5.1 Preserving Tab Stop Alignment

The lines of text in some of the diff output formats are preceded by one or two characters that indicate whether the text is inserted, deleted, or changed. The addition of those characters can cause tabs to move to the next tab stop, throwing off the alignment of columns in the line. GNU diff provides two ways to make tab-aligned columns line up correctly.

The first way is to have diff convert all tabs into the correct number of spaces before outputting them; select this method with the --expand-tabs (-t) option. To use this form of output with patch, you must give patch the -l or --ignore-white-space option (see Applying Patches with Changed White Space, for more information). diff normally assumes that tab stops are set every 8 print columns, but this can be altered by the --tabsize=columns option.

The other method for making tabs line up correctly is to add a tab character instead of a space after the indicator character at the beginning of the line. This ensures that all following tab characters are in the same position relative to tab stops that they were in the original files, so that the output is aligned correctly. Its disadvantage is that it can make long lines too long to fit on one line of the screen or the paper. It also does not work with the unified output format, which does not have a space character after the change type indicator character. Select this method with the -T or --initial-tab option.

5.2 Omitting trailing blanks

When outputting lines in normal or context format, or outputting an unchanged line in unified format, diff normally outputs a blank just before each line. If the line is empty, the output of diff therefore contains trailing blanks even though the input does not contain them. For example, when outputting an unchanged empty line in context format, diff normally outputs a line with two leading spaces.

Some text editors and email agents routinely delete trailing blanks, so it can be a problem to deal with diff output files that contain them. You can avoid this problem with the --suppress-blank-empty option. It causes diff to omit trailing blanks at the end of output lines in normal, context, and unified format, unless the trailing blanks were already present in the input. This changes the output format slightly, so that output lines are guaranteed to never end in a blank unless an input line ends in a blank. This format is less likely to be munged by text editors or by transmission via email. It is accepted by GNU patch as well.

5.3 Paginating diff Output

It can be convenient to have long output page-numbered and time-stamped. The --paginate (-l) option does this by sending the diff output through the pr program. Here is what the page header might look like for ‘diff -lc lao tzu’:

2002-02-22 14:20                 diff -lc lao tzu                 Page 1