ci

ci [options] file...

-f
    Forces checkin even if the file hasn't changed.

-l[revision]
    Locks the checked-in revision. Optionally, a specific revision can be locked.

-u[revision]
    Unlocks the checked-in revision. Optionally, a specific revision can be unlocked.

-m message
    Specifies a log message directly on the command line, avoiding the prompt.

-i
    Initial checkin; creates an RCS file if one doesn't exist.

-r[rev]
    Specify the revision number to check in the file with.

-q
    Quiet mode; suppresses most messages.

-d[date]
    Uses the provided date and time for the checkin. Useful for restoring historical data.

The ci command is used to check in revisions into a Revision Control System (RCS) file. It creates or updates an RCS file and stores the current version of a specified file within it. This allows for tracking changes to a file over time, providing a history of modifications and enabling recovery of previous versions. RCS files typically have a ,v suffix. The ci command prompts the user for a log message describing the changes being checked in. The checked-in version becomes the new head revision of the RCS file. If the file doesn't exist, ci creates a new RCS file, otherwise it adds a new revision to it.
It's crucial to understand that RCS is a relatively old version control system and is generally superseded by modern systems like Git.

RCS is a single-user system. Concurrent modifications are not supported and RCS does not handle merging different versions automatically.
RCS has been superceeded by other solutions like git.

A typical RCS workflow involves checking out a file with co (check out), modifying it, and then checking it back in with ci. The ci command then prompts for a commit message describing the changes.

RCS (Revision Control System) was developed by Walter F. Tichy at Purdue University. It's one of the earliest version control systems, designed for managing individual files, particularly source code. It predates more sophisticated systems like CVS, Subversion, and Git. RCS gained popularity in the 1980s and 1990s for managing smaller projects and individual files. Its simplicity made it easy to use and understand. However, it lacks features needed for complex, collaborative development environments which led to the development of superior systems.

co(1), rcsdiff(1), rcsmerge(1), rcs(1)