Get output from LrTasks.execute(cmd)

Including output redirection to a temp file in the command line is the only documented way of capturing the output.  There is the undocumented LrRemoteCommunication which appears to start a task with pipes for reading and writing, but I haven't seen any reports of anyone figuring out how that works.

Here is the utility function I used for running commands:

--[[----------------------------------------------------------------------------
public int exitCode, string output, string errOutput
safeExecute (string commandLine [, boolean getOutput])
Executes the command line "commandLine"in the platform shell via
LrTasks.execute, working around a bug in execute() on Windows where quoted
program names aren't accepted.
If "getOutput" is true, "output" will contain standard out and standard
error and "errOutput" will be "".  If "getOutput" is "separate", then
"output" will contain standard out and "errOutput" will contain standard
error.  If "getOutput" is false, then both "output" and "errOutput" will be
"".
Returns in "exitCode" the exit code of the command line. If any errors
occur in safeExecute itself, "exitCode" will be -1, and "output" and
"errOutput" will be:
getOuptut == "separate": "", <error message>
otherwise:              <error message>, ""
------------------------------------------------------------------------------]]
function Util.safeExecute (commandLine, getOutput)
return LrFunctionContext.callWithContext ("", function (context)
    local outFile, errFile
    context:addCleanupHandler (function ()
        if outFile then LrFileUtils.delete (outFile) end
        if errFile then LrFileUtils.delete (errFile) end
        end)
       
    if getOutput then
        local uuid = LrUUID.generateUUID ()
        outFile = child (getStandardFilePath ("temp"), uuid .. ".out")
        commandLine = commandLine .. ' > "' .. outFile .. '"'
        if getOutput == "separate" then
            errFile = child (getStandardFilePath ("temp"), uuid .. ".err")
            commandLine = commandLine .. ' 2>"' .. errFile .. '"'
        else
            commandLine = commandLine .. ' 2>&1'
            end
        end
    if WIN_ENV then commandLine = '"' .. commandLine .. '"' end
    local exitStatus = LrTasks.execute (commandLine)
    local output, errOutput, success = "", ""
    local function outputErr (file, output)
        local err = string.format ("Couldn't read output:\n%s\n%s",
            file, output)
        if getOutput == "separate" then
            return -1, "", err
        else
            return -1, err, ""
            end
        end
    if outFile then
        success, output = pcall (LrFileUtils.readFile, outFile)
        if not success then return outputErr (outFile, output) end
        end
    if errFile then
        success, errOutput = pcall (LrFileUtils.readFile, errFile)
        if not success then return outputErr (errFile, errOutput) end
        end
    return exitStatus, output, errOutput
    end) end