Suppressing the output of a command run using 'system' method while running it in a ruby script

Question

I am not sure if this makes sense but I am thinking if there is a way to suppress the output shown for a command when run using the system method in ruby? I mea

Answer 1

If you want to take advantage of the variadic form of Kernel.system, which side-steps the many quoting issues with shells, you can use the same options which Kernel.spawn accepts.

TL;DR - Use :out => File::NULL to silence output from Kernel.system

Arguments with special characters (spaces, etc.) can cause problems with the shell:

irb(main):001:0> filename_with_spaces = "foo bar.txt"
=> "foo bar.txt"

irb(main):002:0> system "ls -l #{filename_with_spaces}"
ls: bar.txt: No such file or directory
ls: foo: No such file or directory
=> false

So if you are interpolating variables into a system call, it is safer to provide the arguments separately:

irb(main):003:0> system "ls", "-l", filename_with_spaces
-rw-r--r--  1 nobody  nobody  9 Feb  1 16:53 foo bar.txt
=> true

But now we have a problem if we want to hide the output.

irb(main):004:0> system "ls", "-l", filename_with_spaces, "> /dev/null"
ls: > /dev/null: No such file or directory
-rw-r--r--  1 nobody  nobody  9 Feb  1 16:53 foo bar.txt
=> false

We could close STDOUT using the :out => :close option:

irb(main):005:0> system "ls", "-l", filename_with_spaces, :out => :close
=> true

However, this might cause issues with certain commands which may try to attach to STDOUT.

irb(main):006:0> system "echo", "hi there", :out => :close
echo: write: Bad file descriptor
=> false

To fix this, we can go back to redirecting our output, using File::NULL to remain portable:

irb(main):007:0> system "echo", "hi there", :out => File::NULL
=> true

Answer 2

After a call to system the exit code is in the special variable $? so if useradd returns different values to indicate if the user was successfully added (e.g. 0 for success) then you can do the following:

system('useradd xx > /dev/null')
if $? == 0
  puts 'added'
else
  puts 'failed'
end

where the redirect to /dev/null will suppress the output.

Alternatively if the program being called does not use its exit code to indicate success or failure you can use backticks and search for a particular substring in the output e.g.

if `useradd xx`.include? 'success'
  puts 'it worked'
else
  puts 'failed to add user'
end

Answer 3

IO.popen

This is another good option:

IO.popen(['echo', 'a']) do |f|
  f.read == "a\n" or raise
end
$?.exitstatus == 0 or raise

Nothing will get output to your stdout.

http://www.ruby-doc.org/core-2.1.4/IO.html#method-c-popen

Answer 4

You can also use backticks or %x

Answer 5

This should work

system ls, STDOUT:'/dev/null'

Answer 6

I was faced with this question as well...

Keep in mind that with ruby system calls, the UNIX command you are trying to use might offer a "silent" option--nullifying the need to suppress terminal output altogether!

For instance:

system 'curl -s "your_params_here"'

Will suppress the output that typically accompanies a curl call.