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Examining And Comparing Old Versions

One of the convenient features of version control is the ability to examine any version of a file, or compare two versions.

C-x v ~ version RET
Examine version version of the visited file, in a buffer of its own.
C-x v =
Compare the current buffer contents with the latest checked-in version of the file.
C-u C-x v = file RET oldvers RET newvers RET
Compare the specified two versions of file.
C-x v g
Display the result of the CVS annotate command using colors.

To examine an old version in toto, visit the file and then type C-x v ~ version RET (vc-version-other-window). This puts the text of version version in a file named `filename.~version~', and visits it in its own buffer in a separate window. (In RCS, you can also select an old version and create a branch from it. See section Multiple Branches of a File.)

But usually it is more convenient to compare two versions of the file, with the command C-x v = (vc-diff). Plain C-x v = compares the current buffer contents (saving them in the file if necessary) with the last checked-in version of the file. C-u C-x v =, with a numeric argument, reads a file name and two version numbers, then compares those versions of the specified file.

If you supply a directory name instead of the name of a registered file, this command compares the two specified versions of all registered files in that directory and its subdirectories.

You can specify a checked-in version by its number; an empty input specifies the current contents of the work file (which may be different from all the checked-in versions). You can also specify a snapshot name (see section Snapshots) instead of one or both version numbers.

This command works by running the diff utility, getting the options from the variable diff-switches. It displays the output in a special buffer in another window. Unlike the M-x diff command, C-x v = does not try to locate the changes in the old and new versions. This is because normally one or both versions do not exist as files when you compare them; they exist only in the records of the master file. See section Comparing Files, for more information about M-x diff.

For CVS-controlled files, you can display the result of the CVS annotate command, using colors to enhance the visual appearance. Use the command M-x vc-annotate to do this. Red means new, blue means old, and intermediate colors indicate intermediate ages. A prefix argument n specifies a stretch factor for the time scale; it makes each color cover a period n times as long.

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