Wandering Thoughts archives

Why I have a little C program to filter a $PATH (more or less)

I use a non-standard shell and have for a long time, which means that I have to write and maintain my own set of dotfiles (which sometimes has advantages). In the long ago days when I started doing this, I had a bunch of accounts on different Unixes around the university (as was the fashion at the time, especially if you were a sysadmin). So I decided that I was going to simplify my life by having one set of dotfiles for rc that I used on all of my accounts, across a wide variety of Unixes and Unix environments. That way, when I made an improvement in a shell function I used, I could get it everywhere by just pushing out a new version of my dotfiles.

(This was long enough ago that my dotfile propagation was mostly manual, although I believe I used rdist for some of it.)

In the old days, one of the problems you faced if you wanted a common set of dotfiles across a wide variety of Unixes was that there were a lot of things that potentially could be in your $PATH. Different Unixes had different sets of standard directories, and local groups put local programs (that I definitely wanted access to) in different places. I could have put everything in $PATH (giving me a gigantic one) or tried to carefully scope out what system environment I was on and set an appropriate $PATH for each one, but I decided to take a more brute force approach. I started with a giant potential $PATH that listed every last directory that could appear in $PATH in any system I had an account on, and then I had a C program that filtered that potential $PATH down to only things that existed on the local system. Because it was written in C and had to stat() things anyways, I made it also keep track of what concrete directories it had seen and filter out duplicates, so that if there were symlinks from one name to another, I wouldn't get it twice in my $PATH.

(Looking at historical copies of the source code for this program, the filtering of duplicates was added a bit later; the very first version only cared about whether a directory existed or not.)

The reason I wrote a C program for this (imaginatively called 'isdirs') instead of using shell builtins to do this filtering (which is entirely possible) is primarily because this was so long ago that running a C program was definitely faster than using shell builtins in my shell. I did have a fallback shell builtin version in case my C program might not be compiled for the current system and architecture, although it didn't do the filtering of duplicates.

(Rc uses a real list for its equivalent of $PATH instead of the awkward ':' separated pseudo-list that other Unix shells use, so both my C program and my shell builtin could simply take a conventional argument list of directories rather than having to try to crack a $PATH apart.)

(This entry was inspired by Ben Zanin's trick(s) to filter out duplicate $PATH entries (also), which prompted me to mention my program.)

PS: rc technically only has one dotfile, .rcrc, but I split my version up into several files that did different parts of the work. One reason for this split was so that I could source only some parts to set up my environment in a non-interactive context (also).

Sidebar: the rc builtin version

Rc has very few builtins and those builtins don't include test, so this is a bit convoluted:

path=`{tpath=() pe=() {
        for (pe in $path)
           builtin cd $pe >[1=] >[2=] && tpath=($tpath $pe)
        echo $tpath
       } >[2]/dev/null}

In a conventional shell with a test builtin, you would just use 'test -d' to see if directories were there. In rc, the only builtin that will tell you if a directory exists is to try to cd to it. That we change directories is harmless because everything is running inside the equivalent of a Bourne shell $(...).

Keen eyed people will have noticed that this version doesn't work if anything in $path has a space in it, because we pass the result back as a whitespace-separated string. This is a limitation shared with how I used the C program, but I never had to use a Unix where one of my $PATH entries needed a space in it.

The profusion of things that could be in your $PATH on old Unixes

In the beginning, which is to say the early days of Bell Labs Research Unix, life was simple and there was only /bin. Soon afterwards that disk ran out of space and we got /usr/bin (and all of /usr), and some people might even have put /etc on their $PATH. When UCB released BSD Unix, they added /usr/ucb as a place for (some of) their new programs and put some more useful programs in /etc (and at some point there was also /usr/etc); now you had three or four $PATH entries. When window systems showed up, people gave them their own directories too, such as /usr/bin/X11 or /usr/openwin/bin, and this pattern was followed by other third party collections of programs, with (for example) /usr/bin/mh holding all of the (N)MH programs (if you installed them there). A bit later, SunOS 4.0 added /sbin and /usr/sbin and other Unixes soon copied them, adding yet more potential $PATH entries.

(Sometimes X11 wound up in /usr/X11/bin, or /usr/X11<release>/bin. OpenBSD still has a /usr/X11R6 directory tree, to my surprise.)

When Unix went out into the field, early system administrators soon learned that they didn't want to put local programs into /usr/bin, /usr/sbin, and so on. Of course there was no particular agreement on where to put things, so people came up with all sorts of options for the local hierarchy, including /usr/local, /local, /slocal, /<group name> (such as /csri or /dgp), and more. Often these /local/bin things had additional subdirectories for things like the locally built version of X11, which might be plain 'bin/X11' or have a version suffix, like 'bin/X11R4', 'bin/X11R5', or 'bin/X11R6'. Some places got more elaborate; rather than putting everything in a single hierarchy, they put separate things into separate directory hierarchies. When people used /opt for this, you could get /opt/gnu/bin, /opt/tk/bin, and so on.

(There were lots of variations, especially for locally built versions of X11. And a lot of people built X11 from source in those days, at least in the university circles I was in.)

Unix vendors didn't sit still either. As they began adding more optional pieces they started splitting them up into various directory trees, both for their own software and for third party software they felt like shipping. Third party software was often planted into either /usr/local or /usr/contrib, although there were other options, and vendor stuff could go in many places. A typical example is Solaris 9's $PATH for sysadmins (and I think that's not even fully complete, since I believe Solaris 9 had some stuff hiding under /usr/xpg4). Energetic Unix vendors could and did put various things in /opt under various names. By this point, commercial software vendors that shipped things for Unixes also often put them in /opt.

This led to three broad things for people using Unixes back in those days. First, you invariably had a large $PATH, between all of the standard locations, the vendor additions, and the local additions on top of those (and possibly personal 'bin' directories in your $HOME). Second, there was a lot of variation in the $PATH you wanted, both from Unix to Unix (with every vendor having their own collection of non-standard $PATH additions) and from site to site (with sysadmins making all sorts of decisions about where to put local things). Third, setting yourself up on a new Unix often required a bunch of exploration and digging. Unix vendors often didn't add everything that you wanted to their standard $PATH, for example. If you were lucky and got an account at a well run site, their local custom new account dotfiles would set you up with a correct and reasonably complete local $PATH. If you were a sysadmin exploring a new to you Unix, you might wind up writing a grumpy blog entry.

(This got much more complicated for sites that had a multi-Unix environment, especially with shared home directories.)

Modern Unix life is usually at least somewhat better. On Linux, you're typically down to two main directories (/usr/bin and /usr/sbin) and possibly some things in /opt, depending on local tastes. The *BSDs are a little more expansive but typically nowhere near the heights of, for example, Solaris 9's $PATH (see the comments on that entry too).