A format string is a text string that contains placeholders for values. These placeholders begin with a '{', contain the name or index of the value, and end with a '}'.

For example:

"Values: {1} ({2})" with values "First" and "My second val" produces "Values: First (My second val)" 

Format strings can contain placeholders in any order and multiple occurrences of the same placeholder.

When setting a format string from the command line that contains spaces, surround it with double quotes:

fastfetch --title-format "Hello, {user-name}" 

Value indices can be meaningful named tags instead of numbers:

"--title-format '{user-name-colored}{at-symbol-colored}{host-name-colored}'" 

This is equivalent to using numerical indices. See module-specific help for supported tags:

fastfetch -h title-format 

Using named tags is strongly recommended because they are meaningful and don't affected by ordering.

You can specify a truncation length using the syntax '{arg:trunc-length}':

"--title-format '{user-name:5}'" → truncates user name to 5 characters 

If 'trunc-length' is negative, an ellipsis (…) will be appended when truncated.

Note: String length is counted in raw bytes; multi-byte Unicode characters and ANSI escape codes may not be counted as expected.

Use '<' or '>' instead of ':' to set left or right padding:

"{user-name<20}" → left-aligned with spaces: "<user-name> " "{user-name>20}" → right-aligned with spaces: " <user-name>" 

Use '{~startIndex,endIndex}' to slice a string:

"{~0,5}" → first five characters "{~-5,}" → last five characters "{~2,-2}" → from third character to second-to-last character 

Negative indices count backward from the end of the string. If an index is omitted, 0 is used.

You can reference constants and environment variables:

"{$NUM}" → reference a constant defined in `display.constants` "{$ENV_VAR}" → reference an environment variable 

If a value index is missing (empty placeholder: "{}"), an internal counter automatically assigns the next sequential value:

"Values: {} ({})" → equivalent to "Values: {1} ({2})" 

Note that this counter only increments for empty placeholders:

"{2} {} {}" → second value, then first value, then second value again 

A double open curly brace ("{{") will be printed as a single open curly brace ('{') and not treated as a placeholder.

To conditionally print content only when a variable is set:

"{?2} Second value: {2}{?}" → prints only if value 2 is set 

To conditionally print content only when a variable is NOT set:

"{/2}Value not available{/}" → prints only if value 2 is NOT set 

Example combining both:

"{?2}{2}{?}{/2}Second value fallback{/}" 

To terminate formatting at any point, use "{-}".

To apply color to text, start a placeholder with '#' followed by terminal color codes:

"{#4;35}Colored Text{#}" → pink and underlined text 

The escape sequence "\033[" at the start and 'm' at the end are automatically added.

"{#}" is equivalent to "{#0}" and resets all formatting to normal.

Named formats are also supported:

"{#underline_magenta}Colored Text{#}" 

See fastfetch -h color for details about supported color codes.

If a format string evaluates to an empty value, the entire line will be omitted from the output.

This can be used to disable specific outputs:

"--host-format ' '" → disables host output 

Note that using an empty string (e.g., "--host-format ''") would be treated as not set, and the built-in format would be used instead.

Format strings can also be used to set fixed values—simply use a string without any placeholders:

"--custom-format Preferred"