Why is $$ returning the same id as the parent process?

Question

I have problem with Bash, and I don't know why.
Under shell, I enter:

echo $$    ## print 2433
(echo $$)  ## also print 2433
(./getpid) ## print 2602

Where getpid is a C program to get current pid, like:

   int main() {
    printf("%d", (int)getpid());
    return 0;
   }

What confuses me is that:

1. I think "(command)" is a sub-process (am i right?), and i think its pid should be different with its parent pid, but they are the same, why...
2. When I use my program to show pid between parenthesis, the pid it shows is different, is it right?
3. Is $$ something like macro?

Can you help me?

Answer 1

$$ is defined to return the process ID of the parent in a subshell; from the man page under "Special Parameters":

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

In bash 4, you can get the process ID of the child with BASHPID.

~ $ echo $$
17601
~ $ ( echo $$; echo $BASHPID )
17601
17634

Answer 2

You can use one of the following.

• $! is the PID of the last backgrounded process.
• kill -0 $PID checks whether it's still running.
• $$ is the PID of the current shell.

Answer 3

1. Parentheses invoke a subshell in Bash. Since it's only a subshell it might have the same PID - depends on implementation.
2. The C program you invoke is a separate process, which has its own unique PID - doesn't matter if it's in a subshell or not.
3. $$ is an alias in Bash to the current script PID. See differences between $$ and $BASHPID here, and right above that the additional variable $BASH_SUBSHELL which contains the nesting level.

Answer 4

Try getppid() if you want your C program to print your shell's PID.

Answer 5

this one univesal way to get correct pid

pid=$(cut -d' ' -f4 < /proc/self/stat)

same nice worked for sub

SUB(){
    pid=$(cut -d' ' -f4 < /proc/self/stat)
    echo "$$ != $pid"
}

echo "pid = $$"

(SUB)

check output

pid = 8099
8099 != 8100

Answer 6

If you were asking how to get the PID of a known command it would resemble something like this:

If you had issued the command below #The command issued was ***

dd if=/dev/diskx of=/dev/disky

---

Then you would use:

PIDs=$(ps | grep dd | grep if | cut -b 1-5)

What happens here is it pipes all needed unique characters to a field and that field can be echoed using

echo $PIDs

Answer 7

if you want a simple shell script for getting the maximum PID with variable, do this

pid=$(cat /proc/sys/kernel/pid_max)
echo $pid

that will print you the maximum PID can be.

Answer 8

Portable way of achieving that

get_own_pid() {
  # This function being called in a subshell,
  # it must returns the pid of the parent of the "cut" command parent
  cut -d' ' -f4 < /proc/self/stat \
    | xargs -I% sh -c 'cut -d" " -f4 < /proc/%/stat'
}

get_parent_pid() {
  # Same thing but repeating the last command once more to get the parent one level above
  cut -d' ' -f4 < /proc/self/stat \
    | xargs -I% sh -c 'cut -d" " -f4 < /proc/%/stat' \
    | xargs -I% sh -c 'cut -d" " -f4 < /proc/%/stat'
}


# Here pid is the same as the $$ pid because called from main process
MY_PID=$(get_own_pid)
echo "$$ == ${MY_PID}"

# Here called in a subprocess, the returned pid is different
(
  MY_CHILD_PID=$(get_own_pid)
  PARENT_PID_FROM_CHILD=$(get_parent_pid)
  echo "$$ != ${MY_CHILD_PID}"
  echo "$$ == ${PARENT_PID_FROM_CHILD}"
)

Inspired from artem lapkin answer, thanks!