How to simultaneously capture external command output and print it to the terminal - powershell

Question

Can I pipe back from:
$OUTPUT = $(flutter build ios --release --no-codesign | tail -1)
I would like to get both the last line from the build AND show progress, something like
$OUTPUT = $(flutter build ios --release --no-codesign | out | tail -1)
where the hypothetical out utility would also send the output to the terminal.
Do you know how?

Answer 1

Note:
On Unix-like platforms, with external-program output, js2010's elegant tee /dev/tty solution is the simplest.
The solutions below, which also work on Windows, may be of interest for processing external-program output line by line in PowerShell.
A general solution that also works with the complex objects that PowerShell-native commands can output, requires different approaches:
In PowerShell (Core) 7+, use the following:
# PS v7+ only. Works on both Windows and Unix
... | Tee-Object ($IsWindows ? 'CON' : '/dev/tty')
In Windows PowerShell, where Tee-Object unfortunately doesn't support targeting CON, a proxy function that utilizes Out-Host is required - see this answer.
A PowerShell solution (given that the code in your question is PowerShell[1]):
I'm not sure how flutter reports its progress, but the following may work:
If everything goes to stdout:
$OUTPUT = flutter build ios --release --no-codesign | % {
Write-Host $_ # print to host (console)
$_ # send through pipeline
} | select -Last 1
Note: % is the built-in alias for ForEach-Object, and select the one for Select-Object.
If progress messages go to stderr:
$OUTPUT = flutter build ios --release --no-codesign 2>&1 | % {
Write-Host $_.ToString() # print to host (console)
if ($_ -is [string]) { $_ } # send only stdout through pipeline
} | select -Last 1
[1] As evidenced by the $ sigil in the variable name in the LHS of an assignment and the spaces around =
($OUTPUT = ), neither of which would work as intended in bash / POSIX-like shells.

Answer 2

I assume you mean bash because to my knowledge there is no tail in powershell.
Here's how you can see a command's output while still capturing it into a variable.
#!/bin/bash
# redirect the file descriptor 3 to 1 (stdout)
exec 3>&1
longRunningCmd="flutter build ios --release --no-codesign"
# use tee to copy the command's output to file descriptor 3 (stdout) while
# capturing 1 (stdout) into a variable
output=$(eval "$longRunningCmd" | tee >(cat - >&3) )
# last line of output
lastline=$(printf "%s" "$output" | tail -n 1)
echo "$lastline"

Answer 3

I use write-progress in the pipeline.
In order to keep readable pipeline, I wrote a function
function Write-PipedProgress{
<#
.SYNOPSIS
Insert this function in a pipeline to display progress bar to user
.EXAMPLE
$Result = (Get-250Items |
Write-PipedProgress -PropertyName Name -Activity "Audit services" -ExpectedCount 250 |
Process-ItemFurther)
>
[cmdletBinding()]
param(
[parameter(Mandatory=$true,ValueFromPipeline=$true)]
$Data,
[string]$PropertyName=$null,
[string]$Activity,
[int]$ExpectedCount=100
)
begin {
Write-Verbose "Starting $($MyInvocation.MyCommand)"
$ItemCounter = 0
}
process {
Write-Verbose "Start processing of $($MyInvocation.MyCommand)($Data)"
try {
$ItemCounter++
# (3) mitigate unexpected additional input volume"
if ($ItemCounter -lt $ExpectedCount) {
$StatusProperty = if ($propertyName) { $Data.$PropertyName } > > else { ""}
$StatusMessage = "Processing $ItemCounter th $StatusProperty"
$statusPercent = 100 * $ItemCounter / $ExpectedCount
Write-Progress -Activity $Activity -Status $StatusMessage -> > PercentComplete $statusPercent
} else {
Write-Progress -Activity $Activity -Status "taking longer than expected" -PercentComplete 99
}
# return input data to next element in pipe
$Data
} catch {
throw
}
finally {
Write-Verbose "Complete processing of $Data in > $($MyInvocation.MyCommand)"
}
}
end {
Write-Progress -Activity $Activity -Completed
Write-Verbose "Complete $($MyInvocation.MyCommand) - processed $ItemCounter items"
}
}
Hope this helps ;-)

Answer 4

I believe this would work, at least in osx or linux powershell (or even Windows Subsystem for Linux) that have these commands available. I tested it with "ls" instead of "flutter". Is there actually an "out" command?
$OUTPUT = bash -c 'flutter build ios --release --no-codesign | tee /dev/tty | tail -1'
Or, assuming tee isn't aliased to tee-object. Actually, tee-object would work too.
$OUTPUT = flutter build ios --release --no-codesign | tee /dev/tty | tail -1
It would work with the $( ) too, but you don't need it. In powershell, it's used to combine multiple pipelines.