Suppose I run this command: coproc nested { sleep 10; } it executes sleep command within a childshell as expected. The process tree look like this when I run ps T --forest command.
PID TTY STAT TIME COMMAND
40305 pts/0 Ss 0:00 bash
46922 pts/0 S 0:00 \_ bash
46923 pts/0 S 0:00 | \_ sleep 10
46924 pts/0 R+ 0:00 \_ ps T --forest
Ok no problem, its exactly what I expected. But then, when i run the command like this: coproc nested { (sleep 10); }, the process tree looks the same when I run ps T --forest command.
PID TTY STAT TIME COMMAND
40305 pts/0 Ss 0:00 bash
47064 pts/0 S 0:00 \_ bash
47065 pts/0 S 0:00 | \_ sleep 10
47066 pts/0 R+ 0:00 \_ ps T --forest
Note that, here, (sleep 10) is a process list and it automatically should create a subshell to run the command sleep 10.
So, what I was expecting was a process tree like this:
bash
\_ bash
| \_bash
| \_ sleep 10
\_ps T --forest
When a subshell just contains one command, bash optimizes and doesn't fork an extra child. Put multiple commands in the subprocess.
barmar@dev:~$ coproc nested { (:; sleep 10); }
[1] 2748073
barmar@dev:~$ ps T --forest
PID TTY STAT TIME COMMAND
3462478 pts/1 Ss 0:00 /bin/bash
2748073 pts/1 S 0:00 \_ /bin/bash
2748074 pts/1 S 0:00 | \_ /bin/bash
2748075 pts/1 S 0:00 | \_ sleep 10
2748079 pts/1 R+ 0:00 \_ ps T --forest
: is a built-in command that does nothing.