Add Override Output Field Separator as a Ruby TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1861 TILs and counting..._
+_1862 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1551,6 +1551,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [OpenStruct Has Bad Performance Characteristics](ruby/open-struct-has-bad-performance-characteristics.md)
 - [Or Operator Precedence](ruby/or-operator-precedence.md)
 - [Output Bytecode For A Ruby Program](ruby/output-bytecode-for-a-ruby-program.md)
+- [Override Output Field Separator](ruby/override-output-field-separator.md)
 - [Override The Initial Sequence Value](ruby/override-the-initial-sequence-value.md)
 - [Parallel Bundle Install](ruby/parallel-bundle-install.md)
 - [Parse JSON Into An OpenStruct](ruby/parse-json-into-an-open-struct.md)

ruby/override-output-field-separator.md

@@ -0,0 +1,52 @@
+# 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.
+
+```ruby
+> [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:
+
+```ruby
+> $, = ' |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:
+
+```ruby
+# 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](https://lnkd.in/p/g6YH5KvQ) which showed this method definition from
+Rails source:
+
+```ruby
+def safe_join(array, sep = $,)
+```
+
+Note that this is [now
+deprecated](https://docs.ruby-lang.org/en/master/language/globals_md.html#deprecated).