I have stumbled across this sintax while reviewing a code in Ruby. The code is:
if __FILE__ == $PROGRAM_NAME
#some code...
end
I suppose __FILE__
is a variable that gets me the name of the file I am in?
But what does $PROGRAM_NAME
simbolize then? Also, why is this if statement necessary, since the program works with or without it?
__FILE__
always returns the path of the source file. It's not a variable so you can't assign value to it. Whether it returns a relative path or an absolute one depends on how you run the script.
$PROGRAM_NAME
or $0
by default returns the command that boots the program (minus the path of ruby interpreter). For example, you have a script file test.rb
like this:
#!/usr/bin/env ruby
puts __FILE__
puts $PROGRAM_NAME
If you run this script with ruby test.rb
, it prints
test.rb
test.rb
If you run the script with ruby /path/to/test.rb
, it prints
/path/to/test.rb
/path/to/test.rb
If you give the script an execution permission and run it with ./test.rb
, it prints
./test.rb
./test.rb
Unlike __FILE__
, $PROGRAM_NAME
and $0
are real global variables, and you can change their values. $PROGRAM_NAME
and $0
are aliases to each other, so you change the value of either one, the value of the other will change accordingly. For example, you have a test2.rb
like this:
#!/usr/bin/env ruby
$0 = 'Hello, world!'
puts $0
puts $PROGRAM_NAME
it prints
Hello, world!
Hello, world!
__FILE__
is the current source file name.
It seems to ruby code wants to make sure that the current file corresponds to the program that needs to executed.
$
before a variable, means its a global variable.
Check here to know more - http://www.zenspider.com/Languages/Ruby/QuickRef.html#18
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