mjhjmtu

I hope the title is correct and not misleading, I've tried to be specific about something which I don't know very much about. To be more general, I wonder why some similar bash commands behave the way they do.

I have a bash script foo:

#!/usr/bin/env bash

while true
do
 echo "reading"
 read data
 echo $data
 echo "stderr msg" >&2
 sleep 1
done

It's an infinite loop which reads one line at a time from stdin and outputs that same line. I also have a bash script bar:

#!/usr/bin/env bash

./foo &

The following commands were run in bash (v. 4.4.19) in a terminal on Ubuntu 18.04 (let's assume the scripts reside in the working directory):

1) ./foo &

• Stops when it tries to read from stdin, in accordance with this answer.




• Killed when controlling terminal is killed, as expected.

2) (./foo &)

• Appears to be automagically continuously fed with new input from stdin. Shouldn't it too stop when attempting to read from stdin? Whatever it's getting it doesn't show when echoed, thus I'm guessing it's EOF characters.




• Continues running even after the controlling terminal is killed. Looking at the output of ps, the controlling terminal changes from pts/x to ?. How did this happen without the use of either disown or nohup? (However, after killing the originally controlling terminal, it yields a "write error: Input/output error" on every write, which I suppose is because its stdout was tied to the now closed terminal. Correct?)

3) bash -c "./foo &"

Appears to behave exactly like 2).

4) ./bar

Appears to behave exactly like 2) and 3).

5) Adding bash -c "~/foo" (no &) as a startup application

Behaves similarly to 2), 3) and 4), with the differences:

• Its controlling terminal is the desktop ttyx.


• You can't (can you?) kill its controlling terminal.

None of those differences seem odd to me, just the fact that it's continuously fed with new input.

My guess is that 2) - 5) all use a subshell of sorts whose stdin is redirected to something like /dev/null, though I'm very unsure and seek a more exact answer.

Your sub shells are non-interactive; only interactive shells are automatically killed.