I'd like to temporarily redirect stderr in a Ruby script for the duration of a block, ensuring that I reset it to its original value at the end of the block.
I had trouble finding how to do this in the ruby docs.
In Ruby, $stderr
refers to the output stream that is currently used as stderr, whereas STDERR
is the default stderr stream. It is easy to temporarily assign a different output stream to $stderr
.
require "stringio"
def capture_stderr
# The output stream must be an IO-like object. In this case we capture it in
# an in-memory IO object so we can return the string value. You can assign any
# IO object here.
previous_stderr, $stderr = $stderr, StringIO.new
yield
$stderr.string
ensure
# Restore the previous value of stderr (typically equal to STDERR).
$stderr = previous_stderr
end
Now you can do the following:
captured_output = capture_stderr do
# Does not output anything directly.
$stderr.puts "test"
end
captured_output
#=> "test\n"
The same principle also works for $stdout
and STDOUT
.