Linux – Can /proc/PID still exist when PID isn’t running

linuxprocprocess

is it possible that a folder like /proc/4587 exists even though there isn't a process that has PID 4587?

Best Answer

If /proc is only the proc mount (and no one is playing tricks with overlays), no, a pid-based folder only exists as long as the corresponding process exists in some state (including as a zombie). In fact, just before returning a directory entry for a process id, the kernel re-validates the process’ existence — so at the instant a directory entry is returned, the corresponding process is still there. Accessing a directory also starts by looking up the corresponding process. (If the line numbers change, look for proc_pident_instantiate and proc_pident_lookup.)

You can run into issues caused by listing /proc and using the results later (even a few microseconds later): a process can be running when you list /proc, and stop before you act on the results.

Related Solutions

How to read any process’ /proc/pid/io

Use iotop.

It should be available in your repo for a Redhat/Centos/Fedora machine (if it is not already installed).

It outputs a similar info as top, but instead of the CPU/memory stats, you will get the IO stats (Disk reads, writes and swapin).

The options -p , -u and --only might be of interest to you.

For example, to see the IO activity of the process with ID 5435, use:

iotop -p 5435

From the man page:

   -p PID, --pid=PID
          A list of processes/threads to monitor (all by default).

   -u USER, --user=USER
          A list of users to monitor (all by default)

   -P, --processes
          Only show processes. Normally iotop shows all threads.

Linux – How does Linux decide the /proc/PID/stat “name” of a process

This is down to a Linux:

When a program starts another, it should use the name of the executable file as command line parameter $0, but it may choose to do otherwise. The Name field of /proc/PID/status is always set to the name of the executable by the kernel (but truncated to 15 characters).

The application itself can change a name. You can get the longer name from /proc/PID/cmdline (read up to the first null byte).

Best Answer

Related Solutions

How to read any process’ /proc/pid/io

Linux – How does Linux decide the /proc/PID/stat “name” of a process

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