X11 Clipboard Utilities

I use xsel.

OS Agnostic

I use the following scripts with Linux, macos and Cygwin under Windows

#!/bin/dash
# copy to clipboard
if [ -n "$DISPLAY" ]; then # X11
  cat "$@" | xsel -i -b
elif [ -d "/Applications" ]; then # macos
  cat "$@" | pbcopy
elif [ -d "/cygdrive/c/cygwin64" ]; then # cygwin
  cat "$@" > /dev/clipboard
else
  echo "Cannot copy as not gui" > /dev/stderr
fi

to copy, and

#!/bin/dash
if [ -n "$DISPLAY" ]; then # X11
  paste() { xsel -o -b; }
elif [ -d "/Applications" ]; then # macos
  paste() { pbpaste; }
elif [ -d "/cygdrive/c/cygwin64" ]; then # cygwin
  paste() { cat /dev/clipboard; }
else
  echo "Cannot paste as not gui" > /dev/stderr
fi
if [ -n "$1" ]; then
  paste | tee "$1"
else
  paste
fi

to paste.