Capture output of running a shell command in Ruby

Preface

I am creating a plugin for Jekyll to perform extra processing of some of my posts. Specifically, for some of my Python posts, I want to include the actual output of the Python snippets I’ve embedded in the Markdown.

I need to

• Create a program to process my Python code
• Create a plugin for Jekyll in Ruby to execute the program
• Come up with a way for my plugin to know which posts to process

This is only about the Ruby command execution.

Run shell commands from Ruby and capture output

The simplest way to do this is to enclose the command in backticks (`), and it will return a string with the outputs.

msg = `echo hello`
puts msg
# Prints: hello

Execute a program from shell with string input to stdin in Ruby

There are some different functions we can use for this purpose in Open3, depending on what control over the input and ouput you need.

I wanted do pass in a string to STDIN to simplify my program. That can be done using the capture* variants.

cmd = "cat > file.txt"
post_content = "hello world"
stdout_str, exit_code = Open3.capture2(cmd, :stdin_data=>post_content)

Capture stdout and stderr of shell command in Ruby

In the code above I’m only caring about the stdout. But if you need to capture STDERR as well, either forward STDERR to STDIN in your shell command

stdout_str, exit_code = Open3.capture2("my_program 2>&1")

or let Ruby take care of exactly the same thing

stdout_and_stderr, status = Open3.capture2e("my_program")

or use capture3 if you want the outputs separated.

post_content = "this input is piped to stdin"
stdout_str, stderr_str, exit_code = Open3.capture3(cmd, :stdin_data=>post_content)

Note that the capture* functions operates with strings, while popen* operates with pipes. See the documentation for Open3 for a complete reference and list of available functions.

References

• https://stackoverflow.com/questions/26601232/preprocessing-markup-files-in-jekyll
• https://jekyllrb.com/docs/plugins/generators/
• https://stackoverflow.com/questions/2232/how-to-call-shell-commands-from-ruby
• https://ruby-doc.org/stdlib-2.4.1/libdoc/open3/rdoc/Open3.html#method-c-capture3
• https://jekyllrb.com/docs/plugins/converters/
• https://blog.yossarian.net/2019/06/09/Pipelines-in-Ruby-with-Open3