Question

In ruby I want to execute some local command which will generate a file,then I want to read the contents of the file,and modify it.

However I always get null.

This is the code:

exec "java -jar xx.jar --output main.txt";
content=IO.read('main.txt');
content="somethinkg:"+content;
File.open('main.txt','w') do |f|
  f.puts content
end

the main.txt is generated,however I can not modify the content.

Then I think it maybe caused by the thread,when the main.txt is being read,the java command is not complete,so the content is empty.

Then I tried this:

t=Thread.new do
  exec "java -jar xx.jar --output main.txt";
end
t.join
content=IO.read('main.txt');
puts content
content="somethinkg:"+content;
File.open('main.txt','w') do |f|
  f.puts content
end

However nothing changed,I can not still get the content.

What is the problem?

Answer 1

I think the problem over here is the exec command If you check the ruby documentation on exec
over here

the code after exec never execute

so

t=Thread.new do
  exec "java -jar xx.jar --output main.txt";
end

So your code never run anything after this

Try with system or backtick command

something like this

t=Thread.new do
  system "java -jar xx.jar --output main.txt";
end
t.join
content=IO.read('main.txt');
puts content
content="somethinkg:"+content;
File.open('main.txt','w') do |f|
  f.puts content
end

Hope it help