4.1 fmt: Reformat paragraph text

fmt fills and joins lines to produce output lines of (at most) a given number of characters (75 by default). Synopsis:

fmt [option]… [file]…

fmt reads from the specified file arguments (or standard input if none are given), and writes to standard output.

By default, blank lines, spaces between words, and indentation are preserved in the output; successive input lines with different indentation are not joined; tabs are expanded on input and introduced on output.

fmt prefers breaking lines at the end of a sentence, and tries to avoid line breaks after the first word of a sentence or before the last word of a sentence. A sentence break is defined as either the end of a paragraph or a word ending in any of ‘.?!’, followed by two spaces or end of line, ignoring any intervening parentheses or quotes. Like TeX, fmt reads entire “paragraphs” before choosing line breaks; the algorithm is a variant of that given by Donald E. Knuth and Michael F. Plass in “Breaking Paragraphs Into Lines”, Software: Practice & Experience 11, 11 (November 1981), 1119–1184.

The program accepts the following options. Also see Common options.


Crown margin mode: preserve the indentation of the first two lines within a paragraph, and align the left margin of each subsequent line with that of the second line.


Tagged paragraph mode: like crown margin mode, except that if indentation of the first line of a paragraph is the same as the indentation of the second, the first line is treated as a one-line paragraph.


Split lines only. Do not join short lines to form longer ones. This prevents sample lines of code, and other such “formatted” text from being unduly combined.


Uniform spacing. Reduce spacing between words to one space, and spacing between sentences to two spaces.

-w width

Fill output lines up to width characters (default 75 or goal plus 10, if goal is provided).

-g goal

fmt initially tries to make lines goal characters wide. By default, this is 7% shorter than width.

-p prefix

Only lines beginning with prefix (possibly preceded by whitespace) are subject to formatting. The prefix and any preceding whitespace are stripped for the formatting and then re-attached to each formatted output line. One use is to format certain kinds of program comments, while leaving the code unchanged.

An exit status of zero indicates success, and a nonzero value indicates failure.