Redirecting stderr to stdout

Normally if you run a file from inside another script, it can be tricky to get get error message sometimes.

For example:

# test.rb



p("{}"

3.2.1 :001 > `ruby test.rb`
test.rb: --> test.rb
Unmatched `(', missing `)' ?          
> 4  p("{}"                           
test.rb:4: syntax error, unexpected end-of-input, expecting ')' (SyntaxError)
p("{}"                                
      ^                               
                                      
 => ""                                
3.2.1 :002 > 

The error message still is displayed in the shell, but it’s missing from the return value, which is the important thing if we’re running this inside another script/app.

The output is being displayed through stderr and not stdout.

There’s a trick you can do to merge stderr into stdout.

ruby test.rb 2>&1 which looks a little silly.

but now you get the error in the return value:

3.2.1 :004 > `ruby test.rb 2>&1`
 => "test.rb: --> test.rb\nUnmatched `(', missing `)' ?\n> 4  p(\"{}\"\ntest.rb:4: syntax error, unexpected end-of-input, expecting ')' (SyntaxError)\np(\"{}\"\n      ^\n\n" 

A breakdown of how it works:

File descriptor 1 is the standard output (stdout).

File descriptor 2 is the standard error (stderr).

At first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as “redirect stderr to a file named 1”.

& indicates that what follows and precedes is a file descriptor, and not a filename. Thus, we use 2>&1. Consider >& to be a redirect merger operator.

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