Sunday, July 6, 2008

Importing Scripts as Libraries, Part Deux

In this post I showed how using the dotsource operator lets us run a script and import all of its named objects from the script scope into the global scope. The effect is as if instead of calling a script you just typed its contents at the prompt.

This set off a lightbulb in my head, and gave me the solution to something I'd been pondering for a while: how to import scripts as libraries (modules, etc, whatever we're calling them these days). You see, if I come up with a really neat function I don't want to have to cut and paste it into every script that uses it, and if I come up with a lot of content, I'd like to be able to share them with people.

This is what I came up with, and it incorporates a few new tricks I picked up along the way to trying to solve this problem. The desired result: I can type in 'import-script scriptname.ps1' and it is automatically imported without having to know the full path to the file.

Some highlights:
  • Test-Path is a nice cmdlet for checking the existence of a path on any PSDrive, so that means any Enviornment variable, registry key, file object, etc.
  • Exceptions work, but are really hard to get straight. Check out this link for some details.
  • The (, (command)) syntax is how I'm forcing the variable to be an array. I might get rid of it later, I think now that I'm using boolean comparison instead of looking for the length of the array to check if any files were found.
  • In most examples people use Write-Error instead of echo to print error messages, but I wanted to have the error show up clearly without all of the red words and extra garbage.
  • Note that I'm searching everything under the folder pointed to by the PSPATH environment variable. I point mine to C:\PSLibs by default.

function Import-Script ([string]$script) {
  $local:ReportErrorShowSource = 0

  # Clean up the Exception messages a bit
  trap [Exception] {
    $errmsg = "`n`tError importing '$script': "
    $errmsg += ($_.Exception.Message + "`n")
echo $errmsg

  # Check PSPATH
  if (-not (Test-Path "Env:pspath")) {
    throw "PSPath environment variable not set."
  }elseif (-not (Test-Path $env:pspath)) {
    throw "PSPATH environment variable points to invalid path." 

  $files = (, (dir $env:pspath $script -Recurse))

  # Make sure we find a single matching file
  if ($files.Length -gt 1) {
    throw ([string]$files.Length + " files of name '$script' found in PSPATH.")
  }elseif (-not $files) {
    throw "No files named '$script' found in PSPATH."

  # Do the needful
  . $files[0].FullName

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