A Bash Quicky

August 30th, 2012 by Allister Banks

In our last episode spelunking a particularly shallow trough of bash goodness, we came across dollar sign substitution, which I said mimics some uses of regular expressions. Regex’s are often thought of as thick or dense with meaning. One of my more favorite descriptions goes something like, if you measured each character used in code for a regex in cups of coffee, you’d find the creators of this particular syntax the most primo, industrial-strength-caffeinated folks around. I’m paraphrasing, of course.

Now copy-pasta-happy, cargo-culting-coders like myself tend to find working code samples and reuse salvaged pieces almost without thinking, often recognizing the shape of the lines of code more than the underlying meaning. Looping back around to dollar sign substitution, we can actually interpret this commonly used value, assigned to a variable meaning the name of the script:
Okay children, what does it all mean? Well, let’s start at the very beginning(a very good place to start):
${0}The dollar sign and curly braces force an evaluation of the symbols contained inside, often used for returning complex series of variables. As an aside, counting in programming languages starts with zero, and each space-separated part of the text is defined with a number per place in the order, also known as positional parameters. The entire path to our script is given the special ‘seat’ of zero, so this puts the focus on that zero position.

Regrouping quickly, our objective is to pull out the path leading up to the script’s name. So we’re essentially gathering up all the stuff up to and including the last forward slash before our scripts filename, and chuckin’ them in the lorry bin.
${0##*}To match all of the instances of a pattern, in our case the forward slashes in our path, we double up the number signs(or pound sign for telcom fans, or hash for our friends on the fairer side of the puddle.) This performs a “greedy” match, gobbling up all instances, with a star “globbing”, to indiscriminately mop up any matching characters encountered along the way.
${0##*/}Then we cap the whole mess off by telling it to stop when it hits the last occurrence of a character, in this case forward slash. And that’s that!

Pardon the tongue-in-cheek tone of this quick detour into a bash-style regex-analogue… but to reward the masochists, here’s another joke from Puppet-gif-contest-award-winner @pmbuko:

Email from a linux user: “Slash is full.” I wanted to respond: “Did he enjoy his meal?”

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