the pipeline, and does not have an alias.
Here’s all three in action, in a very simplistic example:
$names = @("Julie","Abby","Ben","Kevin")
$names | ForEach-Object -Process { Write-Output $_}
###The output
PS C:\Windows\system32> $names = @("Julie","Abby","Ben","Kevin")
PS C:\Windows\system32> $names | ForEach-Object -Process { Write-Output $_}
Julie
Abby
Ben
KevinThe other type of foreach is a statement that once again processes each object in a collection and performs the code in the scriptblock for each item but this time a specific name is used to reference each item in the collection. An example is:
PS C:\Windows\system32> foreach ($name in $names) { Write-Output $name}
Julie
Abby
Ben
KevinThe big difference is that, in the pipeline, ForEach-Object processes one object at a time. That means it can be slower, since that scriptblock must be interpreted on each iteration. It also tends to use less memory, since objects streaming down the pipeline one at a time don’t all have to be bunched up in a variable first.
The scripting construct tends to be faster, but it often has more memory overhead, because you have to give it the entire collection of objects at once, instead of streaming objects into it one at a time.
Both use vaguely similar syntax, but there are differences. It’s important to understand that they are not the same, and that they execute differently. It’s confusing because ForEach is both an alias and a scripting construct. PowerShell determine which you’re using by looking at the context in which you’re using it.
Let’s compare the execution time between ForEach vs ForEach-Object.
$Time1 = (Measure-Command {Get-ChildItem C:\Windows -Recurse | ForEach-Object {$_.Name}}).TotalMilliseconds
$Time2 = (Measure-Command {foreach ($file in (Get-ChildItem C:\Windows -Recurse)) {$file.Name}}).TotalMilliseconds
Write-Output "Time using ForEach-Object is $Time1"
Write-Output "Time using ForEach statement is $Time2"
The total time though is still faster with foreach statement (but would use more memory).
Time using ForEach-Object is 74523.1784
Time using ForEach statement is 47353.5139
