tmux: add a prefix option overruling shortcut if defined

Previously, it was not possible to set an arbitrary tmux prefix since
CTRL was hardcoded in the module.

To avoid breaking existing configs, a new option was implemented that
conveniently uses the tmux terminology but defaults to null and does
not affect previous behavior when set to null.

The behavior for the shortcut option was not completely replicated,
i.e., it does not bind "b" to send-prefix but stick to the default of
the prefix binding sending prefix (C-b C-b instead of C-b b) and it
does not bind repetition of the prefix (C-b C-b) to `last-window`,
both of these bring the option closer to the default tmux
configuration.

Fixes #1237

modules/programs/tmux.nix

@@ -68,13 +68,21 @@ let
       bind -r L resize-pane -R ${toString cfg.resizeAmount}
     ''}
 
-    ${optionalString (cfg.shortcut != defaultShortcut) ''
-      # rebind main key: C-${cfg.shortcut}
-      unbind C-${defaultShortcut}
-      set -g prefix C-${cfg.shortcut}
-      bind ${cfg.shortcut} send-prefix
-      bind C-${cfg.shortcut} last-window
-    ''}
+    ${if cfg.prefix != null
+        then ''
+          # rebind main key: ${cfg.prefix}
+          unbind C-${defaultShortcut}
+          set -g prefix ${cfg.prefix}
+          bind ${cfg.prefix} send-prefix
+        ''
+        else optionalString (cfg.shortcut != defaultShortcut) ''
+          # rebind main key: C-${cfg.shortcut}
+          unbind C-${defaultShortcut}
+          set -g prefix C-${cfg.shortcut}
+          bind ${cfg.shortcut} send-prefix
+          bind C-${cfg.shortcut} last-window
+        ''
+     }
 
     ${optionalString cfg.disableConfirmationPrompt ''
       bind-key & kill-window
@@ -238,6 +246,15 @@ in
         '';
       };
 
+      prefix = mkOption {
+        default = null;
+        example = "C-a";
+        type = types.nullOr types.str;
+        description = ''
+          Set the prefix key. Overrules the "shortcut" option when set.
+        '';
+      };
+
       shortcut = mkOption {
         default = defaultShortcut;
         example = "a";

tests/modules/programs/tmux/default.nix

@@ -5,4 +5,6 @@
   tmux-secure-socket-enabled = ./secure-socket-enabled.nix;
   tmux-disable-confirmation-prompt = ./disable-confirmation-prompt.nix;
   tmux-default-shell = ./default-shell.nix;
+  tmux-shortcut-without-prefix = ./shortcut-without-prefix.nix;
+  tmux-prefix = ./prefix.nix;
 }

tests/modules/programs/tmux/prefix.conf

@@ -0,0 +1,32 @@
+# ============================================= #
+# Start with defaults from the Sensible plugin  #
+# --------------------------------------------- #
+run-shell @sensible_rtp@
+# ============================================= #
+
+set  -g default-terminal "screen"
+set  -g base-index      0
+setw -g pane-base-index 0
+
+
+
+
+
+set -g status-keys emacs
+set -g mode-keys   emacs
+
+
+
+# rebind main key: C-a
+unbind C-b
+set -g prefix C-a
+bind C-a send-prefix
+
+
+
+
+setw -g aggressive-resize off
+setw -g clock-mode-style  12
+set  -s escape-time       500
+set  -g history-limit     2000
+

tests/modules/programs/tmux/prefix.nix

@@ -0,0 +1,26 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.tmux = {
+      enable = true;
+      prefix = "C-a";
+    };
+
+    nixpkgs.overlays = [
+      (self: super: {
+        tmuxPlugins = super.tmuxPlugins // {
+          sensible = super.tmuxPlugins.sensible // { rtp = "@sensible_rtp@"; };
+        };
+      })
+    ];
+
+    nmt.script = ''
+      assertFileExists home-files/.tmux.conf
+      assertFileContent home-files/.tmux.conf \
+        ${./prefix.conf}
+    '';
+  };
+}

tests/modules/programs/tmux/shortcut-without-prefix.conf

@@ -0,0 +1,33 @@
+# ============================================= #
+# Start with defaults from the Sensible plugin  #
+# --------------------------------------------- #
+run-shell @sensible_rtp@
+# ============================================= #
+
+set  -g default-terminal "screen"
+set  -g base-index      0
+setw -g pane-base-index 0
+
+
+
+
+
+set -g status-keys emacs
+set -g mode-keys   emacs
+
+
+
+# rebind main key: C-a
+unbind C-b
+set -g prefix C-a
+bind a send-prefix
+bind C-a last-window
+
+
+
+
+setw -g aggressive-resize off
+setw -g clock-mode-style  12
+set  -s escape-time       500
+set  -g history-limit     2000
+

tests/modules/programs/tmux/shortcut-without-prefix.nix

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.tmux = {
+      enable = true;
+      shortcut = "a";
+      prefix = null;
+    };
+
+    nixpkgs.overlays = [
+      (self: super: {
+        tmuxPlugins = super.tmuxPlugins // {
+          sensible = super.tmuxPlugins.sensible // { rtp = "@sensible_rtp@"; };
+        };
+      })
+    ];
+
+    nmt.script = ''
+      assertFileExists home-files/.tmux.conf
+      assertFileContent home-files/.tmux.conf \
+        ${./shortcut-without-prefix.conf}
+    '';
+  };
+}