进程

Modern operating systems are usually multitasking, meaning that they create the illusion of doing more than one thing at once by rapidly switching from one executing program to another. The Linux kernel manages this through the use of processes. Processes are how Linux organizes the different programs waiting for their turn at the CPU.

通常，现在的操作系统都支持多任务，意味着操作系统通过在一个执行中的程序和另一个 程序之间快速地切换造成了一种它同时能够做多件事情的假象。Linux 内核通过使用进程来 管理多任务。进程，就是 Linux 组织安排正在等待使用 CPU 的各种程序的方式。

Sometimes a computer will become sluggish or an application will stop responding. In this chapter, we will look at some of the tools available at the command line that let us examine what programs are doing, and how to terminate processes that are misbehaving.

有时候，计算机变得呆滞，运行缓慢，或者一个应用程序停止响应。在这一章中，我们将看一些 可用的命令行工具，这些工具帮助我们查看程序的执行状态，以及怎样终止行为不当的进程。

This chapter will introduce the following commands:

这一章将介绍以下命令：

• ps– Report a snapshot of current processes

• top – Display tasks

• jobs – List active jobs

• bg – Place a job in the background

• fg – Place a job in the foreground

• kill – Send a signal to a process

• killall – Kill processes by name

• shutdown – Shutdown or reboot the system

• ps – 报告当前进程快照

• top – 显示任务

• jobs – 列出活跃的任务

• bg – 把一个任务放到后台执行

• fg – 把一个任务放到前台执行

• kill – 给一个进程发送信号

• killall – 杀死指定名字的进程

• shutdown – 关机或重启系统

进程是怎样工作的

When a system starts up, the kernel initiates a few of its own activities as processes and launches a program called init. init, in turn, runs a series of shell scripts (located in /etc) called init scripts, which start all the system services. Many of these services are implemented as daemon programs, programs that just sit in the background and do their thing without having any user interface. So even if we are not logged in, the system is at least a little busy performing routine stuff.

当系统启动的时候，内核先把一些它自己的活动初始化为进程，然后运行一个叫做 init 的程序。init， 依次地，再运行一系列的称为 init 脚本的 shell 脚本（位于 /etc ），它们可以启动所有的系统服务。 其中许多系统服务以守护进程（daemon）的形式实现，守护进程仅在后台运行，没有任何用户接口(User Interface)。 即使我们没有登录系统，系统也在执行一些例行事务。

The fact that a program can launch other programs is expressed in the process scheme as a parent process producing a child process.

从进程的角度而言，一个程序启动另一个程序可以被表述为一个父进程可以产生一个子进程。

The kernel maintains information about each process to help keep things organized. For example, each process is assigned a number called a process ID or PID. PIDs are assigned in ascending order, with init always getting PID 1. The kernel also keeps track of the memory assigned to each process, as well as the processes’ readiness to resume execution. Like files, processes also have owners and user IDs, effective user IDs, etc.

内核维护每个进程的信息，以此来保持事情有序。例如，系统分配给每个进程一个数字，这个数字叫做 进程(process) ID 或 PID。PID 号按升序分配，init 进程的 PID 总是1。内核也对分配给每个进程的内存和就绪状态进行跟踪以便继续执行这个进程。 像文件一样，进程也有所有者和用户 ID，有效用户 ID，等等。

查看进程

The most commonly used command to view processes (there are several) is ps. The ps program has a lot of options, but in it simplest form it is used like this:

查看进程，最常使用地命令（有几个命令）是 ps(process status)。ps 程序有许多选项，它最简单地使用形式是这样的：

[me@linuxbox ~]$ ps
PID TTY           TIME CMD
5198 pts/1    00:00:00 bash
10129 pts/1   00:00:00 ps

The result in this example lists two processes, process 5198 and process 10129, which are bash and ps respectively. As we can see, by default, ps doesn’t show us very much, just the processes associated with the current terminal session. To see more, we need to add some options, but before we do that, let’s look at the other fields produced by ps. TTY is short for “Teletype,” and refers to the controlling terminal for the process. Unix is showing its age here. The TIME field is the amount of CPU time consumed by the process. As we can see, neither process makes the computer work very hard.

上例中，列出了两个进程，进程 5198 和进程 10129，各自代表命令 bash 和 ps。正如我们所看到的， 默认情况下，ps 不会显示很多进程信息，只是列出与当前终端会话相关的进程。为了得到更多信息， 我们需要加上一些选项，但是在这样做之前，我们先看一下 ps 命令运行结果的其它字段。 TTY 是 “Teletype”(直译电传打字机) 的简写，是指进程的控制终端。TTY 体现了 Unix 的年代久远。TIME 字段表示 进程所消耗的 CPU 时间数量。正如我们所看到的，这两个进程使计算机工作起来很轻松。

If we add an option, we can get a bigger picture of what the system is doing:

如果给 ps 命令加上选项，我们可以得到更多关于系统运行状态的信息：

[me@linuxbox ~]$ ps x
PID TTY   STAT   TIME COMMAND
2799 ?    Ssl    0:00 /usr/libexec/bonobo-activation-server –ac
2820 ?    Sl     0:01 /usr/libexec/evolution-data-server-1.10 --

and many more...

Adding the “x” option (note that there is no leading dash) tells ps to show all of our processes regardless of what terminal (if any) they are controlled by. The presence of a “?” in the TTY column indicates no controlling terminal. Using this option, we see a list of every process that we own.

加上 “x” 选项（注意没有开头的 “-“ 字符），告诉 ps 命令，展示所有进程，不管它们由什么 终端（如果有的话）控制。在 TTY 一栏中出现的 “?” ，表示没有控制终端。使用这个 “x” 选项，可以 看到我们所拥有的每个进程的信息。

Since the system is running a lot of processes, ps produces a long list. It is often helpful to pipe the output from ps into less for easier viewing. Some option combinations also produce long lines of output, so maximizing the terminal emulator window may be a good idea, too.

因为系统中正运行着许多进程，所以 ps 命令的输出结果很长。为了方便查看，将 ps 的输出管道 到 less 中通常很有帮助。一些选项组合也会产生很长的输出结果，所以最大化 终端仿真器窗口可能也是一个好主意。

A new column titled STAT has been added to the output. STAT is short for “state” and reveals the current status of the process:

输出结果中，新添加了一栏，标题为 STAT 。STAT 是 “state” 的简写，它揭示了进程当前状态：

The process state may be followed by other characters. These indicate various exotic process characteristics. See the ps man page for more detail.

进程状态信息之后，可能还跟随其他的字符，来表示各种外来进程的特性。详细信息请看 ps 手册页。

Another popular set of options is “aux” (without a leading dash). This gives us even more information:

另一个流行的选项组合是 “aux”（不带开头的”-“字符）。这会给我们更多信息：

[me@linuxbox ~]$ ps aux
USER   PID  %CPU  %MEM     VSZ    RSS  TTY   STAT   START   TIME  COMMAND
root     1   0.0   0.0    2136    644  ?     Ss     Mar05   0:31  init
root     2   0.0   0.0       0      0  ?     S<     Mar05   0:00  [kt]

and many more...

This set of options displays the processes belonging to every user. Using the options without the leading dash invokes the command with “BSD style” behavior. The Linux version of ps can emulate the behavior of the ps program found in several different Unix implementations. With these options, we get these additional columns:

这个选项组合，能够显示属于每个用户的进程信息。使用这个选项，可以唤醒 “BSD 风格” 的输出结果。 Linux 版本的 ps 命令，可以模拟几个不同 Unix 版本中的 ps 程序的行为。通过这些选项，我们得到 这些额外的列。

用 top 命令动态查看进程

While the ps command can reveal a lot about what the machine is doing, it provides only a snapshot of the machine’s state at the moment the ps command is executed. To see a more dynamic view of the machine’s activity, we use the top command:

虽然 ps 命令能够展示许多计算机运行状态的信息，但是它只是提供 ps 命令执行时刻的机器状态快照。 为了看到更多动态的信息，我们使用 top 命令：

[me@linuxbox ~]$ top

The top program displays a continuously updating (by default, every 3 seconds) display of the system processes listed in order of process activity. The name “top” comes from the fact that the top program is used to see the “top” processes on the system. The top display consists of two parts: a system summary at the top of the display, followed by a table of processes sorted by CPU activity:

top 程序以进程活动顺序显示连续更新的系统进程列表。（默认情况下，每三秒钟更新一次），”top”这个名字 来源于 top 程序是用来查看系统中“顶端”进程的。top 显示结果由两部分组成： 最上面是系统概要，下面是进程列表，以 CPU 的使用率排序。

top - 14:59:20 up 6:30, 2 users, load average: 0.07, 0.02, 0.00
Tasks: 109 total,   1 running,  106 sleeping,    0 stopped,    2 zombie
Cpu(s): 0.7%us, 1.0%sy, 0.0%ni, 98.3%id, 0.0%wa, 0.0%hi, 0.0%si
Mem:   319496k total,   314860k used,   4636k free,   19392k buff
Swap:  875500k total,   149128k used,   726372k free,  114676k cach

 PID  USER       PR   NI   VIRT   RES   SHR  S %CPU  %MEM   TIME+    COMMAND
6244  me         39   19  31752  3124  2188  S  6.3   1.0   16:24.42 trackerd
....

The system summary contains a lot of good stuff. Here’s a rundown:

其中系统概要包含许多有用信息。下表是对系统概要的说明：

The top program accepts a number of keyboard commands. The two most interesting are h, which displays the program’s help screen, and q, which quits top.

top 程序接受一系列从键盘输入的命令。两个最有趣的命令是 h 和 q。h，显示程序的帮助屏幕，q， 退出 top 程序。

Both major desktop environments provide graphical applications that display information similar to top (in much the same way that Task Manager in Windows works), but I find that top is better than the graphical versions because it is faster and it consumes far fewer system resources. After all, our system monitor program shouldn’t be the source of the system slowdown that we are trying to track.

两个主要的桌面环境都提供了图形化应用程序，来显示与 top 程序相似的信息 （和 Windows 中的任务管理器类似），但是我觉得 top 程序要好于图形化的版本， 因为它运行速度快，并且消费很少的系统资源。总不至于因为启动了监控界面，让那被监控的系统都变慢。

控制进程

Now that we can see and monitor processes, let’s gain some control over them. For our experiments, we’re going to use a little program called xlogo as our guinea pig. The xlogo program is a sample program supplied with the X Window System (the underlying engine that makes the graphics on our display go) which simply displays a re- sizable window containing the X logo. First, we’ll get to know our test subject:

现在我们可以看到和监测进程，让我们得到一些对它们的控制权。为了我们的实验，我们将使用 一个叫做 xlogo 的小程序，作为我们的实验品。这个 xlogo 程序是 X 窗口系统 （使图形界面显示在屏幕上的底层引擎）提供的示例程序，这个程序仅显示一个大小可调的 包含 X 标志的窗口。首先，我们需要知道测试的实验对象：

[me@linuxbox ~]$ xlogo

After entering the command, a small window containing the logo should appear somewhere on the screen. On some systems, xlogo may print a warning message, but it may be safely ignored.

命令执行之后，一个包含 X 标志的小窗口应该出现在屏幕的某个位置上。在一些系统中，xlogo 命令 会打印一条警告信息，但是不用理会它。

Tip: If your system does not include the xlogo program, try using gedit or kwrite instead.

小贴士：如果你的系统不包含 xlogo 程序，试着用 gedit 或者 kwrite 来代替。

We can verify that xlogo is running by resizing its window. If the logo is redrawn in the new size, the program is running.

通过调整它的窗口大小，我们能够证明 xlogo 程序正在运行。如果这个标志以新的尺寸被重画， 则这个程序正在运行。

Notice how our shell prompt has not returned? This is because the shell is waiting for the program to finish, just like all the other programs we have used so far. If we close the xlogo window, the prompt returns.

注意，为什么我们的 shell 提示符还没有返回？这是因为 shell 正在等待这个程序结束， 就像到目前为止我们用过的其它所有程序一样。如果我们关闭 xlogo 窗口，shell 提示符就返回了。

中断一个进程

Let’s observe what happens when we run xlogo again. First, enter the xlogo command and verify that the program is running. Next, return to the terminal window and type Ctrl-c.

我们再运行 xlogo 程序一次，观察一下发生了什么事。首先，执行 xlogo 命令，并且 证实这个程序正在运行。下一步，回到终端窗口，按下 Ctrl-c。

[me@linuxbox ~]$ xlogo
[me@linuxbox ~]$

In a terminal, typing Ctrl-c, interrupts a program. This means that we politely asked the program to terminate. After typing Ctrl-c, the xlogo window closed and the shell prompt returned.

在一个终端中，输入 Ctrl-c，中断一个程序。这意味着，我们礼貌地要求终止这个程序。 输入 Ctrl-c 之后，xlogo 窗口关闭，shell 提示符返回。

Many (but not all) command line programs can be interrupted by using this technique.

通过这个技巧，许多（但不是全部）命令行程序可以被中断。

把一个进程放置到后台(执行)

Let’s say we wanted to get the shell prompt back without terminating the xlogo program. We’ll do this by placing the program in the background. Think of the terminal as having a foreground (with stuff visible on the surface like the shell prompt) and a background (with hidden stuff behind the surface.) To launch a program so that it is immediately placed in the background, we follow the command with an- “&” character:

假如说我们想让 shell 提示符返回，却不终止 xlogo 程序。我们可以把 这个程序放到后台(background)执行。把终端想象是一个有前台（包含在表层可见的事物，像 shell 提示符） 和后台（包含表层之下的隐藏的事物）的设备。为了启动一个程序并让它立即在后台 运行，我们在程序命令之后，加上”&”字符：

[me@linuxbox ~]$ xlogo &
[1] 28236
[me@linuxbox ~]$

After entering the command, the xlogo window appeared and the shell prompt returned, but some funny numbers were printed too. This message is part of a shell feature called job control. With this message, the shell is telling us that we have started job number 1 (“[1]”) and that it has PID 28236. If we run ps, we can see our process:

执行命令之后，这个 xlogo 窗口出现，并且 shell 提示符返回，同时打印一些有趣的数字。 这条信息是 shell 特性的一部分，叫做任务控制 (job control)。通过这条信息，shell 告诉我们，已经启动了 任务号(job number)为1（“［1］”），PID 为28236的程序。如果我们运行 ps 命令，可以看到我们的进程：

[me@linuxbox ~]$ ps
  PID TTY         TIME   CMD
10603 pts/1   00:00:00   bash
28236 pts/1   00:00:00   xlogo
28239 pts/1   00:00:00   ps

The shell’s job control facility also gives us a way to list the jobs that are have been launched from our terminal. Using the jobs command, we can see this list:

shell 的任务控制功能给出了一种列出从我们终端中启动了的任务的方法。执行 jobs 命令，我们可以看到这个输出列表：

[me@linuxbox ~]$ jobs
[1]+ Running            xlogo &

The results show that we have one job, numbered “1”, that it is running, and that the command was xlogo &.

结果显示我们有一个任务，编号为“1”，它正在运行，并且这个任务的命令是 xlogo ＆。

进程返回到前台

A process in the background is immune from keyboard input, including any attempt interrupt it with a Ctrl-c. To return a process to the foreground, use the fg command, this way:

一个在后台运行的进程对一切来自键盘的输入都免疫，也不能用 Ctrl-c 来中断它。 为了让一个进程返回前台 (foreground)，这样使用 fg 命令：

[me@linuxbox ~]$ jobs
[1]+ Running        xlogo &
[me@linuxbox ~]$ fg %1
xlogo

The command fg followed by a percent sign and the job number (called a jobspec) does the trick. If we only have one background job, the jobspec is optional. To terminate xlogo, type Ctrl-c.

fg 命令之后，跟随着一个百分号和任务序号（叫做 jobspec ，如此处的 %1）就可以了。如果我们只有一个后台任务，那么 jobspec(job specification) 是可有可无的。输入 Ctrl-c 来终止 xlogo 程序。

停止一个进程

Sometimes we’ll want to stop a process without terminating it. This is often done to allow a foreground process to be moved to the background. To stop a foreground process, type Ctrl-z. Let’s try it. At the command prompt, type xlogo, the Enter key, then Ctrl-z:

有时候，我们想要停下一个进程，而不是终止它。我们这么做通常是为了允许前台进程被移动到后台。 输入 Ctrl-z，可以停下一个前台进程。让我们试一下。在命令提示符下，执行 xlogo 命令， 然后输入 Ctrl-z:

[me@linuxbox ~]$ xlogo
[1]+ Stopped                 xlogo
[me@linuxbox ~]$

After stopping xlogo, we can verify that the program has stopped by attempting to resize the xlogo window. We will see that it appears quite dead. We can either restore the program to the foreground, using the fg command, or move the program to the background with the bg command:

停止 xlogo 程序之后，通过调整 xlogo 的窗口大小，我们可以证实这个程序已经停止了。 它看起来像死掉了一样。使用 fg 命令，可以恢复程序到前台运行，或者用 bg 命令把程序移到后台。

[me@linuxbox ~]$ bg %1
[1]+ xlogo &
[me@linuxbox ~]$

As with the fg command, the jobspec is optional if there is only one job.

和 fg 命令一样，如果只有一个任务的话，jobspec 参数是可选的。

Moving a process from the foreground to the background is handy if we launch a graphical program from the command, but forget to place it in the background by appending the trailing “&”.

如果我们从命令行启动一个图形程序，但是忘了在命令后加字符 “&”， 将一个进程从前台移动到后台也是很方便的。

Why would you want to launch a graphical program from the command line? There are two reasons. First, the program you wish to run might not be listed on the window manager’s menus (such as xlogo). Secondly, by launching a program from the command line, you might be able to see error messages that would otherwise be invisible if the program were launched graphically. Sometimes, a program will fail to start up when launched from the graphical menu. By launching it from the command line instead, we may see an error message that will reveal the problem. Also, some graphical programs have many interesting and useful command line options.

为什么要从命令行启动一个图形界面程序呢？有两个原因。第一个，你想要启动的程序，可能 没有在窗口管理器的菜单中列出来（比方说 xlogo）。第二个，从命令行启动一个程序， 你能够看到一些错误信息，如果从图形界面中运行程序的话，这些信息是不可见的。有时候， 一个程序不能从图形界面菜单中启动。通过从命令行中启动它，我们可能会看到 能揭示问题的错误信息。一些图形界面程序还有许多有意思并且有用的命令行选项。

Signals

The kill command is used to “kill” programs. This allows us to terminate programs that need killing. Here’s an example:

kill 命令被用来“杀死”程序。这样我们就可以终止需要杀死的程序。这里有一个例子：

[me@linuxbox ~]$ xlogo &
[1] 28401
[me@linuxbox ~]$ kill 28401
[1]+ Terminated               xlogo

We first launch xlogo in the background. The shell prints the jobspec and the PID of the background process. Next, we use the kill command and specify the PID of the process we want to terminate. We could have also specified the process using a jobspec (for example, “%1”) instead of a PID.

首先，我们在后台启动 xlogo 程序。shell 打印出这个后台进程的 jobspec 和 PID。下一步，我们使用 kill 命令，并且指定我们想要终止的进程 PID。也可以用 jobspec（例如，“％1”）来代替 PID。

While this is all very straightforward, there is more to it than that. The kill command doesn’t exactly “kill” programs, rather it sends them signals. Signals are one of several ways that the operating system communicates with programs. We have already seen signals in action with the use of Ctrl-c and Ctrl-z. When the terminal receives one of these keystrokes, it sends a signal to the program in the foreground. In the case of Ctrl-c, a signal called INT (Interrupt) is sent; with Ctrl-z, a signal called TSTP (Terminal Stop.) Programs, in turn, “listen” for signals and may act upon them as they are received. The fact that a program can listen and act upon signals allows a program to do things like save work in progress when it is sent a termination signal.

虽然这个命令看上去很直白， 但是它的含义不止于此。这个 kill 命令不是真的“杀死”程序，而是给程序 发送信号。信号是操作系统与程序之间进行通信时所采用的几种方式中的一种。 Ctrl-c 和 Ctrl-z 就是信号的实际例子。当终端接受了其中一个按键组合后，它会给在前端运行 的程序发送一个信号。在使用 Ctrl-c 的情况下，会发送一个叫做 INT（Interrupt ，中断）的信号；当使用 Ctrl-z 时，则发送一个叫做 TSTP（Terminal Stop ，终端停止）的信号。程序，相应地，监听信号的到来，当程序 接到信号之后，则做出响应。一个程序能够监听和响应信号这件事允许一个程序做些事情， 比如，当程序接到一个终止信号时，它可以保存所做的工作。

通过 kill 命令给进程发送信号

The kill command is used to send signals to programs. Its most common syntax looks like this:

kill 命令被用来给程序发送信号。它最常见的语法形式看起来像这样：

kill [-signal] PID...

If no signal is specified on the command line, then the TERM (Terminate) signal is sent by default. The kill command is most often used to send the following signals:

如果在命令行中没有指定信号，那么默认情况下，发送 TERM（Terminate，终止）信号。kill 命令被经常 用来发送以下命令：

Let’s try out the kill command:

让我们试一下 kill 命令：

[me@linuxbox ~]$ xlogo &
[1] 13546
[me@linuxbox ~]$ kill -1 13546
[1]+ Hangup         xlogo

In this example, we start the xlogo program in the background and then send it a HUP signal with kill. The xlogo program terminates and the shell indicates that the background process has received a hangup signal. You may need to press the enter key a couple of times before you see the message. Note that signals may be specified either by number or by name, including the name prefixed with the letters “SIG”:

在这个例子里，我们在后台启动 xlogo 程序，然后通过 kill 命令，发送给它一个 HUP 信号。 这个 xlogo 程序终止运行，并且 shell 指示这个后台进程已经接受了一个挂起信号。在看到这条 信息之前，你可能需要多按几次 enter 键。注意，信号既可以用号码，也可以用名字来指定， 包括在前面加上字母 “SIG” 的名字。

[me@linuxbox ~]$ xlogo 1] 13601
[me@linuxbox ~]$ kill -INT 13601
[1]+ Interrupt                    xlogo
[me@linuxbox ~]$ xlogo &
[1] 13608
[me@linuxbox ~]$ kill -SIGINT 13608
[1]+ Interrupt                    xlogo

Repeat the example above and try out the other signals. Remember, you can also use jobspecs in place of PIDs.

重复上面的例子，试着使用其它的信号。记住，你也可以用 jobspecs 来代替 PID。

Processes, like files, have owners, and you must be the owner of a process (or the superuser) in order to send it signals with kill.

进程，和文件一样，拥有所有者，所以为了能够通过 kill 命令来给进程发送信号， 你必须是进程的所有者（或者是超级用户）。

In addition to the list of signals above, which are most often used with kill, there are other signals frequently used by the system. Here is a list of other common signals:

除了上表列出的 kill 命令最常使用的信号之外，还有一些系统频繁使用的信号。以下是其它一些常用 信号列表：

For the curious, a complete list of signals can be seen with the following command:

为了满足读者的好奇心，通过下面的命令可以得到一个完整的信号列表：

[me@linuxbox ~]$ kill -l

通过 killall 命令给多个进程发送信号

It’s also possible to send signals to multiple processes matching a specified program or user name by using the killall command. Here is the syntax:

也有可能通过 killall 命令，给匹配特定程序或用户名的多个进程发送信号。下面是 killall 命令的语法形式：

killall [-u user] [-signal] name...

To demonstrate, we will start a couple of instances of the xlogo program and then terminate them:

为了说明情况，我们将启动一对 xlogo 程序的实例，然后再终止它们：

[me@linuxbox ~]$ xlogo &
[1] 18801
[me@linuxbox ~]$ xlogo &
[2] 18802
[me@linuxbox ~]$ killall xlogo
[1]- Terminated                xlogo
[2]+ Terminated                xlogo

Remember, as with kill, you must have superuser privileges to send signals to processes that do not belong to you.

记住，和 kill 命令一样，你必须拥有超级用户权限才能给不属于你的进程发送信号。

更多和进程相关的命令

Since monitoring processes is an important system administration task, there are a lot of commands for it. Here are some to play with:

因为监测进程是一个很重要的系统管理任务，所以有许多命令与它相关。玩玩下面几个命令：