subshell differences between /bin/sh and /bin/ksh

[Johnny C. Lam]

Could someone please explain the output below?  I have the following
shell script to inspect the PIDs of child processes:

        echo ">>> sleep"
        echo "  > parent: $$"
        /bin/sleep 1 &
        echo "  > child: $!"
        ps -o "pid,ppid,command"
        wait
        
        echo ">>> sleep in subshell"
        echo "  > parent: $$"
        ( /bin/sleep 1 ) &
        echo "  > child: $!"
        ps -o "pid,ppid,command"
        wait
        
        echo ">>> sleep twice in subshell"
        echo "  > parent: $$"
        ( /bin/sleep 1; /bin/sleep 2 ) &
        echo "  > child: $!"
        ps -o "pid,ppid,command"
        wait

If I run this script using /bin/sh, I get the following output:

        >>> sleep
          > parent: 13544
          > child: 4947
          PID  PPID COMMAND
          139 13544 ps -o pid,ppid,command 
         4947 13544 /bin/sleep 1 
         6189 13986 -ksh 
        13544  6189 /bin/sh t.sh 
        >>> sleep in subshell
          > parent: 13544
          > child: 14447
          PID  PPID COMMAND
         4630 13544 ps -o pid,ppid,command 
         6189 13986 -ksh 
        13544  6189 /bin/sh t.sh 
        14447 13544 /bin/sh t.sh 
        17445 14447 /bin/sleep 1 
        >>> sleep twice in subshell
          > parent: 13544
          > child: 1487
          PID  PPID COMMAND
         1487 13544 /bin/sh t.sh 
         1548  3239 /bin/sleep 1 
         3239  1487 /bin/sh t.sh 
         6189 13986 -ksh 
        13544  6189 /bin/sh t.sh 
        24087 13544 ps -o pid,ppid,command 

In the "sleep twice in subshell" (3rd) case, why is there another
subshell process with PID 3239?  The process chain looks like:

        /bin/sh t.sh > /bin/sh t.sh > /bin/sh t.sh > /bin/sleep 1

If I run this script using /bin/ksh, I get the following output:

        >>> sleep
          > parent: 9269
          > child: 5229
         PID  PPID COMMAND
         544  9269 ps -o pid,ppid,command 
        5229  9269 /bin/sleep 1 
        6189 13986 -ksh 
        9269  6189 /bin/ksh t.sh 
        >>> sleep in subshell
          > parent: 9269
          > child: 5139
          PID  PPID COMMAND
         5139  9269 /bin/sleep 1 
         6189 13986 -ksh 
         9269  6189 /bin/ksh t.sh 
        14597  9269 ps -o pid,ppid,command 
        >>> sleep twice in subshell
          > parent: 9269
          > child: 22025
          PID  PPID COMMAND
         5997 22025 /bin/sleep 1 
         6189 13986 -ksh 
         9269  6189 /bin/ksh t.sh 
        16165  9269 ps -o pid,ppid,command 
        22025  9269 /bin/ksh t.sh 

In the "sleep in subshell" (2nd) case, why is there no subshell?

        /bin/sh t.sh > /bin/sleep 1

I checked with bash-4.4.012 and zsh-5.1 and their output matches
with /bin/ksh.  This is on NetBSD-7.0.1.

Regards,

Johnny C. Lam
j...@netbsd.org