How do I redirect stderr and stdout to file for a Ruby script?

Question

How do I redirect stderr and stdout to file for a Ruby script?

Leffen

Related question: #4459830 – Ultimo

Answer 1

From within a Ruby script, you can redirect stdout and stderr with the IO#reopen method.

# a.rb
$stdout.reopen("out.txt", "w")
$stderr.reopen("err.txt", "w")

puts 'normal output'
warn 'something to stderr'

$ ls
a.rb
$ ruby a.rb
$ ls
a.rb    err.txt out.txt
$ cat err.txt 
something to stderr
$ cat out.txt 
normal output

Palaeo

How can normal $stdout be restored again afterwards? – Lottery

why should we use reopen method ? it's mean that if "out.txt" is not exists, it will be created? – Belletrist

Take a look at this answer on how to restore $stdout afterwards. – Guidepost

You'll need to $stdout.sync = true and $stderr.sync = true if you hope to, say, tail the file while lines are being written. Otherwise, the lines won't be written out until the process closes, or something else like that flushes the streams. – Oneil

Answer 2

Note: reopening of the standard streams to /dev/null is a good old method of helping a process to become a daemon. For example:

# daemon.rb
$stdout.reopen("/dev/null", "w")
$stderr.reopen("/dev/null", "w")

Manvil

What do you mean by "helping a process become a daemon"? – Omni

In UNIX in order to make a program a daemon the programmer should not merely fork it into background but also redirect it's standard IO streams. Are you familiar with this? – Manvil

Note: If using built-in Process.daemon it can do this for you. See docs here. – Unset

Answer 3

def silence_stdout
  $stdout = File.new( '/dev/null', 'w' )
  yield
ensure
  $stdout = STDOUT
end

Fosse

Good, but this is better, in case STDOUT was something else. https://mcmap.net/q/354515/-how-do-i-temporarily-redirect-stderr-in-ruby – Drive

Answer 4

./yourscript.rb 2>&1 > log.txt

will redirect stdout and stderr to the same file.

Yiddish

Assuming you are running on *nix – Hallucinogen

@Steve: I think > works on Windows - I'm not sure about 2> though. – Ultimo

This answer also makes assumptions about what shell the user is using, even within *nix. Some shells have different redirection styles, for instance the default behavior of zsh. – Lumberman

It does not. Redirection order matters. It first redirects stderr to stdout, making stderr a COPY of the initial stdout, then redirect stdout to the file. Only stdout is redirected to a file. to redirect both stdout and stderr to the same file, use ./yourscript.rb > log.txt 2>&1. As of 2022, some shells support &> or >& to redirect both descriptors at the same time (bash 5.2 or zsh 5.8 for example) – Almaraz

Answer 5

A full example with $stdout and $stderr redirected to a file and how to restore the initial behavior.

#!/usr/bin/ruby

logfile = "/tmp/testruby.log"

@original_stdout = $stderr.dup
@original_stderr = $stderr.dup
$stdout.reopen(logfile, "w")
$stdout.sync = true
$stderr.reopen($stdout)


def restore_stdout
  $stdout.reopen(@original_stdout)
  $stderr.reopen(@original_stderr)
end

def fail_exit(msg)
  puts "- #{msg}" # to the logfile
  restore_stdout
  $stderr.puts "+ #{msg}" # to standard error
  exit!
end

def success_exit(msg)
  puts "- #{msg}" # to the logfile
  restore_stdout  
  $stdout.puts "+ #{msg}" # to standard output
  exit
end

puts "This message goes to the file"

success_exit "A successful exit message"

Dullard