File Comparison And Merge

Command-Line Options

Comparing 2 files:

   kdiff3 file1 file2

Merging 2 files:

   kdiff3 file1 file2 -m
   kdiff3 file1 file2 -o outputfile

Comparing 3 files:

   kdiff3 file1 file2 file3

Merging 3 files:

   kdiff3 file1 file2 file3 -m
   kdiff3 file1 file2 file3 -o outputfile

Note that file1 will be treated as base of file2 and file3.

Special case: Files with the same name
If all files have the same name but are in different directories, you can reduce typework by specifying the filename only for the first file. E.g.:

   kdiff3 dir1/filename dir2 dir3

Commandline for starting a directory comparison or merge:
This is very similar, but now it's about directories.

   kdiff3 dir1 dir2
   kdiff3 dir1 dir2 -o destdir
   kdiff3 dir1 dir2 dir3
   kdiff3 dir1 dir2 dir3 -o destdir

For directory comparison and merge you can continue to read here.

Other command line options
To see all available command line options type

kdiff3 --help

Example output:

Options:
  -m, --merge               Merge the input.
  -b, --base file           Explicit base file. For compatibility with certain tools.
  -o, --output file         Output file. Implies -m. E.g.: -o newfile.txt
  --out file                Output file, again. (For compatibility with certain tools.)
  --auto                    No GUI if all conflicts are auto-solvable. (Needs -o file)
  --qall                    Don't solve conflicts automatically. (For compatibility...)
  --L1 alias1               Visible name replacement for input file 1 (base).
  --L2 alias2               Visible name replacement for input file 2.
  --L3 alias3               Visible name replacement for input file 3.
  -L, --fname alias         Alternative visible name replacement. Supply this once for every input.
  --cs string               Override a config setting. Use once for every setting. E.g.: --cs "AutoAdvance=1"
  --confighelp              Show list of config settings and current values.
  --config file             Use a different config file.

The option --cs allows you to adjust a configuration value that is otherwise only adjustable via the configure dialogs. But be aware that when KDiff3 then terminates the changed value will be stored along with the other settings. With --confighelp you can find out the names of the available items and current values.
Via --config you can specify a different config file. When you often use KDiff3 with completely different setups this allows you to easily switch between them.

Ignorable command line options
Many people want to use KDiff3 with some version control system. But when that version control system calls KDiff3 using command line parameters that KDiff3 doesn't recognise, then KDiff3 terminates with an error. The integration settings allow to specify command line parameters that should be ignored by KDiff3. They will appear in the usage help like in this example:

  --foo                     Ignored. (User defined.)

Command line options to ignore:	 A list of options, separated via semicolon ';'. When one of these options appears on the commandline, then KDiff3 will ignore it and run without reporting an error. (Default is "u;query;html;abort").
When this isn't enough, then it is recommended to write a shell script that does the option translation.

Open-Dialog
Since many input files must be selectable, the program has a special open dialog:
The open dialog allows you to edit the filenames by hand, selecting a file via the file-browser ("File...") or allows you to choose recent files from the drop-down lists. If you open the dialog again, then the current names still remain there. The third input file is not required. If the entry for "C" remains empty, then only a two file diff analysis will be done.
You can also select a directory via "Dir...". If for A a directory is specified then a directory-comparison/merge starts. If A specifies a file but B, C or the output specify a directory, then KDiff3 uses the filename from A in the specified directories.
If "Merge" is selected, then the "Output"-line becomes editable. But it is not required to specify the output filename immediately. You can also postpone this until saving.
The "Configure..."-button opens the options-dialog, so that you can set the options before running the analysis.

Paste and Drop Input
Sometimes you want to compare parts of a text that is not an own file. KDiff3 also allows you to paste text from the clipboard into the diff input window that has the focus. The diff analysis happens immediately then. In the open dialog you need not specify files then, but just close it via "Cancel".
You can also use drag and drop: Drag a file from a file manager or selected text from an editor and drop it onto a diff input window.
What's the idea? Sometimes a file contains two similar functions, but checking how similar they really are is a big effort if you first must create two files and then load them. Now you can simply copy, paste and compare the relevant sections.
Note: Currently you cannot drag anything from KDiff3. Only dropping in the diff input is supported.
Warning: Some editors still interpret the drag and drop into another program like cut (instead of copy) and paste. Your original data might be lost then.

Comparing Files And Interpreting The Information In The Input Windows

Info Line
At the top of each text window is its "info line". The info lines of the input windows contain a letter "A", "B" or "C", the editable filename, a button for browsing, and the line number of the first visible line in the window. (Note that window "C" is optional.) Each info line appears in a different color.
When you selected another file via browsing or finished editing the filename here by pressing enter, the new file will be loaded and compared with the already loaded file(s).

Coloring
The three input windows are assigned the letters "A", "B" and "C". "A" has color blue, "B" has green and "C" has magenta. (These are the defaults, but can be changed in the Settings-Menu.)
When a difference is detected then the color shows which input file differs. When both other input files differ then the color used to express this is red by default ("Conflict color" in the Settings). This colorscheme is especially useful in the case of three input files, which will be seen in the next section (Merging).

Summary Column
Left of each text is the "summary column". If differences occurred on a line then the summary column shows the respective color. For a white-space-only difference the summary is chequered. For programming languages where white space is not so important this is useful to see at one glance if anything of importance was modified. (In C/C++ white space is only interesting within strings, comments, for the preprocessor, and some only very esoteric situations.)
The vertical line separating the summary column and the text is interrupted if the input file had no lines there. When word-wrap is enabled then this vertical line appears dotted for wrapped lines.

Overview Column
On the right side a "overview"-column is visible left of the vertical scrollbar. It shows the compressed summary column of input "A". All the differences and conflicts are visible at one glance. When only two input windows are used, then all differences appear red here because every difference is also a conflict. A black rectangle frames the visible part of the inputs. For very long input files, when the number of input lines is bigger than the height of the overview column in pixels, then several input lines share one overview line. A conflict then has top priority over simple differences, which have priority over no change, so that no difference or conflict is lost here. By clicking into this overview column the corresponding text will be shown.

Manually Aligning Lines
Sometimes the algorithm places the wrong lines next to each other. Or you want to compare one piece of text with text at a completely different position in the other file. For these situations you can manually instruct KDiff3 to align certain lines. Mark the text for which you want to improve the alignment with the mouse as you would for copy and paste in the first diff view and then choose "Add Manual Diff Alignment" in the "Diffview"-menu (keyboard shortcut Y). An orange bar will appear in the summary column next to the chosen text. Repeat this for the second and (if available) third diff view. KDiff3 will immediately recalculate the differences everytime you do this, and will align the chosen lines. Of course some of the previously matching lines in between might not match anymore.
Currently merging doesn't support the use of manual diff help.

Manually Joining and Splitting Diff Sections
In some cases KDiff3 will see too many or too few diff sections for merging. In such a case you can join or split existing sections.
Add new sections by first selecting text in the lines that belong together in either input window (as for copying to the clipboard). Then choose "Split Diff At Selection" in the "Merge" menu. Splits will be added above the first line and below the last line of the selected text. If you only want to add one section, then select text beginning at another section-split.
For joining sections in either input window select something in the lines from the sections to join. (You can join several sections in one step too.) Then choose "Join selected Diffs" in the "Merge"-menu.

Merging And The Merge Output Editor Window
The merge output editor window (below the diff input windows) also has an info line at the top showing "Output:", the filename and "[Modified]" if you edited something. Usually it will contain some text through the automatic merge facilities, but often it will also contain conflicts.
!!! Saving is disabled until all conflicts are resolved !!! (Use the "Go to prev/next unsolved conflicts"-buttons to find the remaining conflicts.)
With only two input files every difference is also a conflict that must be solved manually.
With three input files the first file is treated as base, while the second and third input files contain modifications. When at any line only either input B or input C have changed but not both then the changed source will automatically be selected. Only when B and C have changed on the same lines, then the tool detects a conflict that must be solved manually. When B and C are the same, but not the same as A, then C is selected.

The Summary Column
The merge output editor window also has a summary column on the left. It shows the letter of the input from which a line was selected or nothing if all three sources where equal on a line. For conflicts it shows a questionmark "?" and the line shows "<Merge Conflict>", all in red. Because solving conflicts line by line would take very long, the lines are grouped into groups that have the same difference and conflict characteristics. But only-white-space-conflicts are separated from non-white-space-conflicts in order to ease the merging of files were the indentation changed for many lines.

Setting The Current Group And Synchronising Merge And Diff View Position
When clicking into the summary column with the left mouse button in either window then the beginning of the group belonging to that line will shown in all windows. This group then becomes the "current group". It is highlighted with the "Current range (diff) background color" and a black bar appears on the left side of the text.

Choosing Inputs A, B or C For Current Conflict And Editing
The button bar below the menubar contains three input selector buttons containing the letters "A", "B" and "C". Click the input selector button to insert (or remove if already inserted) the lines from the respective source. To choose the lines from several inputs click the respective buttons in the needed order. For example if you want that the lines from "B" appear before the lines from "A" in the output, first click "B", then "A".
If you use the auto-advance option ("Automatically go to next unsolved conflict after source selection"), you should disable this before choosing lines from several inputs or if you want to edit the lines after your choice. Otherwise KDiff3 will jump to the next conflict after choosing the first input.
It is often helpful directly edit the merge output. The summary column will show "m" for every line that was manually modified. When for instance the differences are aligned in a way that simply choosing the inputs won't be satisfactory, then you can mark the needed text and use normal copy and paste to put it into the merge output.
Sometimes, when a line is removed either by automatic merge or by editing and no other lines remain in that group, then the text <No src line> will appear in that line. This is just a placeholder for the group for when you might change your mind and select some source again. This text won't appear in the saved file or in any selections you want to copy and paste.
The text "<Merge Conflict>" will appear in the clipboard if you copy and paste some text containing such a line. But still be careful to do so.

Choosing Input A, B, or C for All Conflicts
The normal merge will start by solving simple conflicts automatically. But the "Merge"-menu provides some actions for other common needs. If you have to select the same source for most conflicts, then you can choose "A", "B" or "C" everywhere, or only for the remaining unsolved conflicts, or for unsolved white space conflicts. If you want to decide every single delta yourself, you can "Set deltas to conflicts". Or if you want to return to the automatic choices of KDiff3 then select "Automatically solve simple conflicts". KDiff3 then restarts the merge. For actions that change your previous modifications KDiff3 will ask for your confirmation before proceeding.
Note: When choosing either source for unsolved white space conflicts and the options "Ignore Numbers" or "Ignore C/C++ Comments" are used then changes in numbers or comments will be treated like white space too.

Automatic Merge of Version Control Keywords and History (Log)
Many version control systems support special keywords in the file. (e.g. "$Date$", "$Header$", "$Author$", "$Log$" etc.) During the check-in the version control system (VCS) changes these lines. For instance "$Date$" will turn into "$Date: 2005/03/22 18:45:01 $". Since this line will be different in every version of the file, it would require manual interaction during the merge.
KDiff3 offers automatic merge for these items. For simple lines that match the "Auto merge regular expression"-option in all input-files KDiff3 will choose the line from B or - if available - from C. (Additionally it is necessary that the lines in question line up in the comparison and the previous line contains no conflict.) This auto merge can either be run immediately after a merge starts (activate the option "Run regular expression auto merge on merge start") or later via the merge menu "Run Regular Expression Auto Merge".
Automatic merge for version control history (also called "log") is also supported. The history automerge can either run immediately when the merge starts by activating the option "Merge version control history on merge start" or later via the merge menu "Automatically Solve History Conflicts".
Usually the version control history begins with a line containing the keyword "$Log$". This must be matched by the "History start regular expression"-option. KDiff3 detects which subsequent lines are in the history by analysing the leading characters that came before the "$Log$"-keyword. If the same "leading comment"-characters also appears in the following lines, then they are also included in the history.
During each check-in the VCS writes a unique line specifying version-, date- and time-information followed by lines with user comments. These lines form one history-entry. This history section grows with every check-in and the most recent entries appear at the top (after the history start line).
When for parallel development two or more developers check-in a branch of the file then the merge history will contain several entries that appear as conflicts during the merge of the branches. Since merging these can become very tedious, KDiff3 offers support with two possible strategies: Just insert the history information from both contributors at the top or sort the history information by a user defined key.
The just-insert-all-entries-method is easier to configure. KDiff3 just needs a method to detect, which lines belong to one history entry. Most VCS insert an empty line after each history entry. If there are no other empty lines, this is a sufficient criterion for KDiff3. Just set an empty "History entry start regular expression". If the empty line criterion isn't sufficient, you can specify a regular expression to detect the history entry start.
Note that KDiff3 will remove duplicate history entries. If a history entry appeared several times in the history of a input file, only one entry will remain in the output.
If you want to sort the history, then you have to specify how the sort key should be built. Use parentheses in the "History entry start regular expression" to group parts of the regular expression that should later be used for the sort key. Then specify the "History entry start sort key order" specifying a comma "," separated list of numbers referring to the position of the group in the regular expression.
Because this is not so easy to get right immediately, you are able to test and improve the regular expressions and key-generation in a dedicated dialog by pressing the "Test your regular expressions"-button.
Example: Assume a history that looks like this:

/**************************************************************************
** HISTORY:    $Log: \toms_merge_main_view\MyApplication\src\complexalgorithm.cpp $
**
**     \main\integration_branch_12   2 Apr 2001 10:45:41   tom
**  Merged branch simon_branch_15.
**
**     \main\henry_bugfix_branch_7\1   30 Mar 2001 19:22:05   henry
**  Improved the speed for subroutine convertToMesh().
**  Fixed crash.
**************************************************************************/

The history start line matches the regular expression ".*\$Log.*\$.*". Then follow the history entries.
The line with the "$Log$"-keyword begins with two "*" after which follows a space. KDiff3 uses the first non-white-space string as "leading comment" and assumes that the history ends in the first line without this leading comment. In this example the last line ends with a string that also starts with two "*", but instead of a space character more "*" follow. Hence this line ends the history.
If history sorting isn't required then the history entry start line regular expression could look like this. (This line is split in two because it wouldn't fit otherwise.)

\s*\\main\\\S+\s+[0-9]+ (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)
 [0-9][0-9][0-9][0-9] [0-9][0-9]:[0-9][0-9]:[0-9][0-9]\s+.*

For details about regular expressions please see the regular expression documentation by Trolltech. Note that "\s" (with lowercase "s") matches any white space and "\S" (with uppercase "S") matches any non-white-space. In our example the history entry start contains first the version info with reg. exp. "\\main\\\S+", the date consisting of day "[0-9]+", month "(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)" and year "[0-9][0-9][0-9][0-9]", the time "[0-9][0-9]:[0-9][0-9]:[0-9][0-9]" and finally the developers login name ".*".
Note that the "leading comment"-characters (in the example "**") will already be removed by KDiff3 before trying to match, hence the regular expression begins with a match for none or more white-space characters "\s*". Because comment characters can differ in each file (e.g. C/C++ uses other comment characters than a Perl script) KDiff3 takes care of the leading comment characters and you should not specify them in the regular expression.
If you require a sorted history. Then the sortkey must be calculated. For this the relevant parts in the regular expression must be grouped by parentheses. (The extra parentheses can also stay in if history sorting is disabled.)

\s*\\main\\(\S+)\s+([0-9]+) (Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)
 ([0-9][0-9][0-9][0-9]) ([0-9][0-9]:[0-9][0-9]:[0-9][0-9])\s+(.*)

The parentheses now contain 1. version info, 2. day, 3. month, 4. year, 5. time, 6. name. But if we want to sort by date and time, we need to construct a key with the elements in a different order of appearance: First the year, followed by month, day, time, version info and name. Hence the sortkey order to specify is "4,3,2,5,1,6".
Because month names aren't good for sorting ("Apr" would be first) KDiff3 detects in which order the month names were given and uses that number instead ("Apr"->"04"). And if a pure number is found it will be transformed to a 4-digit value with leading zeros for sorting. Finally the resulting sort key for the first history entry start line will be:

2001 04 0002 10:45:41 integration_branch_12   tom

For more information also see Merge Settings.

Navigation And Editing
Much navigation will be done with the scroll bars and the mouse but you can also navigate with the keys. If you click into either window then you can use the cursor buttons left, right, up, down, page up, page down, home, end, ctrl-home, ctrl-end as you would in other programs. The overview-column next to the vertical scroll bar of the input files can also be used for navigating by clicking into it.
You can also use the wheel mouse to scroll up and down.
In the merge output editor you can also use the other keys for editing. You can toggle between insert and overwrite mode with the insert key. (Default is insert-mode.)
A left-mouse-button-click into any summary column will synchronise all windows to show the beginning of the same group of lines (as explained in section "Setting The Current Group And Synchronising Merge And Diff View Position").
The button bar also contains nine navigation buttons with which you can jump to the current/first/last difference, to the next/previous difference (ctrl-down/ctrl-up), to the next/previous conflict (ctrl-pgdown/ctrl-pgup), or to the next/previous unsolved conflict. Note that for KDiff3 a "conflict" that was not automatically solved at the start of the merge stays a "conflict" even if it is solved. Hence the necessity to distinguish "unsolved conflicts".

Auto-Advance
There also is a button "Automatically go to next unsolved conflict after source selection" (Auto-Advance). If you enable this, then, when one source is selected, KDiff3 will jump to and select the next unsolved conflict automatically. This can help when you always want to choose one source only. When you need both sources, or you want to edit after selecting, then you probably want to switch this off. Before proceeding to the next unsolved conflict KDiff3 shows you the effect of your choice for a short time. This delay is adjustable in the Diff- & Merge-Settings: You can specify the "Auto-Advance delay" in milli seconds between 0 and 2000. Hint: Tired of many clicks? - Use a small Auto-Advance-delay and the shortcuts Ctrl-1/2/3 to select A/B/C for many conflicts.

Select, Copy And Paste
The input windows don't show a cursor, so selections must be made with the mouse by clicking with the left mouse button at the start, holding down the mousebutton and moving to the end, where you release the mouse button again. You can also select a word by double clicking it. In the merge output editor you can also select via the keyboard by holding the "shift"-button and navigation with the cursor keys.
If the selection exceeds the visible range you can move the mouse over the window borders which causes KDiff3 to scroll in that direction.
For very large selections you can also use the navigation keys while holding down the mouse. E.g. use page up and page down to quickly go to a certain position. At the end position release the mouse button.
In order to select everything in the current window use menu "Edit"->"Select All" (Ctrl-A).
To copy to the clipboard you must press the "Copy"-button (Ctrl-C or Ctrl-Insert). But there exists an option "Auto Copy Selection". If this is enabled, then whatever you select is copied immediately and you don't need to explicitly copy. But pay attention when using this because the contents of the clipboard might then be destroyed accidentally.
"Cut" (Ctrl-X or Shift-Delete) copies to the clipboard and deletes the selected text.
"Paste" (Ctrl-V or Shift-Insert) inserts the text in the clipboard at the cursorposition or over the current selection. If you paste to either diff input window the contents of the clipboard will be shown in that window and the comparison will restart immediately. This is useful if you want to quickly grab a piece of text from somewhere and compare it with something else without first creating files.

Saving
Saving will only be allowed, when all conflicts were solved. If the file already exists and the "Backup files"-option is enabled then the existing file will be renamed with an ".orig"-extension, but if such a file exists it will be deleted. When you exit or start another diff-analysis and data wasn't saved yet, then KDiff3 will ask if you want to save, cancel or proceed without saving. (KDiff3 does not catch any signals. So if you "kill" KDiff3 then your data will be lost.)
Line endings are saved according to the normal method on the underlying operating system. For Unices each line ends with an linefeed-character "\n", while for Win32-based systems each line ends with a carriage-return + a linefeed "\r\n". KDiff3 does not preserve the line-endings of the input files, which also means that you shouldn't use KDiff3 with binary files.

Finding Strings
You can search for a string in any text-window of KDiff3. The "Find ..."-command (Ctrl-F) in the Edit-menu opens a dialog that lets you specify the string to search for. You can also select the windows which should be searched. Searching will always start at the top. Use the "Find Next"-command (F3) to proceed to the next occurrence. If you select to search several windows then the first window will be searched from top to bottom before the search starts in the next window at the top again, etc.

Printing
KDiff3 supports printing for textfile differences. The "Print..."-command (Ctrl-P) in the File-menu opens a dialog that allows you to select the printer and to adjust other options.
There are several possibilities to adjust the range. Due to different printing dialogs on different operating systems, the method to achieve certain range selections varies.
All:	Print everything.
Current:	Print a page starting at the first visible line in the window. (On systems without this option this can be achieved by specifying page number 10000 for printing.)
Selection:	 Before choosing to print select text with the mouse (like for copy and paste) in one of the diff input windows to define the start and end line. If no text in one of the diff input windows was selected, then this won't be an available choice. (On systems without this option this can be achived by specifying page number 9999 for printing.)
Range:	Specify the first and last page.
Other important options for printing will be taken from the normal options:
Font, font size
Show line numbers
Word wrap
Colors
etc.
Landscape formatting is also recommended for printing.

Options
Options and the recent-file-list will be saved when you exit the program, and reloaded when you start it. (Menu Settings->Configure KDiff3 ...)

Font
Select a fixed width font. (On some systems this dialog will also present variable width fonts, but you should not use them.)
Italic Font for Deltas:	 If you select this, then text differences will be drawn with the italic version of the selected font. If the font doesn't support italic, then this does nothing.

Colors
Foreground color:	 Usually black.
Background color:	 Usually white.
Diff Background color:	 Usually light gray.
Color A:	 Usually dark blue.
Color B:	 Usually dark green.
Color C:	 Usually dark magenta.
Conflict Color:	 Usually red.
Current range background color:	 Usually light yellow.
Current range diff background color:	 Usually dark yellow.
Color for manually selected diff ranges:	 Usually orange.
Newest file color in directory comparison:	 Usually green.
Oldest file color in directory comparison:	 Usually red.
Middle age file color in directory comparison:	 Usually dark yellow.
Color for missing files in directory comparison:	 Usually black.
Changing the colors for directory comparison will be effective only when starting the next directory comparison.
On systems with only 16 or 256 colors some colors are not available in pure form. On such systems the "Defaults"-button will choose a pure color.

Editor Settings
Tab inserts spaces:	 If this is disabled and you press the tabulator key, a tab-character is inserted, otherwise the appropriate amount of characters is inserted.
Tab size:	 Can be adjusted for your specific needs. Default is 8.
Auto indentation:	 When pressing Enter or Return the indentation of the previous line is used for the new line.
Auto copy selection:	 Every selection is immediately copied to the clipboard when active and you needn't explicitly copy it.
Line end style:	 When saving you can select what line end style you prefer. The default setting is the common choice for the used operating system.

Diff Settings
When comparing files, KDiff3 first it tries to match lines that are equal in all input files. Only during this step it might ignore white space. The second step compares each line. In this step white space will not be ignored. Also during the merge white space will not be ignored.
Ignore numbers:	 Default is off. Number characters ('0'-'9', '.', '-') will be ignored in the first part of the analysis in which the line matching is done. In the result the differences will be shown nevertheless, but they are treated as white space.
Ignore C/C++ comments:	 Default is off. Changes in comments will be treated like changes in white space.
Ignore case:	 Default is off. Case-differences of characters (like 'A' vs. 'a') will be treated like changes in white space.
Preprocessor-Command:	 See next section.
Line-Matching Preprocessor-Command:	 See next section.
Try Hard:	 Try hard to find an even smaller delta. (Default is on.) This will probably be effective for complicated and big files. And slow for very big files.

Merge Settings
Auto Advance Delay (ms):	 When in auto-advance-mode this setting specifies how long to show the result of the selection before jumping to the next unsolved conflict.
White space 2/3-file merge default:	 Automatically solve all white-space conflict by choosing the specified file. (Default is manual choice.) Useful if white space really isn't important in many files. If you need this only occasionally better use "Choose A/B/C For All Unsolved Whitespace Conflicts" in the merge menu. Note that if you enable either "Ignore numbers" or "Ignore C/C++ comments" then this auto-choice also applies for conflicts in numbers or comments.
Auto merge regular expression:	 Regular expression for lines where KDiff3 should automatically choose one source. See also Automatic Merge ...
Run regular expression auto merge on merge start:	 If activated KDiff3 runs the automatic merge using the "Auto merge regular expression" when a merge is started.
History start regular expression:	 Regular expression for the start of the merge history entry. Usually this line contains the "$Log$"-keyword. Default value: ".*\$Log.*\$.*"
History entry start regular expression:	 A merge history entry consists of several lines. Specify the regular expression to detect the first line (without the leading comment). Use parentheses to group the keys you want to use for sorting. If left empty, then KDiff3 assumes that empty lines separate history entries. See also Automatic Merge ...
History merge sorting:	 Enable version control history sorting.
History entry start sort key order:	 Each parentheses used in the regular expression for the history start entry groups a key that can be used for sorting. Specify the list of keys (that are numbered in order of occurrence starting with 1) using ',' as separator (e.g. "4,5,6,1,2,3,7"). If left empty, then no sorting will be done. See also Automatic Merge ...
Merge version control history on merge start:	 If activated KDiff3 runs the automatic history merging using aforementioned options when a merge is started.
Max number of history entries:	 KDiff3 truncates the history list after the specified number of entries. Use -1 to avoid truncation. (Default is -1).
Test your regular expressions	 This button shows a dialog that allows you to improve and test the regular expressions above. Just copy the respective data from your files into the example lines. The "Match results" will immediately show whether the match succeeds or not. The "Sort key result" will display the key used for history merge sorting.
Irrelevant merge command:	 Specify a command of your own that should be called when KDiff3 detects that for a three file merge the file from B doesn't contribute any relevant data that isn't already contained in the file from C. The command is called with the three filenames as parameters. Data matched by the "Auto merge regular expression" or in the history isn't considered relevant.

Directory Merge
These options are concerned with scanning the directory and handling the merge: See the Directory Comparison/Merge Docs for details.
Yet there is one option here that is also relevant for saving single files:
Backup files:	 When a file is saved and an older version already exists, then the original version will be renamed with an ".orig" extension. If an old backup file with ".orig" extension already exists then this will be deleted without backup.

Regional and Language Options
Language:	Adjust the language of the user interface. Changing this option doesn't affect the running program. You have to exit and restart KDiff3 so that the language is changed. (This option is not available in the Frameworks version of KDiff3.)
Use the same encoding for everything:	 The following encoding options can be adjusted separately for each item or if this option is true, all values will take the first value.
Local Encoding:	Above the codec-selectors appears a note that tells you what the local encoding is. (This is not adjustable but for your information just in case you don't know your local encoding, but need to select it.)
File Encoding for A/B/C:	 Adjust the file encoding for input files. This has an effect on how the special characters are interpreted. Since you can adjust each codec separately you can even compare and merge files that were saved using different codecs.
File Encoding for Merge Output and Saving:	 When you have edited a file, then you can adjust which encoding will be used when saving to disk.
File Encoding for Preprocessor Files:	When you define preprocessors then they might not be able to operate on your codec. (e.g.: Your files are 16-bit-unicode and your preprocessor can only take 8-bit-ascii.) With this option you can define the encoding of preprocessor output.
Right To Left Language:	Some languages are written right to left. When this option is enabled, KDiff3 draws the text from right to left in the diff input windows and in the merge output window. Note that if you start KDiff3 with the command line option "--reverse" then all layouting will be done right to left too. (This is a feature provided by Qt.) This documentation was written assuming that "Right To Left Language" or reverse layout are disabled. So some references to "left" or "right" must be replaced by their respective counterpart if you use these options.

Miscellaneous
(These options and actions are available in menus or the buttonbar.)
Show line numbers:	 You can select if line numbers should be shown for the input files.
Show space and tabulator characters for differences:	 Sometimes the visible spaces and tabs are disturbing. You can turn this off.
Show white space:	 Turn this off to suppress any highlighting of white-space-only changes in the text or overview-columns. (Note that this also applies to changes in numbers or comments if the options "Ignore numbers" or "Ignore C/C++ comments" are active.)
Overview options:	 These choices are only available when you compare three files. In normal mode all differences are shown in one color-coded overview-column. But sometimes you are especially interested in the differences between only two of these three files. Selecting "A vs. B", "A vs. C" or "B vs. C"-overview will show a second overview column with the required information next to the normal overview.
Word wrap diff windows:	 Wrap lines when their length would exceed the width of a window.
Show Window A/B/C:	 Sometimes you want to use the space on the screen better for long lines. Hide the windows that are not important. (In the Windows-menu.)
Toggle Split Orientation:	 Switch between diff windows shown next to each other (A left of B left of C) or above each other (A above B above C). This should also help for long lines. (In the Windows-menu.)
Start a merge quickly:	 Sometimes you are viewing the deltas and decide to merge. "Merge current file" in the Directory-menu also works if you only compare two files. A single click starts the merge and uses the filename of the last input-file as the default output filename. (When this is used to restart a merge, then the output filename will be preserved.)

Configuring Keyboard-Shortcuts
Currently only the Frameworks-version supports user-configurable keyboard-shortcuts. (Menu Settings->Configure Shortcuts...)

Preprocessor Commands
KDiff3 supports two preprocessor options.
Preprocessor-Command:	 When any file is read, it will be piped through this external command. The output of this command will be visible instead of the original file. You can write your own preprocessor that fulfills your specific needs. Use this to cut away disturbing parts of the file, or to automatically correct the indentation etc.
Line-Matching Preprocessor-Command:	 When any file is read, it will be piped through this external command. If a preprocessor-command (see above) is also specified, then the output of the preprocessor is the input of the line-matching preprocessor. The output will only be used during the line matching phase of the analysis. You can write your own preprocessor that fulfills your specific needs. Each input line must have a corresponding output line.
The idea is to allow the user greater flexibility while configuring the diff-result. But this requires an external program, and many users don't want to write one themselves. The good news is that very often sed or perl will do the job.
Example: Simple testcase: Consider file a.txt (6 lines):

      aa
      ba
      ca
      da
      ea
      fa

And file b.txt (3 lines):

      cg
      dg
      eg

Without a preprocessor the following lines would be placed next to each other:

      aa - cg
      ba - dg
      ca - eg
      da
      ea
      fa

This is probably not wanted since the first letter contains the actually interesting information. To help the matching algorithm to ignore the second letter we can use a line matching preprocessor command, that replaces 'g' with 'a':

   sed 's/g/a/'

With this command the result of the comparison would be:

      aa
      ba
      ca - cg
      da - dg
      ea - eg
      fa

Internally the matching algorithm sees the files after running the line matching preprocessor, but on the screen the file is unchanged. (The normal preprocessor would change the data also on the screen.)

sed Basics
This section only introduces some very basic features of sed. For more information see info:/sed or http://www.gnu.org/software/sed/manual/html_mono/sed.html. A precompiled version for Windows can be found at http://unxutils.sourceforge.net. Note that the following examples assume that the sed-command is in some directory in the PATH-environment variable. If this is not the case, you have to specify the full absolute path for the command.
In this context only the sed-substitute-command is used:

   sed 's/REGEXP/REPLACEMENT/FLAGS'

Before you use a new command within KDiff3, you should first test it in a console. Here the echo-command is useful. Example:

   echo abrakadabra | sed 's/a/o/'
   -> obrakadabra

This example shows a very simple sed-command that replaces the first occurance of "a" with "o". If you want to replace all occurances then you need the "g"-flag:

   echo abrakadabra | sed 's/a/o/g'
   -> obrokodobro

The "|"-symbol is the pipe-command that transfers the output of the previous command to the input of the following command. If you want to test with a longer file then you can use cat on Unix-like systems or type on Windows-like systems. sed will do the substitution for each line.

   cat filename | sed options

Examples For sed-Use In KDiff3

Ignoring Other Types Of Comments
Currently KDiff3 understands only C/C++ comments. Using the Line-Matching-Preprocessor-Command you can also ignore other types of comments, by converting them into C/C++-comments. Example: To ignore comments starting with "#", you would like to convert them to "//". Note that you also must enable the "Ignore C/C++-Comments" option to get an effect. An appropriate Line-Matching-Preprocessor-Command would be:

   sed 's/#/\/\//'

Since for sed the "/"-character has a special meaning, it is necessary to place the "\"-character before each "/" in the replacement-string. Sometimes the "\" is required to add or remove a special meaning of certain characters. The single quotation marks (') are only important when testing on the command shell as it will otherwise attempt to process some characters. KDiff3 does not do this except for the escape sequences '\"' and '\\'.

Caseinsensitive Diff
Use the following Line-Matching-Preprocessor-Command to convert all input to uppercase:

   sed 's/\(.*\)/\U\1/'

Here the ".*" is a regular expression that matches any string and in this context matches all characters in the line. The "\1" in the replacement string refers to the matched text within the first pair of "\(" and "\)". The "\U" converts the inserted text to uppercase.

Ignoring Version Control Keywords
CVS and other version control systems use several keywords to insert automatically generated strings (info:/cvs/Keyword substitution). All of them follow the pattern "$KEYWORD generated text$". We now need a Line-Matching-Preprocessor-Command that removes only the generated text:

   sed 's/\$\(Revision\|Author\|Log\|Header\|Date\).*\$/\$\1\$/'

The "\|" separates the possible keywords. You might want to modify this list according to your needs. The "\" before the "$" is necessary because otherwise the "$" matches the end of the line.
While experimenting with sed you might come to understand and even like these regular expressions. They are useful because there are many other programs that also support similar things.

Ignoring Numbers
Ignoring numbers actually is a built-in option. But as another example, this is how it would look as a Line-Matching-Preprocessor-command.

   sed 's/[0123456789.-]//g'

Any character within '[' and ']' is a match and will be replaced with nothing.

Ignoring Certain Columns
Sometimes a text is very strictly formatted, and contains columns that you always want to ignore, while there are other columns you want to preserve for analysis. In the following example the first five columns (characters) are ignored, the next ten columns are preserved, then again five columns are ignored and the rest of the line is preserved.

   sed 's/.....\(..........\).....\(.*\)/\1\2/'

Each dot '.' matches any single character. The "\1" and "\2" in the replacement string refer to the matched text within the first and second pair of "\(" and "\)" denoting the text to be preserved.

Combining Several Substitutions
Sometimes you want to apply several substitutions at once. You can then use the semicolon ';' to separate these from each other. Example:

   echo abrakadabra | sed 's/a/o/g;s/\(.*\)/\U\1/'
   -> OBROKODOBRO

Using perl instead of sed
Instead of sed you might want to use something else like perl.

   perl -p -e 's/REGEXP/REPLACEMENT/FLAGS'

But some details are different in perl. Note that where sed needed "\(" and "\)" perl requires the simpler "(" and ")" without preceding '\'. Example:

   sed 's/\(.*\)/\U\1/'
   perl -p -e 's/(.*)/\U\1/'

Order Of Preprocessor Execution
The data is piped through all internal and external preprocessors in the following order:
Normal preprocessor,
Line-Matching-Preprocessor,
Ignore case (conversion to uppercase),
Detection of C/C++ comments,
Ignore numbers,
Ignore white space
The data after the normal preprocessor will be preserved for display and merging. The other operations only modify the data that the line-matching-diff-algorithm sees.
In the rare cases where you use a normal preprocessor note that the line-matching-preprocessor sees the output of the normal preprocessor as input.

Warning
The preprocessor-commands are often very useful, but as with any option that modifies your texts or hides away certain differences automatically, you might accidentally overlook certain differences and in the worst case destroy important data.
For this reason during a merge if a normal preprocessor-command is being used KDiff3 will tell you so and ask you if it should be disabled or not. But it won't warn you if a Line-Matching-Preprocessor-command is active. The merge will not complete until all conflicts are solved. If you disabled "Show White Space" then the differences that were removed with the Line-Matching-Preprocessor-command will also be invisible. If the Save-button remains disabled during a merge (because of remaining conflicts), make sure to enable "Show White Space". If you don't want to merge these less important differences manually you can select "Choose [A|B|C] For All Unsolved White space Conflicts" in the Merge-menu.