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I've noticed that calling xdg-open from a subshell will reliably block until the launched process is closed. I suspect there may be a reason for this, but I'm not sure as to why. For example, launching Nautilus doesn't block when calling xdg-open directly from the command line:

xdg-open ~/dir ; echo foo              # doesn't block

but invoking xdg-open from a subshell will reliably block the terminal

var=$(xdg-open ~/dir ; echo foo)       # blocks

{ xdg-open ~/dir ; echo foo ; } | cat  # blocks.

My understanding is that xdg-open detaches the launched process from the shell session so that it's no longer a subprocess. I'd therefore expect this to be different to e.g. invoking sleep 1 & in a subshell for which it seems reasonable that the terminating subshell will block until all subprocess have completed, i.e.

var=$(sleep 1 & echo foo)      # also blocks, but understandable.

But if xdg-open is detaching the process, what's causing the subshell to wait?

In what may (?) be a partial answer, I've noticed that running

{ xdg-open <file> ; ps ; } | cat

shows that depending on program launched by <file>, those that block are also the ones that keep the tty as their controlling terminal. That begs the question why this happens, why this happens only in a subshell and ultimately what's a good way to a launch desktop process from the terminal that will fully and reliably detach from it?

Edit: fix syntax on bash.

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  • Which shell are you using? If bash, note that subshells are started with (...). – Eduardo Trápani Dec 12 '20 at 21:48
  • I'm using zsh, but I observe the same with bash. Note the use of {...} was to simplify the example - the output is piped to cat so my understanding is it is still invoked in a subshell. You get the same behavior in bash if you wrap in a function from which you pipe the output. – wardw Dec 12 '20 at 21:57
  • Ok, but, what is it you're trying to do? xdg-open will end up calling the appropriate application, so your results may vary, they may even depend on whether there's already an open window for that application or not... – Eduardo Trápani Dec 12 '20 at 22:01
  • I'm trying to launch a program via xdg-open from a subshell without blocking the shell that invokes xdg-open. When xdg-open is invoked outside of a subshell the shell doesn't block - I would like to do this reliably in both cases, and better understand why there's a difference. – wardw Dec 12 '20 at 22:15
  • An ampersand at the end of the command is not enough? xdg-open ... & – Eduardo Trápani Dec 12 '20 at 22:21
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First of all, in your examples,

var=$(sleep 1 & ; echo foo)

should not work in bash, because & already serves as a line end.
(-bash: command substitution: line 1: syntax error near unexpected token `;')


Try

var=$(sleep 1 & echo foo) # no ';' !!

(echo $var: foo)

In my understanding, a simple

xdg-open <something> & # optional: echo foo

should do the trick, either in a sub-shell or outside.

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  • You're right - I checked in bash and the example fails. I was doing this in zsh which seems more forgiving, but your approach is portable and probably correct regardless. But I think the use of & here is a distraction, since running a command in the background (of the subshell) is still insufficient since I assume the parent process must still wait for the child (subshell) to fully complete before execution of the parent can progress. But xdg-open should fully detach the launched process and complete? Perhaps it's a just a quirk of xdg-open or something I'm not considering. – wardw Dec 13 '20 at 0:33
  • @wardw You can also try starting xdg-open without '&', then go back to the terminal, press Ctrl+Z and type bg X (Replace X with the number from the prompt, e.g. [X]+ Stopped xdc-open <something>). That should manually send it to background and finish detached from your active terminal. – Andre Wildberg Dec 13 '20 at 1:08

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