til/ruby/override-output-field-separator.md

Override Output Field Separator

Have you ever seen $, as a specific piece of syntax before? I've never actually seen any Ruby code that uses this in the wild. It's called the output field separator. By default it is nil. Like other keywords prefixed with $ it is a global variable.

It can be used to control what gets used by default as a separator in variety of contexts, like when calling #join without any argument.

> [1,2,3].join
=> "123"
> $,
=> nil
> $, = '~'
=> "~"
> [1,2,3].join
=> "1~2~3"

It is also what gets used when calling print with multiple arguments:

> $, = ' |v| '
=> " |v| "
> print "hello", "world"
hello |v| world=> nil
> $, = "\t"
=> "\t"
> print "hello", "world"
hello	world=> nil

This seems pretty esoteric so far, but might be useful in a one-liner where we're doing something like parsing data from a CSV:

# awk-style: pull fields 1 and 3 from a CSV, emit them tab-separated
ruby -F, -ane 'BEGIN { $, = "\t"; $\ = "\n" }; print $F[0], $F[2]' data.csv

My attention was originally drawn to $, by a post from Chris Oliver which showed this method definition from Rails source:

def safe_join(array, sep = $,)

Note that this is now deprecated.