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

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.