Test if executable is in path in PowerShell

In my script I'm about to run a command

pandoc -Ss readme.txt -o readme.html

But I'm not sure if pandoc is installed. So I would like to do (pseudocode)

if (pandoc in the path)
{
    pandoc -Ss readme.txt -o readme.html
}

How can I do this for real?

You can test through Get-Command (gcm)

if (Get-Command "pandoc.exe" -ErrorAction SilentlyContinue) 
{ 
   pandoc -Ss readme.txt -o readme.html
}

If you'd like to test the non-existence of a command in your path, for example to show an error message or download the executable (think NuGet):

if ((Get-Command "pandoc.exe" -ErrorAction SilentlyContinue) -eq $null) 
{ 
   Write-Host "Unable to find pandoc.exe in your PATH"
}

Try

(Get-Help gcm).description

in a PowerShell session to get information about Get-Command.

Here is a function in the spirit of David Brabant's answer with a check for minimum version numbers.

Function Ensure-ExecutableExists
{
    Param
    (
        [Parameter(Mandatory = $True)]
        [string]
        $Executable,

        [string]
        $MinimumVersion = ""
    )

    $CurrentVersion = (Get-Command -Name $Executable -ErrorAction Stop).Version

    If ($MinimumVersion)
    {
        $RequiredVersion = [version]$MinimumVersion

        If ($CurrentVersion -lt $RequiredVersion)
        {
            Throw "$($Executable) version $($CurrentVersion) does not meet requirements"
        }
    }
}

This allows you to do the following:

Ensure-ExecutableExists -Executable pscp -MinimumVersion "0.62.0.0"

It does nothing if the requirement is met or throws an error it isn't.