Btukfyl

How can I get the pid of a subshell?

For example:

$ echo $$

16808

This doesn't work, because the original shell expands $$:

$ ( echo $$ )

16808

Why does single quoting not work? After the original shell removes the single quote, does the subshell not expand $$ in itself?

$ ( echo '$$' )

$$

Why does eval not work either? Is eval run by the subshell? Why does it give me the original shell's PID?

$ ( eval echo '$$' )

16808

Thanks.

In addition to bash's $BASHPID, you can do it portably with:

pid=$(exec sh -c 'echo "$PPID"')

Example:

(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")

You can make it into a function:

# usage getpid [varname]

getpid(){

    pid=$(exec sh -c 'echo "$PPID"')

    test "$1" && eval "$1=$pid"

}

Notice that some shells (eg. zsh or ksh93) do NOT start a subprocess for each subshell created with (...); in that case, $pid may be end up being the same as $$, which is just right, because that's the PID of the process getpid was called from.

$ echo $BASHPID

37152

$ ( echo $BASHPID )

18633

From the manual:

BASHPID

Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.

$

Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the
subshell.

Related:

• 
  Do parentheses really put the command in a subshell?, especially parts of Gilles' answer.