RFC791.org processes help

In 1957, Ken Olsen and a group of his buddies got tired of the sales practices of IBM corporation and started their own computer company, Digital Equipment Corporation. They started building much cheaper mainframes which became really popular. Unix was originally designed on a Digital Equipment machine.

[ rfc791.ORG : Unix Help : Unix Processes ]

ps, kill - process management programs

Unix is a multitasking operating system, which means it can run multiple processes at once. A process is a running program.

The first process you'll see when you log in is the shell. The shell allows you to start other processes. Most shells allow you to start a process in the background using the '&' operator.

[18]Tethys% rm -fr * &
[1] 90463
[19]Tethys%

This deletes all the files in the current directory in the background. Being in the background as it is, you can interact with the shell as the background job does it's thing. The [1] tells you the job number so you can pull the job back into the foreground if neccesary. The 90463 tells you the Process ID, which is the system-wide unique number that identifies your process. You can see all the processes, complete with their IDs by running the ps command.

[19]Tethys% ps
  PID  TT  STAT      TIME COMMAND
90280  p1  Ss     0:00.08 -csh (csh)
90463  p1  T      0:00.20 rm -fr *
90470  p1  R+     0:00.00 ps

Here we see our shell (csh), our background file deletion (rm), and the process listing our processes which we just ran (ps).

Processes under Unix can intercommunicate by sending signals - special messages that command a process to perform a certain action.
There are several signals that can be sent to a process. A program can decide how to react when it receives a certain signal, however usually a given signal will produce the same sort of result in any program.
You can send signals to processes yourself using the kill program.

[20]Tethys% ps
  PID  TT  STAT      TIME COMMAND
90280  p1  Ss     0:00.08 -csh (csh)
90463  p1  T      0:00.20 rm -fr *
90470  p1  R+     0:00.00 ps
[21]Tethys% kill -KILL 90463
[22]Tethys%

The -KILL part of the previous command tells the kill program that the signal you want to send it is the KILL signal. Here are some of the signals that processes usually can receive:

HUP - The HUP signal's original purpose was to tell a program that it's controlling terminal had hung up (ie - a dialup shell connection had dropped). It's now also used to tell system daemon (background) processes (which don't have a controlling terminal) to restart and reread their configuration files.
INT - the INT signal tells a process that the user has hit the interrupt key sequence (usually CTRL+C, but can be reconfigured using the `stty' program). Usually programs exit upon receiving this signal.
KILL - This is the signal after which the `kill' program was named, as it's usually used to send the KILL signal. KILL is one of two signals a process doesn't get to choose how to react to - KILL makes a process exit.
QUIT - Another signal you can send to a process using a keyboard combination - CTRL+\ (depending on whether you've reconfigured it with the `stty' program). Programs almost always exit in response to a QUIT signal, even if they don't respond to an INT signal.
TERM - This is the default signal the `kill' program sends:
```
[22]Tethys% kill 216
```
This command sends a TERM signal to process 216. Programs that need to "shutdown" properly before exiting hope to receive this signal before a KILL signal, because they can't respond to a KILL signal.