== A Unix shell trick

This is the sort of trick where I present the code first and explain
it afterwards:

> _arch=`(PATH=$HOME/bin:/local/bin:$PATH arch || echo unknown) 2>/dev/null`_ 

(This has been slightly modified to keep the line length down; in the
real version, more things are added to _$PATH_. Note that this may be
word-wrapped in your browser.)

This sets the _$arch_ variable to the output of the _arch_ command,
if there is one to be found in any number of places, and otherwise
gives _$arch_ the value of 'unknown', all in one line.
It's worth unpacking this into its component parts (somewhat
abbreviated) in order to see how it works:

- _PATH=$HOME/bin:/local/bin:$PATH arch_: Search for an _arch_
  command using a temporarily augmented and altered _$PATH_.
- _... || echo unknown_: If we failed to find an _arch_, supply a
 default value; a command that isn't found is a false condition, just
 as if the command itself failed. (We assume that _arch_ itself will
 never fail.)
- _(...) 2>/dev/null_: Discard any complaint from the shell about being
  unable to find _arch_.

(Using just '_$PATH=... arch 2>/dev/null_' wouldn't suppress the
'_arch: not found_' error message, because the error message comes from
the shell, not from a theoretical _arch_. We have to use a subshell in
order to be able to redirect the shell's own error message.)

This is the kind of thing that makes programming in the Bourne shell
[[fun and interesting ../programming/BourneListMatch]], and I say that
non-sarcastically.

=== Sidebar: why I do this

Why I do this requires slightly more explanation. I have a single
generic _.profile_ that I use on all of the systems I have accounts
on, and on some of them my home directory is shared across multiple
architectures, which means that I need to add an architecture-specific
directory to my _$PATH_ (for architecture dependent binaries), which
means that I need to know what the local architecture is.

(By local tradition, 'architecture' here includes the operating system
as well as the chipset; typical values are things like 'solaris-sparc'.)

There is a vague local tradition that there is an _arch_ command that
prints out a string suitable for this. However, not all systems follow
this tradition, and not all systems put _arch_ in the same place, and
sometimes I need to fake it with a _$HOME/bin/arch_ (if, for example,
I am in a multi-architecture environment that doesn't follow this
tradition). So determining the local architecture requires dealing
with all of these possibilities, hence the one-liner.