put this in a header or in your do file:

cap mata : mata drop ps()
cap mata : mata drop pj()

cap mata:

function pj(string scalar a, string scalar b, string scalar result )
{
p = pathjoin(a,b)
st_local(result, p)
}

function ps(string scalar path, string scalar result)
{
string scalar base
string scalar tip
pathsplit(path,base,tip)
st_local(result, base)
}
end

then you can portably create paths like this:

mata: pj("root","dir","path")
mata: pj("`path'","dofile","path")
di "`path'"

would be less than a one liner in many other languages, but it works!

One thing I'd found is that as Stata and Stata users have expanded the files & directory tools, you can do most of this stuff within Stata without 'shelling out' to the OS. The advantage of using the Stata commands, rather than the OS commands, would be that your do-file would be cross-platform compatible (except for the file paths, which I'll talk about later).....not that this is something that everyone is worried about, but I work in a multi-platform environment, so I like to think about it for these types of set-ups...so, I guess your posting gives me a reason to think this through a bit.

So, you could use the Stata commands -copy- , -erase-, -rmdir- directly from the do-file rather than the OS-specific commands after a "!" .
The remaining issue for cross-platform compatibility of your script above would be the file paths. This makes me think that you could ask Stata to detect the OS (Windows or Mac) at the start of the do-file and then adjust accordingly, here's an example using your code:

// FIRST, DETECT THE OS, THEN CHOOSE THE APPROPRIATE FILE PATHS
if "`c(os)'" == "MacOSX" {
local data_from "/users/username/data/"
local data_to "\\nairobi\archive\\"
local data_folder1 experiment
}
if "`c(os)'" == "Windows" {
local data_from "D:\data\\"
local data_to "\\nairobi\archive\\"
local data_folder1 experiment
}

// assemble `from_here', `to_here' file paths
foreach where in from to {
local `where'_here "`data_`where''`data_folder1'"
/*
---> I think you could use the -pathjoin()- command too..but I havent tried it (?)
*/
di "path `where': ``where'_here'" // this is just for checking
}

// now move stuff
copy "`from_here'" "`to_here'" , replace //--> using the Stata command instead of OS command
erase "`from_here'" // --> You can use -rmdir- if the folder is empty

//Now, it should work on both MacOS/Linux & Windows.