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Overview

tmux includes a sophisticated terminal feature detection system that automatically identifies and enables advanced terminal capabilities. This allows tmux to provide enhanced functionality when the underlying terminal supports it.

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Terminal Features System

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Feature Detection

tmux detects terminal features through multiple mechanisms:
  • terminfo capabilities: Standard terminal database entries
  • Terminal type identification: Recognizing specific terminal emulators
  • Device Attributes (DA): Querying the terminal at runtime
  • Manual specification: Using the terminal-features option
The feature detection code is implemented in tty-features.c.

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Available Terminal Features

tmux supports the following terminal features:

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Color Support

# Enable 256 color support
set -g terminal-features "xterm*:256"
From tty-features.c:116-128, the RGB feature includes:
  • setrgbf: Set RGB foreground color (\E[38;2;%p1%d;%p2%d;%p3%dm)
  • setrgbb: Set RGB background color (\E[48;2;%p1%d;%p2%d;%p3%dm)
  • Support for 256-color palette alongside RGB

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Text Attributes

Enables strikethrough text formatting:
set -g terminal-features "*:strikethrough"
Implemented as escape sequence: \E[9m
Enables overline text attribute:
set -g terminal-features "*:overline"
Sequence: Smol=\E[53m (from tty-features.c:144-152)
Advanced underscore styling with colors:
set -g terminal-features "*:usstyle"
Supports:
  • Different underscore types (single, double, curly, dotted, dashed)
  • Underscore colors independent of text color
  • RGB colors for underscores
From tty-features.c:155-166:
  • Smulx: Set underscore style
  • Setulc: Set underscore RGB color
  • Setulc1: Set underscore palette color

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Advanced Features

Title Setting

Window title escape sequences:
set -g terminal-features "*:title"
From tty-features.c:51-60:
  • tsl=\E]0; - To status line
  • fsl=\a - From status line

Clipboard

OSC 52 clipboard integration:
set -g terminal-features "*:clipboard"
Sequence: Ms=\E]52;%p1%s;%p2%s\a (tty-features.c:86-94)

Mouse Support

Xterm-style mouse tracking:
set -g terminal-features "*:mouse"
Capability: kmous=\E[M (tty-features.c:75-83)

Focus Reporting

Focus in/out events:
set -g terminal-features "*:focus"
From tty-features.c:181-190:
  • Enable: Enfcs=\E[?1004h
  • Disable: Dsfcs=\E[?1004l

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Cursor Customization

Cursor Styles
# Enable cursor style changes
set -g terminal-features "*:cstyle"
From tty-features.c:193-202:
  • Ss=\E[%p1%d q - Set cursor style
  • Se=\E[2 q - Reset cursor style
Supports various cursor shapes: block, underline, bar, with blinking variants.
Cursor Colors
# Enable cursor color customization
set -g terminal-features "*:ccolour"
From tty-features.c:205-214:
  • Cs=\E]12;%p1%s\a - Set cursor color
  • Cr=\E]112\a - Reset cursor color

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Modern Terminal Features

Enhanced keyboard protocol support:
set -g terminal-features "*:extkeys"
From tty-features.c:239-248:
  • Enable: Eneks=\E[>4;2m
  • Disable: Dseks=\E[>4m
Allows distinguishing more key combinations.

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Graphics Features

Graphics features require terminal support and compile-time configuration.
Sixel Graphics
set -g terminal-features "*:sixel"
From tty-features.c:350-358, enables sixel image support when compiled with the appropriate libraries.
DECFRA Rectangle Fill
set -g terminal-features "*:rectfill"
From tty-features.c:265-273, enables fast rectangle filling operations:
  • Capability: Rect
  • Used for optimized screen redraws
DECSLRM Margins
set -g terminal-features "*:margins"
From tty-features.c:251-262, scroll region margins:
  • Enable: Enmg=\E[?69h
  • Disable: Dsmg=\E[?69l
  • Set margins: Cmg=\E[%i%p1%d;%p2%ds

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Default Terminal Configurations

tmux includes predefined feature sets for popular terminals (from tty-features.c:468-514):

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Modern Terminals Base Set

Most modern terminals support: 
256,RGB,bpaste,clipboard,mouse,strikethrough,title
When running tmux inside tmux:
256,RGB,bpaste,clipboard,mouse,strikethrough,title,
ccolour,cstyle,focus,overline,usstyle,hyperlinks

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Configuration Examples

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Override Terminal Type

Force specific features for all terminals:
~/.tmux.conf
# Enable all supported features for any xterm-compatible terminal
set -g terminal-features "xterm*:256:RGB:cstyle:ccolour:clipboard:title:mouse:focus"

# Enable features for tmux terminals specifically
set -ga terminal-features "tmux*:hyperlinks:sync:usstyle:overline"

# Enable strikethrough for all terminals
set -ga terminal-features "*:strikethrough"

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Working Directory Tracking

OSC 7 Support
# Enable working directory tracking
set -g terminal-features "*:osc7"

# Capability from tty-features.c:63-72:
# Swd=\E]7; fsl=\a

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Disable Specific Features

To disable features that cause problems: 
# Don't use terminal's function keys (use tmux built-ins)
set -g terminal-features "rxvt*:ignorefkeys"
From tty-features.c:276-347, this disables kf0 through kf63 terminal capabilities.

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Feature Application

1

Feature Detection

tmux applies features in this order:
  1. Default features based on $TERM environment variable
  2. Device Attributes response (if available)
  3. User-specified terminal-features option
2

Capability Override

From tty-features.c:434-464, when features are applied:
for (i = 0; i < nitems(tty_features); i++) {
    if ((term->features & (1 << i)) || (~feat & (1 << i)))
        continue;
    tf = tty_features[i];
    
    log_debug("applying terminal feature: %s", tf->name);
    if (tf->capabilities != NULL) {
        capability = tf->capabilities;
        while (*capability != NULL) {
            log_debug("adding capability: %s", *capability);
            tty_term_apply(term, *capability, 1);
            capability++;
        }
    }
    term->flags |= tf->flags;
}
3

Verification

Check active features:
# From outside tmux
tmux show -g terminal-features

# Check detected features in server info
tmux info | grep features

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Best Practices

Let tmux auto-detect features when possible - manual overrides may cause issues
Test feature changes thoroughly - some terminals report capabilities they don’t fully support
Use wildcards carefully - * matches all terminal types
Some features interact with terminal settings. For example:
  • extkeys requires terminal support for the extended keyboard protocol
  • sync may cause artifacts on slow connections
  • margins can conflict with certain terminal scrollback implementations

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Debugging Features

Enable tmux logging to see feature detection: 
# Start tmux with verbose logging
tmux -vvv new-session

# Check tmux-client-*.log for lines like:
# "adding terminal feature: RGB"
# "adding capability: setrgbf=..."