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linuxbashprocesspidproc

My linux bash $$ doesn't match /proc/self link?

I'm on RHEL 5 and tried /proc like this:

$echo $$
50040

$ls -ld /proc/self
lrwxrwxrwx 1 root root 64 Jan 22 15:25 /proc/self -> 22485

I expected that /proc/self link to a subdirectory of /proc that's my current process id. But $$ shows process id is 50040, not 22485. Why is that?

Solution

  • The PID you are seeing when issuing the ls command is the one for the ls command, not that of your shell. If you do it many times in a row, you will see it is different each time:

    fred> ls -ld /proc/self
lrwxrwxrwx 1 root root 0 Jan 12 13:13 /proc/self -> 5075

fred> ls -ld /proc/self
lrwxrwxrwx 1 root root 0 Jan 12 13:13 /proc/self -> 5076

fred> ls -ld /proc/self
lrwxrwxrwx 1 root root 0 Jan 12 13:13 /proc/self -> 5077

    If you want to get the PID of the shell, you need to make sure you use /proc/self when the shell is running, not one of its sub-processes:

    cd /proc/self ; pid=$(awk '{print $1}' stat) ; cd -

    (the cd is a bash internal command so, at the point where you access /proc/self, you're still running in the shell process itself).