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

177
votes

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

"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?

7
bashshellpidsubshell
Note that getpid would show a different process ID even if it weren't run in a subshell. - chepner
@Marian echo $$ $BASHPID ; ( echo $$ $BASHPID ) demonstrates that it does. Round brackets create a subshell. The statements may change variable values, and the parent shell must not see those changes. This is implemented as a fork() operation. - Ben

7 Answers

238
votes

$$ 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
83
votes

You can use one of the following.

  • $! is the PID of the last backgrounded process.
  • kill -0 $PID checks whether it's still running.
  • $$ is the PID of the current shell.
26
votes
  1. Parentheses invoke a subshell in Bash. Since it's only a subshell it might have the same PID - depends on implementation.
  2. The C program you invoke is a separate process, which has its own unique PID - doesn't matter if it's in a subshell or not.
  3. $$ 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.
7
votes

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

3
votes

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
2
votes

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

1
votes

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.