person Tute World Team schedule Updated April 28, 2026

Linux Process States

At any moment, every process is in one of a handful of states. Reading these states from ps or top is one of the first things you do when diagnosing a problem.

The Process States

Code	Name	Meaning
R	Running/Runnable	On CPU, or ready and waiting for CPU time
S	Interruptible Sleep	Waiting for event (I/O, timer). Can be woken by signals.
D	Uninterruptible Sleep	Waiting for I/O. Cannot be killed — not even SIGKILL.
Z	Zombie	Finished, parent hasn't called wait() yet.
T	Stopped	Paused by SIGSTOP or Ctrl+Z. Resumes with SIGCONT.
I	Idle kernel thread	Similar to D but for kernel threads, truly idle

S vs D — The Critical Difference

Both S and D are "sleeping" — but they're very different:

Why can't you kill a process in D state? D (uninterruptible sleep) means the process is in the middle of a critical kernel operation — usually waiting for disk or network I/O at the kernel level. Killing it mid-operation could corrupt data. The kernel ignores SIGKILL in this state. The process will wake up when the I/O completes (or the kernel detects a hang).

Common causes of D state: slow NFS mount, failing disk, kernel bug. If many processes are stuck in D, look for disk issues first:

dmesg | grep -i error
iostat -x 1
iotop

Zombie Processes

What is a zombie process and why does it exist? When a process exits, it can't fully disappear yet. The parent might need its exit status (return code). So the kernel keeps a minimal "tombstone" — the process table entry with the exit status. This is the zombie state. Once the parent calls wait(), the zombie is cleaned up.

A few zombies are normal. Thousands of zombies means a parent process is leaking — it's forking children but never calling wait().

# Find zombie processes
ps aux | grep Z

# Find zombie's parent (to fix the real problem)
ps -o ppid= -p ZOMBIE_PID
# Then investigate or restart that parent process

State Modifiers in ps Output

Modifier	Meaning
`s`	Session leader (controls a terminal)
`+`	In the foreground process group
`l`	Multi-threaded
`N`	Low priority (nice > 0)
`<`	High priority (nice < 0)

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