Shell programming specifications and variables

Shell script concept

• The command to be executed in order to save a text file
• Executable permissions to the file, you can run
• Shell various control statements may be combined to perform more complex operations

Shell script scenarios

• Repetitive operations
• Batch Transactions
• Automated operation and maintenance
• Service health monitoring
• Timing task execution
• .......

Shell role - command interpreter, "translator"

• Interposed between the kernel and user, is responsible for interpreting the command line

The user's login Shell

• After logging in default of Shell program, usually/bin/bash

• Different Shell internal instructions, operating environment, etc. will be different

  [root@localhost ~]# cat /etc/shells
  /bin/sh
  /bin/bash
  /sbin/nologin
  ......

  Write script code

• Use via text editor
• One per line Linuxcommand, executed in the order written

[root@localhost ~]# vim first.sh
cd /boot/
pwd
ls -lh vml*

Given executable permissions

• Making your script executable property

[root@localhost ~]# chmod +x first.sh        

Execute the script file

• Method One: Script file path (absolute and relative paths)

  [root@localhost ~]# ./first.sh          //必须拥有x（执行）权限，执行文件，但不会改变所在路径

• Method Two: sh script file path

  [root@localhost ~]# sh /first.sh      //执行文件，但不会改变所在路径

• Method three: source script file path

  [root@localhost ~]# source /first.sh

• Method four: the script file path

  [root@localhost ~]# . /first.sh

Better script constitution

• Script statements
• Comment Information
• Executable statement

  [root@localhost ~]# vim /first.sh
  #!/bin/bash
  # This is my first Shell-Script.
  cd /boot
  echo "当前的目录位于:”                 //输出友好提示信息
  pwd
  echo "其中以vmI开头的文件包括:”
  Is -Ih vml*

  Redirection and pipeline operations

Interactive hardware

• Standard Input: receiving input data from a user of the device
• Standard Output: output data to the user through the apparatus
• Standard Error: execution error messages through the device report

Types of	Device file	File Description Number	Default Device
Standard Input	/dev/stdin	0	keyboard
Standard output	/dev/stdout	1	monitor
Standard error	/dev/stderr	2	monitor

Redirection

Types of	Operators	use
Redirect input	<	Reads data from the specified file, rather than from the keyboard
Redirect output	>	Save the output to the specified file (overwrite the original content)
	>>	The output is appended to the specified file
Standard error output	2>	Save the error information to the specified file (overwrite the original content)
	2>>	The error message is appended to the specified file
Mix output	&>	Save the standard output and standard error to the same file content

Pipeline Operation Symbol“|”

• The left side of the output command, the command processing target as the right

  cmd1 | cmd2 [...| camdn]

  [root@locadhost ~]# grep "bash$" /etc/passwd
  root:x:0:0:root:/root:/bin/bash
  Teacher:/home/teacher:/bin/bash
  [root@localhost ~]# grep“bash$”letc/passwd I awk -F: '{print $1,$7}'
  root /bin/bash
  teacher /bin/bash

The role of Shell variables, type

The role of variables

• For the flexible management Linuxto provide specific parameters of the system, there are two meanings

  • Variable names: use a fixed name, preset by the system or user-defined

  • Variable values: it can be set varies depending on the user, the system environment

    Type of the variable

• Custom variable: defined by the user, modify and use
• Environment variables: maintained by the system for setting the work environment
• Location variables: to pass parameters to the script from the command line
• Predefined variables: Bashbuilt for a class of variables can not be directly modified

  Custom Variables

• Variable names begin with a letter or an underscore, case sensitive, all-caps recommendations

  变量名=变量值

  View a variable value

  echo $变量名

  [root@locadhost ~]# Rroduct=Python
  Version=2.7.13
  [root@locadhost ~]# ehco $Product
  Python
  [root@locadhost ~]# ehco $Product $Version
  Python 2.7.13

  Use quotation marks when assignment

• Double quotes: Allows the $reference variable value other symbols
• Single quotes: Do not quote the value of other variables, $treated as ordinary characters
• Anti apostrophe: Replace command, the extract command output

  Content is entered from the keyboard variable assignment

  resd [-p "提示信息"] 变量名

  Object-oriented: good object will be built over a direct call using
  process-oriented: each process have their own input

Setting a variable scope

格式1：export 变量名 ...
格式2：export 变量名=变量值 ...

• Both formats can be mixed

  [root@locadhost ~]# echo "$Producvt $Version"
  Benet 6.0
  [root@locadhost ~]# export Product Version
  [root@locadhost ~]# bash
  [root@locadhost ~]# echo "$Product $Version"
  Benet 6.0

  Integer variable computing

  expr 变量1 运算符 变量2  [运算符 变量3]...

  Common operator

• Adding:+
• Subtraction:-
• Multiplication:\*
• Division:/
• Modulus (remainder) operator:%

Special Shell Variables

Environment Variables

• Created by the system in advance, to set the user's working environment
• Profile:/etc/profile ,~/.bash_ profile

Common environmental variables

• PWD、PATH
• USER、SHELL、 HOME

[root@localhost ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/mysql/bin:/root/bin
[root@localhost ~]# PATH="$PATH:/root"
[root@localhost ~]# echo $PATH
 /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/mysql/bin:/root/bin:/root

Position variable

• He expressed as $ n, n is a number between 1 to 9

[root@localhost ~]# ./myprog.sh one two three four five six //$1,第1个位置参数，$2,第2个位置参数

+ Format requirements back representatives

Predefined variables

• $ #; The number of variables in the command line position
• $ *: The contents of all position variables
• $:? The status returned after the execution of the previous command, when the return value of 0 indicates the state to perform normal, non-zero value indicates that the implementation of an exception or error
• $ 0: currently executing process / program name

[root@localhost ~]# vim mybak.sh
#!/bin/bash
TARFILE=beifen-'date +%s .tgz
tar zcf $TARFILE $* &> /dev/null
echo "已执行$0脚本,“
echo“共完成$#个对象的备份”
echo”具体内容包括: $*”
[root@localhost ~]# chmod +x mybak.sh
[root@localhost ~]# ./mybak.sh /etc/passwd /etc/shadow
已执行./mybak.sh脚本，
共完成2个对象的备份
具体包括:/etc/passwd /etc/shadow