系列文章目录

ShellWhat is system learningShell

ShellThe system learns to create oneShell程序

Shell系统学习之向Shell脚本传递参数

ShellHow the system learns to performShell程序

Shell系统学习之Shell变量和引用

 Shell系统学习之Shell条件测试,Judgment Statements and Operators

 Shell系统学习之循环结构

 ShellA function of system learning

ShellAn array of systematic learning

系统学习Shell之正则表达式

ShellFile operations for system learning

ShellThe son of systematic learningShelldeal with the process

目录

系列文章目录

文章目录

前言

子Shell

什么是子Shell

Shell内部命令、External commands and reserved words

在子Shell中执行命令 

进程处理

What are processes and jobs

相关命令fd,jobs,disown

通过信号和trapCommand control process 

小结 

前言

当我们在执行Shell脚本时,会感觉到ShellThe script is currentShellInterpreted at the command line prompt,其实不是的,,when the script is executed,当前的ShellWill start a child process to execute the current oneShell脚本.This article will explain itShell和进程处理

子Shell

什么是子Shell

Let's get to know the first one firstShell是如何启动的？

当用户登录Linux后,The operating system will vary according to the user/etc/passwdThe configuration in the file starts oneShell进程,该ShellProcesses are executed by the current userShell命令的父进程.Each user can specify an automatic defaultShell程序,The sections are shown below/etc/passwd的内容：

[email protected]:~/Documents/Shell/12$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin

其中/etc/passwdEach line describes a user information,一共有7个字段,每个字段的含义如下：

用户名:x:用户ID:用户组ID:备注信息:用户主目录:默认Shell程序 

默认ShellPrograms use absolute paths.if this fieldShell程序不存在,不合法,You cannot log in to the host.If the user does not need to log in,then you can set the path to  /usr/sbin/nologin.

The diagram below depicts father and sonShell的工作过程：

Shell内部命令、External commands and reserved words

在一些其他书籍中,We often see the so-called internal orders and external orders of this term,Here's an explanation of their differences.

Internal commands are meant to be contained withinShell工具包中的命令,Internal commands are embedded in Shell程序中,Not as a separate disk file(可执行文件)存在.比如cd, bg, exit,exec, eval,pwd等.

外部命令是Linux系统中的实用程序部分,These programs exist on disk as disk files.在用户登录时,External commands do not come with defaults like internal commandsShell加载到内存中,Instead, it is called into memory when needed.ShellThe program manages path lookups and code loading when external commands are executed,并控制命令的执行.比如ls,at,du,host等都是外部命令.External commands are usually located in /usr/bin以及/usr/sbin等目录中,其中/usr/sbinThe commands in are generally related to system administration.

ShellThe reserved words are generally Shell语法中的关键字,这里不一一列出了.

在子Shell中执行命令 

 (command1;command2;...)

 (command1;command2;...)&

When the command is executed within a parenthesis,The set of commands will be in the subShell中执行 ,If added after parentheses&,表示该子Shell后台执行,Does not block the current parent process.

Know this method first,There are places to look at later,I don't feel like it's working yet,但是要了解,Know when you see this kind of code.

进程处理

What are processes and jobs

什么是进程,这里就不多做介绍了,Let's first understand the concept of homework：A job can contain multiple processes,Jobs are for users,A process started by a user to complete a task,1jobs can contain1个,也可以包含多个进程,These processes work together,共同完成任务.The process is for the operating system,It is the basic unit of operating system program execution.

比如：

ls -l | grep 13 | more 

The above set of commands can be called a job,But it will still start3个进程.

when we execute in the backgroundShell脚本时,比如：

[email protected]:~/Documents/Shell/13$ ./13-10.sh &
[1] 16386

会显示一个[1] 和 16386

其中[1]中的1表示作业号,16386Indicates the number of the process executing the script.

You will be prompted when the job is complete：

[1]+  Done                    ./13-10.sh 

The above prompt includes the job number 状态 The script command executed

When executing the foreground job again,可以执行Ctrl+Z将当前任务挂起,比如我执行vi 命令编辑一个文件,然后按Ctrl+Z的效果：

[email protected]:~/Documents/Shell/13$ vi 13-2.sh

[2]+  Stopped                 vi 13-2.sh

 The job number indicated above is 2's job has been suspended.

相关命令fd,jobs,disown

使用fd命令,是一个内部命令,Move the job from the background to the foreground to run

fd [jobspec]

jobsepcUsed to specify the job to switch,fdSpecifies the method for switching jobs：

jobs命令View a list of jobs that are executing in the background,基本语法如下：

jobs [options]

[email protected]:~/Documents/Shell/13$ jobs
[2]-  Stopped                 vi 13-2.sh
[3]+  Stopped                 vi 13-2.sh 

其中[3]旁边的+Indicates the most recent job,[2]旁边的-The sign indicates the second-to-last job.

作业的状态：

disown命令Used to delete a job in the job list

disown [jobsepc]

用法与fd命令一样.

通过信号和trapCommand control process 

kill Commands can pass signals to processes.

kill -lThe command displays the signals supported by the current system：

[email protected]:~/Documents/Shell/13$ kill -l
 1) SIGHUP     2) SIGINT     3) SIGQUIT     4) SIGILL     5) SIGTRAP
 6) SIGABRT     7) SIGBUS     8) SIGFPE     9) SIGKILL    10) SIGUSR1
11) SIGSEGV    12) SIGUSR2    13) SIGPIPE    14) SIGALRM    15) SIGTERM
16) SIGSTKFLT    17) SIGCHLD    18) SIGCONT    19) SIGSTOP    20) SIGTSTP
21) SIGTTIN    22) SIGTTOU    23) SIGURG    24) SIGXCPU    25) SIGXFSZ
26) SIGVTALRM    27) SIGPROF    28) SIGWINCH    29) SIGIO    30) SIGPWR
31) SIGSYS    34) SIGRTMIN    35) SIGRTMIN+1    36) SIGRTMIN+2    37) SIGRTMIN+3
38) SIGRTMIN+4    39) SIGRTMIN+5    40) SIGRTMIN+6    41) SIGRTMIN+7    42) SIGRTMIN+8
43) SIGRTMIN+9    44) SIGRTMIN+10    45) SIGRTMIN+11    46) SIGRTMIN+12    47) SIGRTMIN+13
48) SIGRTMIN+14    49) SIGRTMIN+15    50) SIGRTMAX-14    51) SIGRTMAX-13    52) SIGRTMAX-12
53) SIGRTMAX-11    54) SIGRTMAX-10    55) SIGRTMAX-9    56) SIGRTMAX-8    57) SIGRTMAX-7
58) SIGRTMAX-6    59) SIGRTMAX-5    60) SIGRTMAX-4    61) SIGRTMAX-3    62) SIGRTMAX-2
63) SIGRTMAX-1    64) SIGRTMAX 

 也可以通过man 7 signalCheck out the help manual for signals.

Use belowkillCommand syntax for signaling a process：

kill [-s signal] pid

for some signals,The process will take the default response action,And some signal processes may be ignored directly.在Shell中用户可以通过trap命令to set an action in response to a signal.trap是一个内部命令,语法如下：

trap [[arg] signspec ...]

举例：

#!/bin/bash
function signal_handler()
{
    echo "Good bye"
}

trap signal_handler 0

 在POSIXStandard center handle0Defined as a null signal,在程序退出时会触发,该程序的执行结果：

[email protected]:~/Documents/Shell/13$ ./13-11.sh 
Good bye

小结 

This article focuses on father and sonShell,内部命令和外部命令,前后台运行,Processes and jobs and control over them.