Suppressing stubborn console output on system calls in Ruby

Question

I'm calling imagemagik's convert program from within ruby to convert image types. I'm redirecting both stdout and stderr to /dev/null but I'm still getting console text. It only occurs when converting FROM webp so I suspect it's output from the Ubuntu webp package.

buffer = `convert -quiet "#{temp_dir}#{tmp_image_filename}" "#{temp_dir}#{new_image_filename}" > /dev/null 2>&1`

Output:

Decoded /tmp/magick-3658rrhNn7wh4IW2. Dimensions: 580 x 300 . Format: lossy. Now saving...
Saved file /tmp/magick-3658nGuNL-bzCkRA

Is this tty output? I can't figure out how to capture and suppress it. I added the quiet attribute to convert command line but it had no affect (another reason I suspect webp is the culprit). I've tried several other tips in my stackoverflow search with system, IO, wrapping in : $(...), etc to no avail. Any help?

Thanks! Eric

Have you tried using Open3? Backticks are only suitable for quick'n'dirty hacks IMO. — mu is too short
– mu is too short, Commented Nov 6, 2020 at 22:03
No I haven't, looks like a good option. There's actually an example with convert in the docs LoL! I'll report back... THX! — Nowhere Fast
– Nowhere Fast, Commented Nov 6, 2020 at 22:09
Open3 will also save you from launching a shell (which will parse the command line and then exec convert) and leave you open to unpleasant quoting and escaping issues that can cause a lot of problems unless you're very careful. Open3 won't involve a shell so a lot of problems just go away. — mu is too short
– mu is too short, Commented Nov 6, 2020 at 22:14
Open3 is the way to go. Backticks are a dirty hack and honestly shouldn't even be allowed in Ruby without a special flag (ruby --yolo perhaps?). They're exceptionally dangerous if used incorrectly. — tadman
– tadman, Commented Nov 6, 2020 at 23:15
using Open3.capture3 produces the same results. The convert output (more likely the webp output) bleeds thru to the console... — Nowhere Fast
– Nowhere Fast, Commented Nov 7, 2020 at 1:17

Sam Soffes · Accepted Answer · 2025-08-31 18:01:46Z

1

Question was answered in the comments. I moved to Open3.capture3 using a format like below and the console text from webp is captured.

require "open3"
stdout, stderr, status = Open3.capture3("convert -flatten \"#{$temp_dir}#{tmp_image_filename}\" \"#{$temp_dir}#{@image_filename}\"")

edited Aug 31 at 18:01

Sam Soffes

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answered Nov 22, 2020 at 21:59

Nowhere Fast

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