User vs kernel space
A process is executing either in user space or in kernel space.
When executing in user space, the process can or cannot do certain things according to its privileges.
A process switches to kernel space by calling a
syscall.
Apparently user space is also called user mode, kernel space is also called kernel mode.
Process ID (PID)
Each process is identified by a numerical id, the so called process id, sometimes abbreviated as PID.
Process IDs are unique per system at a given time, but an ID might be reused after a process has terminated.
Displaying details about a given process
The shell utility
ps allows to print the details of a process given its id:
$ ps p 894
PID TTY STAT TIME COMMAND
894 pts/0 S 0:00 -bash
Process 1
The first process that the kernel starts is the process with PID = 1.
This process is the direct or indirect parent for all other processes.
By default, the kernel tries to start
/sbin/init as this first process. Historically, this is the init process of
System V which in more recent times is replaced with upstart or
systemd.
Threads
Threads are similar to processes but the threads of a given process share one memory address space.
However, each thread has its own sets of registers and its own execution stack and perhaps private memory
Execution domains/personalities
Linux supports different execution domains, or personalities, for each process.
Among other thins, an execution domains (or personality?) tells Linux how to map signal numbers into signal actions.
The execution domain system allows Linux to provide limited support for binaries compiled under other UNIX-like operating systems.
See also
nproc reports the number of available
CPUs to the current process.
prtstat prints statistics about a process.
ps reports a snapshot of the current processes. With
pstree, the processes are shown as a tree.
pidof finds a process's PID.
pgrep finds processes by their name and attributes.
pstack prints a stack trace.