font_rules

When textual output in the terminal is styled with bold, italic or other attributes, wezterm uses font_rules to decide how to render that text.

By default, unstyled text will use the font specified by the font configuration, and wezterm will use that as a base, and then automatically generate appropriate font_rules that use heavier weight fonts for bold text, lighter weight fonts for dim text and italic fonts for italic text.

Most users won't need to specify any explicit value for font_rules, as the defaults should be sufficient.

If you have some unusual fonts or mixtures of fonts that you'd like to use, such as using your favourite monospace font for the base and a fancy italic font from a different font family for italics, then font_rules will allow you to do so.

font_rules is comprised of a list of rule entries with fields that are split into matcher fields and action fields. Matcher fields specify which textual attributes you want to match on, while action fields specify how you want to render them.

The following fields are matcher fields:

Name	Associated Attribute	Possible Values
italic	italic	true (italic) or false (not italic)
intensity	bold/bright or dim/half-bright	"Normal" (neither bold nor dim), "Bold", "Half"
underline	underline	"None" (not underlined), "Single" (single underline), "Double" (double underline)
blink	blinking	"None" (not blinking), "Rapid" (regular rapid blinking), "Slow" (slow blinking)
reverse	reverse/inverse	true (reversed) or false (not reversed)
strikethrough	strikethrough	true (struck-through) or false (not struck-through)
invisible	invisible	true (invisible) or false (not invisible)

If a matcher field is omitted, then the associated attribute has no impact on the match: the rule doesn't care about that attribute and will match based on the attributes that were listed.

The following fields are action fields:

Name	Action
font	Specify the font that should be used

The way that font_rules are processed is:

1. Take the list of font_rules from your configuration
2. For each rule in the order listed in font_rules:
3. Consider each matcher field explicitly specified in the entry. If the associated attribute does not match the value specified in the entry, move on to the next rule.
4. If all of the matcher fields explicitly specified in the entry match the attributes of the text, then:
5. The font action field, if specified, will override the base font configuration
6. No further font_rules will be considered: the matching is complete
7. If none of the rules you specify matched, then a set of default rules based on your base font will be used in the same way as above.

Here's an example from my configuration file, which I use with a variant of Operator Mono that is patched to add ligatures. This particular font has font-weights that are either too bold or too light for the default rules to produce great results, hence this set of rules.

config.font = wezterm.font_with_fallback 'Operator Mono SSm Lig Medium'
config.font_rules = {
  -- For Bold-but-not-italic text, use this relatively bold font, and override
  -- its color to a tomato-red color to make bold text really stand out.
  {
    intensity = 'Bold',
    italic = false,
    font = wezterm.font_with_fallback(
      'Operator Mono SSm Lig',
      -- Override the color specified by the terminal output and force
      -- it to be tomato-red.
      -- The color value you set here can be any CSS color name or
      -- RGB color string.
      { foreground = 'tomato' }
    ),
  },

  -- Bold-and-italic
  {
    intensity = 'Bold',
    italic = true,
    font = wezterm.font_with_fallback {
      family = 'Operator Mono SSm Lig',
      italic = true,
    },
  },

  -- normal-intensity-and-italic
  {
    intensity = 'Normal',
    italic = true,
    font = wezterm.font_with_fallback {
      family = 'Operator Mono SSm Lig',
      weight = 'DemiLight',
      italic = true,
    },
  },

  -- half-intensity-and-italic (half-bright or dim); use a lighter weight font
  {
    intensity = 'Half',
    italic = true,
    font = wezterm.font_with_fallback {
      family = 'Operator Mono SSm Lig',
      weight = 'Light',
      italic = true,
    },
  },

  -- half-intensity-and-not-italic
  {
    intensity = 'Half',
    italic = false,
    font = wezterm.font_with_fallback {
      family = 'Operator Mono SSm Lig',
      weight = 'Light',
    },
  },
}

Here's another example combining FiraCode with Victor Mono, using Victor Mono only for italics:

config.font = wezterm.font { family = 'FiraCode' }

config.font_rules = {
  {
    intensity = 'Bold',
    italic = true,
    font = wezterm.font {
      family = 'VictorMono',
      weight = 'Bold',
      style = 'Italic',
    },
  },
  {
    italic = true,
    intensity = 'Half',
    font = wezterm.font {
      family = 'VictorMono',
      weight = 'DemiBold',
      style = 'Italic',
    },
  },
  {
    italic = true,
    intensity = 'Normal',
    font = wezterm.font {
      family = 'VictorMono',
      style = 'Italic',
    },
  },
}

Debugging Font Rules

You can run wezterm ls-fonts to summarize the font rules and the fonts that match them:

$ wezterm ls-fonts
Primary font:
wezterm.font_with_fallback({
  -- <built-in>, BuiltIn
  "JetBrains Mono",

  -- /home/wez/.fonts/NotoColorEmoji.ttf, FontConfig
  "Noto Color Emoji",
})


When Intensity=Half Italic=true:
wezterm.font_with_fallback({
  -- <built-in>, BuiltIn
  {family="JetBrains Mono", weight="ExtraLight", italic=true},

  -- /home/wez/.fonts/NotoColorEmoji.ttf, FontConfig
  "Noto Color Emoji",

  -- <built-in>, BuiltIn
  "JetBrains Mono",
})


When Intensity=Half Italic=false:
wezterm.font_with_fallback({
  -- <built-in>, BuiltIn
  {family="JetBrains Mono", weight="ExtraLight"},

  -- /home/wez/.fonts/NotoColorEmoji.ttf, FontConfig
  "Noto Color Emoji",

  -- <built-in>, BuiltIn
  "JetBrains Mono",
})


When Intensity=Bold Italic=false:
wezterm.font_with_fallback({
  -- <built-in>, BuiltIn
  {family="JetBrains Mono", weight="Bold"},

  -- /home/wez/.fonts/NotoColorEmoji.ttf, FontConfig
  "Noto Color Emoji",

  -- <built-in>, BuiltIn
  "JetBrains Mono",
})


When Intensity=Bold Italic=true:
wezterm.font_with_fallback({
  -- <built-in>, BuiltIn
  {family="JetBrains Mono", weight="Bold", italic=true},

  -- /home/wez/.fonts/NotoColorEmoji.ttf, FontConfig
  "Noto Color Emoji",

  -- <built-in>, BuiltIn
  "JetBrains Mono",
})


When Intensity=Normal Italic=true:
wezterm.font_with_fallback({
  -- <built-in>, BuiltIn
  {family="JetBrains Mono", italic=true},

  -- /home/wez/.fonts/NotoColorEmoji.ttf, FontConfig
  "Noto Color Emoji",

  -- <built-in>, BuiltIn
  "JetBrains Mono",
})