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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.
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Understanding the concept of redirections and file descriptors is very important when working on the command line. To redirect stderr and stdout , use the 2>&1 or &> constructs.
Redirecting stdout and stderr to a file: The I/O streams can be redirected by putting the n> operator in use, where n is the file descriptor number. For redirecting stdout, we use “1>” and for stderr, “2>” is added as an operator.
The regular output is sent to Standard Out (STDOUT) and the error messages are sent to Standard Error (STDERR). When you redirect console output using the > symbol, you are only redirecting STDOUT. In order to redirect STDERR, you have to specify 2> for the redirection symbol.
The $stdout is a global variable which holds the standard output stream. printing2.rb. #!/usr/bin/ruby $stdout.print "Ruby language\n" $stdout.puts "Python language" We print two lines using the $stdout variable. Ruby has another three methods for printing output.
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.
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