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

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?

Solution

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 ($IsWindows ? '\\.\CON' : '/dev/tty')
```
  - GitHub issue #19827 is a green-lit, but as yet (as of PowerShell (Core) 7.3.x) unimplemented improvement that will allow simplifying
    ($IsWindows ? '\\.\CON': '/dev/tty') to -Host.
- 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.}