Copy selection to a clipboard in tmux
Tmux 2.4+ with vi copy mode bindings and xclip:
set-option -g mouse on
set-option -s set-clipboard off
bind-key -T copy-mode-vi MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel "xclip -se c -i"
For older tmux versions, emacs copy mode bindings (the default), or non-X platforms (i.e., no xclip), see the explanation below.
Explanation: First we need to enable the mouse option so tmux will capture the mouse and let us bind mouse events:
set-option -g mouse on
Gnome-terminal doesn't support setting the clipboard using xterm escape sequences so we should ensure the set-clipboard option is off:
set-option -s set-clipboard off
This option might be supported and enabled by default on iTerm2 (see set-clipboard in the tmux manual), which would explain the behavior on there.
We can then bind the copy mode MouseDragEnd1Pane "key", i.e., when the first mouse button is released after clicking and dragging in a pane, to a tmux command which takes the current copy mode selection (made by the default binding for MouseDrag1Pane) and pipes it to a shell command. This tmux command was copy-pipe before tmux 2.4, and has since changed to send-keys -X copy-pipe[-and-cancel]. As for the shell command, we simply need something which will set the contents of the system clipboard to whatever is piped to it; xclip is used to do this in the following commands. Some equivalent replacements for "xclip -selection clipboard -i" below on non-X platforms are "wl-copy" (Wayland), "pbcopy" (macOS), "clip.exe" (Windows, WSL), and "cat /dev/clipboard" (Cygwin, MinGW).
Tmux 2.4+:
# For vi copy mode bindings
bind-key -T copy-mode-vi MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel "xclip -selection clipboard -i"
# For emacs copy mode bindings
bind-key -T copy-mode MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel "xclip -selection clipboard -i"
Tmux 2.2 to 2.4:
# For vi copy mode bindings
bind-key -t vi-copy MouseDragEnd1Pane copy-pipe "xclip -selection clipboard -i"
# For emacs copy mode bindings
bind-key -t emacs-copy MouseDragEnd1Pane copy-pipe "xclip -selection clipboard -i"
Before tmux 2.2:
Copy after mouse drag support was originally added in Tmux 1.3 through setting the new mode-mouse option to on. Tmux 2.1 changed the mouse support to the familiar mouse key bindings, but did not have DragEnd bindings, which were introduced in 2.2. Thus, before 2.2 I believe the only method of setting the system clipboard on mouse drag was through the built-in use of xterm escape sequences (the set-clipboard option). This means that it's necessary to update to at least tmux 2.2 to obtain the drag-and-copy behavior for terminals that don't support set-clipboard, such as GNOME Terminal.
As an extension to the accepted answer, people often find that disturbing when releasing the mouse button exits the copy-mode (this is what happens with copy-pipe-and-cancel). See: https://github.com/tmux/tmux/issues/140 .
Therefore, in newer tmux (I use 2.6) we can use copy-pipe together with clear selection to copy to selection but do not exit the copy-mode:
bind-key -T copy-mode-vi MouseDragEnd1Pane send -X copy-pipe "xclip -selection clipboard -i" \; send -X clear-selection
Yet another extension is about using Shift key.
Hold Shift while selecting text with mouse. Now you get a standard right-click menu (keep holding or press Shift again) and you can use Ctrl+Shift+C and Ctrl+Shift+V to copy and paste. Copied text will be also available in system clipboard.
Tested on Ubuntu 18.04.1 with tmux 2.6.
Source: https://forum.upcase.com/t/tmux-ctrl-shift-c-and-ctrl-shift-v-bindings/1208.