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2024-05-13 - Unix version control lore: what, ident

  * ⇐ 2024-05-12 ⫤
  *  
  * ⊨ ☆ ⇒

There are a couple of version control commands that deserve wider
appreciation: SCCS what and RCS ident. They allow you to find out
what source a binary was built from, without having to run it - handy
if it is a library! They basically scan a file looking for magic
strings that contain version control metadata and print out what they
discover.

keyword expansion

SCCS, RCS, cvs, and svn all have a way to expand keywords in a file
when it is checked out of version control.

The POSIX SCCS get documentation describes its runes under the
"identification keywords" heading. The relevant one is %W% which
inserts the magic marker @(#) used by what.

RCS / cvs / svn keyword substitution uses more descriptive markers
like $Revision$.

a berkeley example

It was a lonstanding BSD practice to use keyword expansion
everywhere. I first encountered it when I got involved in the Apache
httpd project in the late 1990s - Apache's CVS repository was hosted
on a FreeBSD box and used a version of FreeBSD's CVS administrative
scripts.

Here's an example from res_send.c in FreeBSD's libc resolver.

static const char sccsid[] = "@(#)res_send.c    8.1 (Berkeley) 6/4/93";
static const char rcsid[] = "$Id: res_send.c,v 1.22 2009/01/22 23:49:23 tbox Exp $";
__FBSDID("$FreeBSD$");

There are geological strata of version control ident strings here:

  * the sccsid from the Berkeley CSRG SCCS repository
  * the rcsid from ISC's BIND repository (tbox was ISC's tinderbox
    build / CI system)
  * the FBSDID from FreeBSD's cvs and later svn repositories (which
    has not been expanded)

an unifdef example

When unifdef was uplifted to git, I wanted to keep its embedded
version control keywords - I have a sentimental liking for this old
tradition. If you've installed cssc, rcs, and unifdef on a Debian
box, you can run,

    :; sccs what /usr/bin/unifdef
    :; ident /usr/bin/unifdef

Both of those will produce similar output to

    :; unifdef -V

On a Mac with the developer command-line tools installed,

    :; what /Library/Developer/CommandLineTools/usr/bin/unifdef

You get the output twice because it's a fat binary!

versioning three ways

In unifdef.c, the embedded version string looks like,

    static const char copyright[] =
        "@(#) $Version: unifdef-2.12 $\n"
        "@(#) $Date: 2020-02-14 16:49:56 +0000 $\n"
        "@(#) $Author: Tony Finch (dot@dotat.at) $\n"
        "@(#) $URL: http://dotat.at/prog/unifdef $\n"
    ;

Each line is prefixed with an SCCS magic marker @(#) so that what can
find it, and wrapped in an RCS-style $Keyword$ so that ident can find
it. There's a fairly trivial version() function that spits out the
copyright[] string when you run unifdef -V.

embedding versions from git

My projects have various horrible build scripts for embedding the
version number from git. The basic idea is,

  * use an annotated or signed tag to mark a release, i.e. git tag -a
    or git tag -s

  * use git describe to get a version string that includes an extra
    number counting commits since the last release

  * maybe use git show --pretty=format:%ai -s HEAD to get a release
    date

  * stuff the outputs from git into the $Version$ and $Date$ RCS
    keywords

retro cool

I enjoy keeping this old feature working, even though it isn't very
useful if no-one knows about it! Maybe if I blog about it, it'll
become more widespread?

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Tony Finch <dot@dotat.at>