问题
Usually be seen in .gemspec file. eg. i18n.gemspec.
$: << File.expand_path('../lib', __FILE__)
回答1:
Pre-defined variables
$! The exception information message set by 'raise'.
$@ Array of backtrace of the last exception thrown.
$& The string matched by the last successful match.
$` The string to the left of the last successful match.
$' The string to the right of the last successful match.
$+ The highest group matched by the last successful match.
$1 The Nth group of the last successful match. May be > 1.
$~ The information about the last match in the current scope.
$= The flag for case insensitive, nil by default.
$/ The input record separator, newline by default.
$\ The output record separator for the print and IO#write. Default is nil.
$, The output field separator for the print and Array#join.
$; The default separator for String#split.
$. The current input line number of the last file that was read.
$< The virtual concatenation file of the files given on command line (or from $stdin if no files were given).
$> The default output for print, printf. $stdout by default.
$_ The last input line of string by gets or readline.
$0 Contains the name of the script being executed. May be assignable.
$* Command line arguments given for the script sans args.
$$ The process number of the Ruby running this script.
$? The status of the last executed child process.
$: Load path for scripts and binary modules by load or require.
$" The array contains the module names loaded by require.
$DEBUG The status of the -d switch.
$FILENAME Current input file from $<. Same as $<.filename.
$LOAD_PATH The alias to the $:.
$stderr The current standard error output.
$stdin The current standard input.
$stdout The current standard output.
$VERBOSE The verbose flag, which is set by the -v switch.
$-0 The alias to $/.
$-a True if option -a is set. Read-only variable.
$-d The alias to $DEBUG.
$-F The alias to $;.
$-i In in-place-edit mode, this variable holds the extension, otherwise nil.
$-I The alias to $:.
$-l True if option -l is set. Read-only variable.
$-p True if option -p is set. Read-only variable.
$-v The alias to $VERBOSE.
$ -w True if option -w is set.
the above shortcuts are an unfortunate Perl anachronism! There are "English Names" available for some of them, which are self-explanatory, and which should be used if available.
see also: http://ruby.runpaint.org/globals
see also: http://www.zenspider.com/Languages/Ruby/QuickRef.html
回答2:
$: is the load path and $LOAD_PATH is its alias. When you load/require libraries/files within your ruby application, ruby searches in the directories listed in $: for them. With $: << File.expand_path('../lib', __FILE__) you add a specific directory to the load path.
回答3:
$: is equivalent to $LOAD_PATH, i.e. the list of directories that you can require files from without giving a more specific path. You'll find the Ruby QuickRef useful for other predefined variables you may run into.
来源：https://stackoverflow.com/questions/7846426/what-is-in-ruby