Example of subshell without child-shell process

Question

I just recently learned that "subshell" is not the same as "child shell process" (see for example What is the exact difference between a "subshell" and a "child process"? and the POSIX definitions of subshell and child process).

To convince myself of this, I am looking for a command that illustrates (proves) that a subshell is created without a child-shell being spawned.

For now, everything I tried seemed to spawn a child-shell whenever a subshell is created:

$ echo $BASHPID; (pwd; cd ..; echo $BASHPID; pwd); pwd      # `( ...)` executed in a subshell
                                                            # and in a child-shell process

$ >&2 ps | ps       # Theoretically executed in two subshells and apparently without child-shells
                    # but I cannot be sure due to the outcome of the next example

$ $ >&2 echo $BASHPID | ps      # `ps` doesn't display a child-shell for the execution of `echo`
953790                          # but `echo $BASHPID` shows a new process that is necessarily
    PID TTY         TIME CMD    # a child-shell since echo is a built-in 
 948538 pts/2   00:00:00 bash
 953791 pts/2   00:00:00 ps

I am looking for a way to demonstrate that having a subshell doesn't necessarily imply having a child-shell...

Bash 5.0.17

Answer 1

In the bash shell, subshells are implemented by forking a child process, so you won't see a case of a subshell not running in a child process in that shell.

ksh93 is the only shell that I know that skips the forking when possible for subshells (an optimisation that is still quite buggy and that the successive people that have tried to maintain it after AT&T disbanded the team that had written it have considered removing).

If you do for instance:

 strace ksh93 -c 'pwd; (cd /; umask 0; pwd; exit 2); pwd'

You'll see ksh93 not forking any process but do something like this instead:

openat(AT_FDCWD, ".", O_RDONLY|O_PATH)  = 3
fcntl(3, F_DUPFD, 10)                   = 10
close(3)                                = 0
fcntl(10, F_SETFD, FD_CLOEXEC)          = 0
[...]

Which saves the current directory on fd 10. Then:

chdir("/")                              = 0
umask(000)                              = 002

Which changes the current directory and umask in the subshell. And upon termination of the subshell (the exit 2 not calling the _exit() system call):

fchdir(10)                              = 0
close(10)                               = 0

To restore the current working directory and:

umask(002)                              = 000

To restore the umask.

Some shells like FreeBSD's sh can skip the fork in very specific cases, like in:

var=$(printf %04d "$n")

(here with a printf builtin, and no change to the environment is being done in there).

In a pipeline, all components have to run concurrently, so they have to run in separate processes, even in ksh93.

In bash, they all run in child processes. In AT&T ksh or zsh, or with bash -O lastpipe (when non-interactive), the rightmost one doesn't (of course, you still need to fork a child process to run external commands such as ps).

You don't see an extra bash process in ps >&2 | ps or (ps) because ps is executed directly in that child process, which before executing ps was bash interpreting the pipeline component: the subshell. For instance, in:

n=0; /bin/true "$((n=1))" | /bin/echo "$((n=2))"; echo "$n"

You'll see 2 and 0 in bash, and 2 and 2 in zsh/ksh93. /bin/true and /bin/echo are executed in child processes, /bin/true directly in the subshell process that had done n=1 earlier, same in bash for /bin/echo (and n=2), but in zsh/ksh/bash -O lastpipe, the n=2 was done in the main shell process, and a child only forked to execute that external utility, just like when you run /bin/echo "$((n=2))" not as part of a pipeline.

In bash (contrary to zsh/ksh), you do see an extra bash process in (: anything; ps), the optimisation is only done if the subshell has only one external command, you'd need to use exec to do that optimisation by hand there: (: anything; exec ps).

Same goes for { ps; } | cat.

Answer 2

I believe you are misinterpreting "subshell" and "subprocess" (aka child process) and the bash man page doesn't really help much in clearing up the confusion.

A subshell is loosely what you get when the current shell calls fork(). Fork creates subprocesses; so the subshell is a subprocess.

The bash man page says that bash "executes commands in a subshell" but what isn't clear from this is that to run external processes (like ps), it then calls exec() in the subshell, replacing the running bash subshell with the new command's executable. In some cases, like when () is used in bash, the subshell is started by ( and exits at ), and commands in between may be started in their own subshells/subprocesses.

The only way that subshell and child process are not the same is that the child process may or may not (anymore) be another instance of the same executable.