Why is $$ returning the same id as the parent process?
BashShellPidSubshellBash Problem Overview
I have problem with Bash, and I don't know why.
Under shell, I enter:
echo $$ ## print 2433
(echo $$) ## also print 2433
(./getpid) ## print 2602
Where getpid
is a C program to get current pid, like:
> int main() { > printf("%d", (int)getpid()); > return 0; > }
What confuses me is that:
- I think "(command)" is a sub-process (am i right?), and i think its pid should be different with its parent pid, but they are the same, why...
- When I use my program to show pid between parenthesis, the pid it shows is different, is it right?
- Is
$$
something like macro?
Can you help me?
Bash Solutions
Solution 1 - Bash
$$
is defined to return the process ID of the parent in a subshell; from the man page under "Special Parameters":
> $ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
In bash
4, you can get the process ID of the child with BASHPID
.
~ $ echo $$
17601
~ $ ( echo $$; echo $BASHPID )
17601
17634
Solution 2 - Bash
You can use one of the following.
$!
is the PID of the last backgrounded process.kill -0 $PID
checks whether it's still running.$$
is the PID of the current shell.
Solution 3 - Bash
- Parentheses invoke a subshell in Bash. Since it's only a subshell it might have the same PID - depends on implementation.
- 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.
$$
is an alias in Bash to the current script PID. See differences between$$
and$BASHPID
here, and right above that the additional variable$BASH_SUBSHELL
which contains the nesting level.
Solution 4 - Bash
Try getppid()
if you want your C program to print your shell's PID.
Solution 5 - Bash
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
Solution 6 - Bash
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 $PIDs
Solution 7 - Bash
if you want a simple shell script for getting the maximum PID with variable, do this
pid=$(cat /proc/sys/kernel/pid_max)
echo $pid
that will print you the maximum PID can be.