How to copy a string to the clipboard from a script?

CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your system's clipboard tool.

#!/usr/bin/env wolframscript

string = "word";
RunProcess[{"bash", "-c", "echo '" <> string <> "'|pbcopy"}]

This copies string to the clipboard.
Additionally, Paste[] requires the FrontEnd, but there's a similar workaround. All together (name from @ConorCosnett):

#!/usr/bin/env wolframscript

string = "Programming is fun!";

copyToClipboardFromString[string_]:=RunProcess[{"bash", "-c", "echo '" <> string <> "'|pbcopy"}]
getClipboard[]:=RunProcess[{"bash", "-c", "pbpaste"}, "StandardOutput"]

copyToClipboardFromString[string]
Print[getClipboard[]]

Implemented on macOS. See here for implementations in Ubuntu (thanks to @ConorCosnett).

Wrapping the CopyToClipboard[] in a UsingFrontEnd[] allows us to use it in scripts:

#!/usr/bin/env wolframscript

string = "Pshhhkkkkkrrrrkakingkakingkakingtshchchchchchchchchdingdingding";
UsingFrontEnd[CopyToClipboard[string]];

If a FrontEnd is already running you may have to quit it, and then rerun this script

Windows 10 implementation

To simply steal from the answers of @conorcosnett and @maxcoplan the following seems to work for Windows:

string = "word";
Run["echo " <> string <> " | clip"];