Order of printf?

Question

I'm wondering why the following prints the first command last?

file=$(printf .tar.gz & printf test)
echo $file

It outputs test.tar.gz but I'd expect .tar.gztest.

Answer 1

The order of asynchronous commands is not guaranteed and shall depend on the shell and operating system scheduler.

A simple test (run with zsh on Mac OSX El Capital):

for shell in bash ksh zsh mksh yash dash; do
  printf '[%s]\n' "$shell"
  "$shell" -c 'file=$(printf .tar.gz & printf test); echo "$file"'
done
[bash]
test.tar.gz
[ksh]
test.tar.gz
[zsh]
.tar.gztest
[mksh]
.tar.gztest
[yash]
test.tar.gz
[dash]
test.tar.gz

On my Ubuntu 14.04 VM, all of those shells produce test.tar.gz.

Answer 2

This is because & sends the first command to background, and after finishing printf test the background command from the subshell shows the info before the subshell exits. producing a test.tar.gz

in debug mode you can see the order of the commands as

++ printf test
++ printf .tar.gz
+ file=test.tar.gz
+ echo test.tar.gz

Hope is a bit clearer now,