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 '$$' )
16808
Thanks.
$ echo $BASHPID
37152
$ ( echo $BASHPID )
18633
From the manual:
BASHPIDExpands to the process ID of the current bash process. This differs from
$$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:
$BASHPID in a variable and use that in the subshell. There is $PPID, but that's the parent PID of the shell in the same sense that $$ is the PID of the shell (it's not reset in a subshell). There is no $BASHPPID variable.
BASHPID was introduced in release 4.0. You are either unknowingly running some other shell, or an older version of bash.
In addition to bash's $BASHPID, you can do it portably with:
pid=$(exec sh -c 'echo "$PPID"')
Example:
(pid=$(exec sh -c 'echo "$PPID"'); echo "$$ $pid")
You can make it into a function:
# usage getpid [varname]
getpid(){
pid=$(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 $$, which is just right, because that's the PID of the process getpid was called from.
/bin/sh (dash) and in busybox (ash).
ksh93, for instance.
(...) from the example which may not spawn a separate process, as it does in bash.
zsh or yash optimise out a fork() for the last command in a subshell. They may even optimise out the fork for the subshell if it's the last command in a script so your getpid could even report the parent of $$. You could define getpid as: getpid(){ sh -c 'echo "$PPID"'; return; } to disable avoid the problem.
exec or without that optimization, the sh -c ... process will be a grandchild, instead of a child of the process where a $(...) command substitution is used, and $PPID will be the pid of the $(...) subshell. That's exactly what happens in the set -E + trap ERR bash example above.
On Linux a cross-shell solution (at least dash, bash, zsh) which does not spawn a new process is
read -r this_pid < /proc/self/stat; echo ${this_pid%% *}
At least in bash and zsh we can also use space as read-delimeter:
read -d ' ' this_pid < /proc/self/stat ; echo $this_pid
See also man 5 proc section /proc/[pid]/stat
read this_pid _rest < /proc/self/stat could also be used (e.g. with dash). bash can do that 128-byte optimize then file is seekable (which is (kinda?) nice). that read syscall per byte is (in other cases) a bit "unfortunate"
To answer the original "why" question - why doesn't $$ expands to subshell pid, even lazily evaluated by eval?
Quoting POSIX doc:
$
Expands to the decimal process ID of the invoked shell. In a subshell (see Shell Execution Environment ), '$' shall expand to the same value as that of the current shell.
$$expansion" vs. "different pid in subshell").