$BASHPID And $$ differ in some cases
An example is provided in the BASHPID
description of the bash manpage:
BASHPID
Expands to the process id of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.
Here is an example of a subshell outputting the contents of the variable, along with $$
and the contents of BASHPID
outside of the subshell.
$ echo $(echo $BASHPID $$) $$ $BASHPID
25680 16920 16920 16920
# | | | |
# | | | -- $BASHPID outside of the subshell
# | | -- $$ outside of the subshell
# | -- $$ inside of the subshell
# -- $BASHPID inside of the subshell
Subshells. $$
is specified by POSIX and always remains the value of the original shell process. $BASHPID
is a Bash-specific variable, and is always the value of the process from which the variable is dereferenced, counting subshells.
$ f() { printf '%s: %d, %d\n' "$1" $$ $BASHPID; };
$ ${BASH_VERSION+shopt -s lastpipe}; set +m;
$ f 1 >&2 | f 2
2: 31490, 31490
1: 31490, 32545
I did manage to convince the mksh maintainer to add BASHPID
to the most recent version, so it is somewhat portable. It is also possible to implement BASHPID
in ksh93 yourself on many platforms.