rcsdiff

rcsdiff [options] file ...

-r[rev]
    Specifies the revision(s) to compare. If one -r is given, it compares the specified rev with the working file. If two -r options are given, it compares the first rev with the second rev.

-q
    Operates in quiet mode, suppressing diagnostic output and warnings.

-c[date]
    Compares with the revision checked in on or before the specified date.

-n[name]
    Compares with the revision specified by the symbolic name.

-kkv
    Uses the kv keyword expansion mode when retrieving revisions for comparison.

-T
    Preserves the modification time of the file when retrieving revisions, useful for timestamp-sensitive operations.

-u, -U[num]
    Generates output in the unified context diff format (similar to diff -u or diff -U).

-C[num]
    Generates output in the context diff format (similar to diff -c), with num lines of context.

-p
    Generates output in a format suitable for input to the patch command.

-s
    Suppresses output if the two revisions or files being compared are identical.

-t
    Preserves the modification time of the new revision when comparing.

-V[rev]
    Specifies a compatible RCS version to use for processing files, for backward compatibility.

-V
    Prints the RCS version number and exits.

-x[suffixes]
    Processes only files whose names end with one of the specified suffixes (e.g., `-x.c/.h`).

-z[zone]
    Sets the default time zone for date comparisons (e.g., `-zUTC`).

diff-options
    Many standard diff options can be passed directly to rcsdiff, such as -b (ignore changes in amount of whitespace), -w (ignore all whitespace), -i (ignore case), -Iregexp (ignore lines matching regexp), -E (ignore blank lines), and -a (treat all files as text).

The rcsdiff command is a utility from the Revision Control System (RCS) package, designed to compare different revisions of files managed under RCS.
It essentially acts as a wrapper around the standard diff command, allowing users to easily see the differences between various historical versions of a file, or between a working file and a specific RCS revision.

By default, if you run rcsdiff on a file, it compares the current working copy of that file with the latest revision stored in the corresponding RCS file (typically 1.1 on the default branch).
Users can specify one or two revisions using the -r option to compare any two arbitrary revisions from the RCS history, or a specific revision against the working file.

rcsdiff is invaluable for understanding changes made over time, reviewing code, or identifying what modifications have been introduced in a working directory before checking them in.

rcsdiff is part of the older RCS system, which is file-based and lacks some features common in modern distributed version control systems (like Git or Mercurial), such as atomic commits across multiple files or robust branching/merging.
Its output relies heavily on the underlying diff utility, so understanding diff's output format is essential. While it can handle different file types, its primary strength is with text files. Binary file comparisons might yield less meaningful results, although the -a option can force text interpretation.

rcsdiff file.c: Compares the current working file file.c with its latest checked-in revision in the RCS file.
rcsdiff -r1.1 file.c: Compares the working file file.c with revision 1.1.
rcsdiff -r1.1 -r1.2 file.c: Compares revision 1.1 of file.c with revision 1.2 of file.c.
rcsdiff -u file.c: Shows the unified diff between the working file and the latest revision.

The Revision Control System (RCS) and its utilities like rcsdiff originated in the early 1980s, primarily developed by Walter F. Tichy. It became one of the first widely adopted version control systems, especially prominent in Unix-like environments. rcsdiff has been a core component since RCS's inception, providing the crucial capability to visualize changes between file revisions, a fundamental aspect of any version control workflow. While newer systems have largely superseded RCS for daily development, it remains valuable for simple file versioning and understanding the history of software development tools.

rcs(1), ci(1), co(1), diff(1), patch(1)