Control and Shift modifiers: why does C-H get executed as C-h?

[thomastthai]

Using the information and configuration below, why does C-H (pressing Ctrl-Shift-h) is interpreted as C-h (pressing Ctrl-h), i.e. "Resized pane left 1" is displayed and the resize to the left is only by 1 instead of 15?

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Update: I may have found the answer. Limitations of Ctrl keys mentioned there are only 32 control characters. Which means C-H doesn't exist and probably gets translated by the terminal emulator to C-h.

There are only 32 ASCII control characters, so in most terminals there are only 32 Ctrl keys:

C-@ is ASCII 0;

C-a to C-z are ASCII 1 to ASCII 26;

C-[, C-, C-], C-^ and C-- are ASCII 27 to 31.

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The wiki section How tmux describes keys with modifiers mentioned multiple modifier keys can be combined:

How tmux describes keys with modifiers

tmux describes keys with modifiers with one of three prefixes:

C- for Ctrl keys;

M- for Meta keys;

S- for Shift keys.

These may be combined, so Ctrl and Meta and Left is C-M-Left.

Many keys with Shift have an alternative name, tmux uses this if it exists. So there is no S-a - it is just in uppercase: A. Similarly, S-Tab is BTab. The S- prefix is used for some function keys which have only one form, for example S-Left and S-Right.

A snippet of my tmux.conf file:

unbind-key -T prefix C-h
unbind-key -T prefix C-j
unbind-key -T prefix C-k
unbind-key -T prefix C-l
unbind-key -T prefix C-H
unbind-key -T prefix C-J
unbind-key -T prefix C-K
unbind-key -T prefix C-L
bind-key -r -T prefix C-h resize-pane -L 1 \; display-message "Resized pane left 1"
bind-key -r -T prefix C-j resize-pane -D 1 \; display-message "Resized pane down 1"
bind-key -r -T prefix C-k resize-pane -U 1 \; display-message "Resized pane up 1"
bind-key -r -T prefix C-l resize-pane -R 1 \; display-message "Resized pane right 1"
bind-key -r -T prefix C-H resize-pane -L 15 \; display-message "Resized pane left 15"
bind-key -r -T prefix C-J resize-pane -D 15 \; display-message "Resized pane down 15"
bind-key -r -T prefix C-K resize-pane -U 15 \; display-message "Resized pane up 15"
bind-key -r -T prefix C-L resize-pane -R 15 \; display-message "Resized pane right 15"

Limitations of Shift keys was also referenced.

## Limitations of Shift keys

Most ASCII keys have a Shift form marked on the keyboard which is sent when the key is pressed with Shift. For example on a UK QWERTY keyboard, pressing S-1 will send !. tmux doesn't know the keyboard layout, so it treats ! as ! not S-1. There is no way to express the key S-1 - ! is used instead.

Shift modifiers and the S- prefix are mostly reserved for function keys such as S-F1 or S-Left.

Using prefix2 table and setting up a similar configuration, the resize and message work as intended for all three combinations. Notice there is no C-H, C-J, C-K, and C-L, i.e. Ctrl-Shift-[h, j, k, l].

bind -T root M-Space switch-client -T prefix2

bind-key -r -T prefix2 h resize-pane -L
bind-key -r -T prefix2 j resize-pane -D
bind-key -r -T prefix2 k resize-pane -U
bind-key -r -T prefix2 l resize-pane -R

bind-key -r -T prefix2 H resize-pane -L 5
bind-key -r -T prefix2 J resize-pane -D 5
bind-key -r -T prefix2 K resize-pane -U 5
bind-key -r -T prefix2 L resize-pane -R 5

bind-key -r -T prefix2 C-h resize-pane -L 10
bind-key -r -T prefix2 C-j resize-pane -D 10
bind-key -r -T prefix2 C-k resize-pane -U 10
bind-key -r -T prefix2 C-l resize-pane -R 10