Saturday, July 31, 2010

The timeless beauty of shell scripts

Long ago, Doug McIlroy wrote: “This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.”

Years later, Rob Pike observed: “Those days are dead and gone and the eulogy was delivered by Perl.” His statement mostly stands, except Python is the new Perl.

I refuse to abandon the old ways. So when a friend pointed me to the intriguing Python Challenge, I avoided Python as much as I could. Instead, I used the Bash shell to string together special-purpose tools.

The FAQ states the purpose of the challenge is to “provide an entertaining way to explore the Python Programming Language”, and “demonstrate the great power of Python’s batteries”. However, I feel it is better suited for training shell script muscles: most of the problems are of the one-shot trivial variety that suit Unix tools.

A full-featured language is often overkill. Abraham Maslow’s quote comes to mind: “It is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail.” I don’t mean to disparage Python, but I feel shell scripts are often overlooked and under-appreciated, especially as they are so accessible. Technically, you’re already shell programming when you run a program from the command-line. Why not learn a bit of Bash or similar, and increase the power available at your fingertips?

Brevity is the soul of wit

While general-purpose scripting languages have their place, judging by the posted solutions, for typical riddles in the Python Challenge, a short Python script is often outdone by a shorter still Bash incantation. In fact, in the first challenge you can stay in your shell. Did you know Bash natively handles (fixed-width precision) arithmetic? For example:

$ echo $((2**42))

Naturally, if arbitrary precision were needed, we could invoke a specialized tool:

$ echo 10^100 | bc

Humble Unix tools yield the most succint solution for several challenges. For example, a Caesar shift is probably terser with tr than any popular language:

$ tr a-z l-za-m

Or extracting lowercase letters from a file:

$ tr -cd a-z

When regular expressions are involved, even though the code may look similar, the old guard such as awk, sed, and grep that feature regularly in Bash scripts have an inherent advantage over Python (and Perl, PHP, Ruby, …). Python takes exponential time to match some regular expressions whereas the classic Unix tools take polynomial time to match the same expressions.

On the downside, Bash makes some tasks tiresome. I can’t think of an easy way to convert an decimal number to an ASCII character. This Bash FAQ suggests the cumbersome:

$ for a in 66 101 110; do printf \\$(printf '%03o' $a); done; echo

Another chore is repeating a character a given number of times. Other than a loop, perhaps the easiest hack is something like:

$ printf "%042d" 0 | tr 0 x

A tiny elegant Haskell solution exists for problem 14, thanks to the transpose function and the language’s concise notation for recursion and composition. A search revealed Bash fans often employ a simple but tedious Awk script for matrix transposition, suggesting a Bash solution is necessarily significantly longer.

Happily, these blemishes are dwarfed by the successes of the Unix philosophy. More than once, my script has been simpler and briefer than any other posted solution because the complexity is hidden within a tool that does one thing, and does it well. My proudest achievement is a one-liner to compute the look-and-say sequence [hint: uniq -c].


O(1) said...

Check out Object Shell: It applies the idea of command-lines comprising small commands combined by piping to Python. You run the command line in a standard shell, but the commands are snippets of python, and python objects are what's piped between commands.

Ferdinand Jamitzky said...

python and bash are not the only languages that can help you to solve the puzzles... I tried R and it works fine. But probably perl and ruby would do fine too...
Is there a solution sheet in several different languages? Would be interesting to see.

Resuna said...

You don't need a "better shell" to incorporate snippets of other languages in a shell script, all you need is the ability to pass the snippet on the command line. You can do that with perl, awk, even applescript.