The Linux Boot Process

The Linux boot process consists of several stages each represented by a different component. The following list briefly summarizes the boot process and features all the major components involved.

• getty - BSD Unix man page (get tty)
  
• ttymon - Solaris man page
• agetty(8) - alternative Linux agetty
• mgetty(8) - mgetty (modem aware getty)
• mingetty(8) - mingetty (minimal getty)

From Wikipedia, the free encyclopedia

On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table. This entry is still needed to allow the process that started the (now zombie) process to read its exit status. The term zombie process derives from the common definition of zombie—an undead person. In the term's colorful metaphor, the child process has died but has not yet been reaped.

When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes. However, the process's entry in the process table remains. The parent can read the child's exit status by executing the wait system call, at which stage the zombie is removed. The wait call may be executed in sequential code, but it is commonly executed in a handler for the SIGCHLD signal, which the parent receives whenever a child has died.

After the zombie is removed, its process ID and entry in the process table can then be reused. However, if a parent fails to call wait, the zombie will be left in the process table. In some situations this may be desirable, for example if the parent creates another child process it ensures that it will not be allocated the same process ID. On modern UNIX-like systems (that comply with SUSv3 specification in this respect), the following special case applies: if the parent explicitly ignores SIGCHLD by setting its handler to SIG_IGN (rather than simply ignoring the signal by default) or has the SA_NOCLDWAIT flag set, all child exit status information will be discarded and no zombie processes will be left.

A zombie process is not the same as an orphan process. An orphan process is a process that is still executing, but whose parent has died. They do not become zombie processes; instead, they are adopted by init (process ID 1), which waits on its children.

Zombies can be identified in the output from the Unix ps command by the presence of a “Z” in the “STAT” column. Zombies that exist for more than a short period of time typically indicate a bug in the parent program, or just an uncommon decision to reap children (see example). As with other leaks, the presence of a few zombies is not worrisome in itself, but may indicate a problem that would grow serious under heavier loads. Since there is no memory allocated to zombie processes except for the process table entry itself, the primary concern with many zombies is not running out of memory, but rather running out of process ID numbers.

To remove zombies from a system, the SIGCHLD signal can be sent to the parent manually, using the kill command. If the parent process still refuses to reap the zombie, the next step would be to remove the parent process. When a process loses its parent, init becomes its new parent. Init periodically executes the wait system call to reap any zombies with init as parent.

[edit] Examples

Synchronously waiting for specific child processes in a (specific) order may leave zombies present longer than the above-mentioned “short period of time”. It is not necessarily a program bug, but rather a programming paradigm that is not seen very often in the wild.

#include 
#include 
#include 

int main(void)
{
       pid_t pids[5];
       int i;

       for (i = 4; i >= 0; --i) {
               pids[i] = fork();
               if (pids[i] == 0) {
                       sleep(i+1);
                       _exit(0);
               }
       }

       for (i = 4; i >= 0; --i)
               waitpid(pids[i], NULL, 0);

       return 0;
}

• X client: a process (like firefox or xemacs) which uses the X client API to display things and receive mouse/keyboard events.
• X server: a process (usually just "X") which X clients connect to. At times (as we'll see below) other processes can act as X servers.
  

• There is no X server @remote, so why didn't xcalc just fail on startup?
• Why was the display number "11".
• How did xcalc show something @home?

1. You type "ssh -X user@remote" in your terminal
2. The ssh process connects to the sshd server @remote.
3. sshd spawns a new process that is an X-server-emulator listening on some display number, e.g. "11"
   
4. sshd sets the $DISPLAY to point to that local "X-server" (e.g. ":11")
5. xcalc reads this $DISPLAY and conncects to this X-server. xcalc thinks it's displaying to the local machine.
6. the X-server-emulator simply forwards the X commands from xcalc through the ssh connection, to the original ssh process.
7. The ssh process @home now acts as a normal X-client and sends those commands to the X-server @home.
   

• Normally, X-traffic (like your keystrokes) is sent unencrypted from X-client to X-server.
• Ssh nicely sends that data through an encrypted channel, so it doesn't go over the internet in the clear.
• "xhost +remote" is putting a lot of trust in 'remote' being a nice guy. If remote ever gets hacked, it could connect to the X-server @home and listen to all its keystrokes.

set completion-query-items 100

set completion-ignore-case on
set show-all-if-ambiguous off

set input-meta on
set output-meta on
set convert-meta off
set bell-style none

"\e[2~": paste-from-clipboard
"\e[3~": delete-char
"\e[5~": beginning-of-history
"\e[6~": end-of-history
"\e[7~": beginning-of-line
"\e[8~": end-of-line

msys中文显示

注意：中文注释可以保留，但文件必须采用linux文件格式，即用\n作换行符。可以用notepad++, vim等工具创建

.bash_profile

# 让ls和dir命令显示中文和颜色
alias ls='ls --show-control-chars --color'
alias dir='dir -N --color'
alias less='less -r'
# 设置为中文环境，使提示成为中文
export LANG="zh_CN.GBK"
# 输出为中文编码
export OUTPUT_CHARSET="GBK"

# 关闭bash命令行8字节字符转义符的转换
set convert-meta off

# 使bash命令行支持8字节字符输入
set input-meta on

# 使bash命令行支持8字节字符输出
set output-meta on

set meta-flag on

set completion-ignore-case on

Internationalization is a complex issue. The short answer is that Cygwin is not Unicode-aware, so things that might work in Linux will not necessarily work on Cygwin. However, some things do work. To type international characters (£äö) in bash, add the following lines to your ~/.inputrc file and restart bash:

set meta-flag on
set convert-meta off
set output-meta on
set input-meta on
set kanji-code sjis

These are options to the readline library, which you can read about in the bash(1) and readline(3) man pages. Other tools that do not use readline for display, such as less and ls, require additional settings, which could be put in your ~/.bashrc:

alias less='/bin/less -r'
alias ls='/bin/ls -F --color=tty --show-control-chars'
export LANG="ja_JP.SJIS"
export OUTPUT_CHARSET="sjis"

These examples use the Japanese Shift-JIS character set, obviously you will want to change them for your own locale.