POSIX Process Group !

If you want to kill a process by its pid with all its sub process and dont want the child process to continue under init process ? Then process group is the one you may want to look at.

p1->p2->p3

By using p1, if you want to kill entire p1,p2 and p3 then we can look into process group. Remember that linux kill command has a option to send signals to entire process group like this :

kill -s sig_num -pid

Also, if you want to start a process through a cgi script and detach it completely from httpd process this would be helpful.

A small program to showcase the use of POSIX setsid.

use POSIX qw(setsid);

chdir '/'                 or die "Can't chdir to /: $!";
umask 0;
open STDIN, '/dev/null'   or die "Can't read /dev/null: $!";
#open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!";
open STDERR, '>/dev/null' or die "Can't write to /dev/null: $!";
defined(my $pid = fork)   or die "Can't fork: $!";
exit if $pid;
setsid                    or die "Can't start a new session: $!";

while(1) {
sleep(5);
print "Hello...\n";
}

Further details on the process groups ...

Process Groups and Tty Management

One of the areas least-understood by most UNIX programmers is process-group management, a topic that is inseparable from signal-handling.

To understand why process-groups exist, think back to the world before windowing systems.

Your average developer wants to run several programs simultaneously -- usually at least an editor and a compilation, although often a debugger as well. Obviously you cannot have two processes reading from the same tty at the same time -- they'll each get some of the characters you type, a useless situation. Likewise output should be managed so that your editor's output doesn't get the output of a background compile intermixed, destroying the screen.

This has been a problem with many operating systems. One solution, used by Tenex and TOPS-20, was to use process stacks. You could interrupt a process to run another process, and when the new process was finished the old would restart.

While this was useful it didn't allow you to switch back and forth between processes (like a debugger and editor) without exiting one of them. Clearly there must be a better way.

The Berkeley Approach

The Berkeley UNIX folks came up with a different idea, called process groups. Whenever the shell starts a new command each process in the command (there can be more than one, eg "ls | more") is placed in its own process group, which is identified by a number. The tty has a concept of "foreground process group", the group of processes which is allowed to do input and output to the tty. The shell sets the foreground process group when starting a new set of processes; by convention the new process group number is the same as the process ID of one of the members of the group. A set of processes has a tty device to which it belongs, called its "controlling tty". This tty device is what is returned when /dev/tty is opened.

Because you want to be able to interrupt the foreground processes, the tty watches for particular keypresses (^Z is the most common one) and sends an interrupt signal to the foreground process group when it sees one. All processes in the process group see the signal, and all stop -- returning control to the shell.

At this point the shell can place any of the active process groups back in the foreground and restart the processes, or start a new process group.

To handle the case where a background process tries to read or write from the tty, the tty driver will send a SIGTTIN or SIGTTOU signal to any background process which attempts to perform such an operation. Under normal circumstances, therefore, only the foreground process(es) can use the tty.

The set of commands to handle process groups is small and straightforward. Under BSD, the commands are:

int setpgrp(int process_id, int group_number);

Move a process into a process group. If you are creating a new process group the group_number should be the same as process_id. If process_id is zero, the current process is moved.

int getpgrp(int process_id);

Find the process group of the indicated process. If process_id is zero, the current process is inspected.

int killpgrp(int signal_number, int group_number);

Send a signal to all members of the indicated process group.

int ioctl(int tty, TIOCSETPGRP, int foreground_group);

Change the foreground process group of a tty.

int ioctl(int tty, TIOCGETPGRP, int *foreground_group);

Find the foreground process group of a tty.

int ioctl(int tty, TIOCNOTTY, 0);

Disassociate this process from its controlling tty. The next tty device that is opened will become the new controlling tty.

The POSIX Approach

The BSD process-group API is rarely used today, although most of the concepts survive. The POSIX specification has provided new interfaces for handling process groups, and even overloaded some existing ones. It also limits several of the calls in ways which BSD did not.

The POSIX process-group API is:

int setpgid(int process_id, int process_group);

Move a process into a new process group. Process_id is the process to move, process_group is the new process group.

int getpgid(int process_id);

Find the process group of a process. Process_id is the process to inspect.

int getpgrp(void);

Find the process group of the current process. This is identical to getpgrp(getpid()).

int tcsetpgrp(int tty, int foreground_group);

Change the foreground process group of a tty. Tty is the file descriptor of the tty to change, foreground_group is the new foreground process group.

int tcgetpgrp(int tty, int *foreground_group);

Find the foreground process group of a tty. Tty is the file descriptor of the tty to inspect, foreground_group is returned filled with the foreground process group of the tty.

int kill(int -process_group, int signal_number);

Send a signal to a process group. Note that process_group must be passed as a negative value, otherwise the signal goes to the indicated process.

Differences between POSIX and BSD Process Group Management

The setpgrp() function is called setpgid() under POSIX and is essentially identical. You must be careful under POSIX not to use the setpgrp() function -- usually it exists, but performs the operation of setsid().

The getpgrp() function was renamed getpgid(), and getpgid() can only inspect the current process' process group.

The killpgrp() function doesn't exist at all. Instead, a negative value passed to the kill() function is taken to mean the process group. Thus you'd perform killpgrp(process_group) by calling kill(-process_group).

The ioctl() commands for querying and changing the foreground process group are replaced with first-class functions:

• int tcsetpgrp(int tty, int process_group);
• int tcgetpgrp(int tty, int *process_group);

While the original BSD ioctl() functions would allow any tty to take on any process group (or even nonexistant process groups) as its foreground tty, POSIX allows only process groups which have the tty as their controlling tty. This limitation disallows some ambiguous (and potentially security-undermining) cases present in BSD.

The TIOCNOTTY ioctl used in BSD is replaced with the setsid() function, which is essentially identical to:

if (getpgrp() != getpid()) { ioctl(tty, TIOCNOTTY, 0); setpgrp(getpid(), getpid()); }

It releases the current tty and puts the calling process into its own process group. Notice that nothing is done if the calling process is already in its own process group -- this is another new limitation, and eliminates some ambiguous cases that existed in BSD (along with some of BSD's flexibility).

Reference : http://www.cs.ucsb.edu/~almeroth/classes/W99.276/assignment1/signals.html

Vim - Cheatsheet

A cheat sheet of some useful and most often used Vim commands. This Vim cheat sheet isn't trying to include all the Vim commands in the known universe, but should list the most essential ones.

Author: Nana Långstedt <>
tuXfile created: 18 January 2003
Last modified: 22 September 2005

< The list of Vim commands >

Working with files
Vim command	Action
:e filename	Open a new file. You can use the Tab key for automatic file name completion, just like at the shell command prompt.
:w filename	Save changes to a file. If you don't specify a file name, Vim saves as the file name you were editing. For saving the file under a different name, specify the file name.
:q	Quit Vim. If you have unsaved changes, Vim refuses to exit.
:q!	Exit Vim without saving changes.
:wq	Write the file and exit.
:x	Almost the same as :wq, write the file and exit if you've made changes to the file. If you haven't made any changes to the file, Vim exits without writing the file.
Moving in the file
These Vim commands and keys work both in command mode and visual mode.
Vim command	Action
j or Up Arrow	Move the cursor up one line.
k or Down Arrow	Down one line.
l or Right Arrow	Right one character.
h or Left Arrow	Left one character.
e	To the end of a word.
E	To the end of a whitespace-delimited word.
b	To the beginning of a word.
B	To the beginning of a whitespace-delimited word.
0	To the beginning of a line.
^	To the first non-whitespace character of a line.
$	To the end of a line.
H	To the first line of the screen.
M	To the middle line of the screen.
L	To the the last line of the screen.
:n	Jump to line number n. For example, to jump to line 42, you'd type :42
Inserting and overwriting text
Vim command	Action
i	Insert before cursor.
I	Insert to the start of the current line.
a	Append after cursor.
A	Append to the end of the current line.
o	Open a new line below and insert.
O	Open a new line above and insert.
C	Change the rest of the current line.
r	Overwrite one character. After overwriting the single character, go back to command mode.
R	Enter insert mode but replace characters rather than inserting.
The ESC key	Exit insert/overwrite mode and go back to command mode.
Deleting text
Vim command	Action
x	Delete characters under the cursor.
X	Delete characters before the cursor.
dd or :d	Delete the current line.
Entering visual mode
Vim command	Action
v	Start highlighting characters. Use the normal movement keys and commands to select text for highlighting.
V	Start highlighting lines.
The ESC key	Exit visual mode and return to command mode.
Editing blocks of text
The Vim commands marked with (V) work in visual mode, when you've selected some text. The other commands work in the command mode, when you haven't selected any text.
Vim command	Action
~	Change the case of characters. This works both in visual and command mode. In visual mode, change the case of highlighted characters. In command mode, change the case of the character uder cursor.
> (V)	Shift right.
< (V)	Shift left.
c (V)	Change the highlighted text.
y (V)	Yank the highlighted text. In Winblows terms, "copy the selected text to clipboard."
d (V)	Delete the highlighted text. In Winblows terms, "cut the selected text to clipboard."
yy or :y or Y	Yank the current line. You don't need to highlight it first.
dd or :d	Delete the current line. Again, you don't need to highlight it first.
p	In Winblows terms, "paste" the contents of the "clipboard". In Vim terms, you "put" the text you yanked or deleted. Put characters after the cursor. Put lines below the current line.
P	Put characters before the cursor. Put lines above the current line.
Undo and redo
Vim command	Action
u	Undo the last action.
U	Undo all the latest changes that were made to the current line.
Ctrl + r	Redo.
Search
Vim command	Action
/pattern	Search the file for pattern.
n	Scan for next search match in the same direction.
N	Scan for next search match but opposite direction.
Replace
Vim command	Action
:rs/foo/bar/a	Substitute foo with bar. r determines the range and a determines the arguments.
The range (r) can be
nothing	Work on current line only.
number	Work on the line whose number you give.
%	The whole file.
Arguments (a) can be
g	Replace all occurrences in the line. Without this, Vim replaces only the first occurrences in each line.
i	Ignore case for the search pattern.
I	Don't ignore case.
c	Confirm each substitution. You can type y to substitute this match, n to skip this match, a to substitute this and all the remaining matches ("Yes to all"), and q to quit substitution.
Examples
:452s/foo/bar/	Replace the first occurrence of the word foo with bar on line number 452.
:s/foo/bar/g	Replace every occurrence of the word foo with bar on current line.
:%s/foo/bar/g	Replace every occurrence of the word foo with bar in the whole file.
:%s/foo/bar/gi	The same as above, but ignore the case of the pattern you want to substitute. This replaces foo, FOO, Foo, and so on.
:%s/foo/bar/gc	Confirm every substitution.
:%s/foo/bar/c	For each line on the file, replace the first occurrence of foo with bar and confirm every substitution.

Play Windows Games on Linux - Try Cedega Today !

So, you are 1 out of many people who use windows just for gaming ??? If so, you can try this cedega tool and check whether your favourite game works on linux ! There are a few options when it comes to playing Windows games on Linux. Cedega is a great tool to allow you to play these games under Linux.

You can check whether your favourite game is supported by cedega here. (http://www.cedega.com/gamesdb/)

Remember that latest versions of cedega is not free. You may need to subscribe it here http://www.cedega.com/subscription/subscribe.html

But, older stable version can be downloaded from their cvs and compiled :)

Compilation Instructions : http://www.linux-gamers.net/modules/wiwimod/index.php?page=HOWTO+Cedega+CVS

Help Script : http://winecvs.linux-gamers.net/index.php/Main_Page

If you are scared about compiling by yourself, dont worry ! Your favourite linux distribution (Ubuntu, Suse, Fedora ...) might have already packaged it for you. Just check in their repositories :)