Technology Sharing

한어Русский языкEnglishFrançaisIndonesianSanskrit日本語DeutschPortuguêsΕλληνικάespañolItalianoSuomalainenLatina

---

1. Local variables

• Define Shell variables. The variable name does not need to be prefixed with$
• Local variablesOnly valid during the user's current shell lifetime

---

2. Variable definition

• Variable name requirements: composed of letters, numbers, and underscores. It can start with a letter or an underscore.
• Variable names are strictly case-sensitive

---

3. Get the variable value

• apostrophe: What you see is what you get, strong reference
• Double quotes: Output all contents in quotation marks, identify special symbols, weak quotes
• No quotes: Continuous symbols can be enclosed without quotation marks. Spaces will cause ambiguity. It is best to use double quotation marks.
• Backtick: Quote the command execution result, equal to$()Usage

---

4. Special variables

• Special shell variables, used in scripts, functions, etc. to pass parameters

• There are special, positional parameter variables as follows

  variable	effect
  $0	Get the shell script file name and script path
  $n	Get the nth parameter of the shell script, n is between 1 and 9, if it is greater than 9, some{},like{$10}
  $#	Get the total number of parameters after the executed shell script
  $*	Get all the parameters of the shell script. Without quotes, it is equivalent to$@The effect of adding quotation marks "$*" isReceives all parameters as a single string
  $@	Without quotation marks, the effect is the same as above. With quotation marks,Receives all parameters as separate strings

---

5. Special state variables

• Search method：man bash,searchSpecial Parameters

  variable	effect
  $?	The return value of the last command execution status, 0 for success, non-0 for failure
  $$	The process number of the current shell script
  $!	PID of the last background process
  $_	Get the last parameter of the last command

---

6. Shell built-in variable commands

1.echo

• parameter：
  • -n: Output content without line break
  • -e: Interpret escape characters
    • n: Line break
    • r: Enter
    • t：tab
    • b:backspace
    • v: vertical tab character

2.eval

• Function: Execute multiple commands
• Example：eval ls; cd ~

3.exec

• Function: Do not create a child process and execute subsequent commands.execAfter execution, automaticallyexit

---

7. Intercepting strings

• grammar：

  grammar	effect
  #	Remove the shortest match from the beginning
  ##	Remove the longest match from the beginning
  %	Remove the shortest match from the end
  %%	Remove the longest match from the end
  #	Specify character content interception
  a*c	Matches a string that starts with a, has any characters in the middle, and ends with c
  ${变量}	Returning the value of a variable
  ${#变量}	Returns variable length, character length
  ${变量:start}	Returns the character after the value of the variable offset
  ${变量:start:length}	Extract the length-limited characters after offset
  ${变量#word}	Delete the shortest matching word substring from the beginning of the variable
  ${变量##word}	Delete the longest matching word from the beginning of the variable
  ${变量%word}	Delete the shortest matching word substring from the end of the variable
  ${变量%%word}	Delete the longest matching word substring from the end of the variable
  ${变量/pattern/string}	Replace the first match of pattern with string
  ${变量//pattern/string}	Replace all patterns with string

• Operation Example: Delete the file name

  # 通过⼦串的替换⽅式
  snowk@SnowKYP# f=stu_102999_1_finished.jpg
  
  # 变量的⼦串功能，去掉后缀
  snowk@SnowKYP# echo ${f//_finished/}
  stu_102999_1.jpg
  
  # 利⽤变量的反引⽤替换⽂件名
  snowk@SnowKYP# mv $f `echo ${f//_finished/}`
  
  # 剩下的⽂件，利⽤循环操作
  # 找出剩下所有需要替换的jpg⽂件
  snowk@SnowKYP# ls *fin*.jpg
  stu_102999_2_finished.jpg stu_102999_3_finished.jpg stu_102999_4_finished.jpg
  stu_102999_5_finished.jpg
  
  # 写shell循环代码，循环操作
  # 去掉所有jpg⽂件的_finished后缀
  snowk@SnowKYP# for file in `ls *fin*.jpg`;do mv $file `echo ${file//_finished/}`;done
  
  snowk@SnowKYP# ls *.jpg
  stu_102999_1.jpg stu_102999_2.jpg stu_102999_3.jpg stu_102999_4.jpg stu_102999_5.jpg
  1
  2
  3
  4
  5
  6
  7
  8
  9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22

---

8. Special shell expansion variable processing

1. Grammar

• Function: These four extended variables are all used to judge and process the values ​​of variables
• grammar：parameter，参数，范围
  • ${parameter:-word}:ifparameterThe variable value is empty, returnwordString
  • ${parameter:=word}:ifparameterIf the variable value is empty,wordSubstitutes a variable value and returns its value
  • ${parameter:?word}:ifparameterThe variable value is empty.wordAsstderrOutput, otherwise output variable value
    • Used to set the error message returned when the variable is empty and an error occurs
  • ${parameter:+word}:ifparameterIf the variable value is empty, do nothing.wordreturn

2. Application scenarios

• Data backup, script to delete expired data

  # 原脚本命令，有bug，有歧义
  find ${file_path} -name '*.tar.gz' -type f -mtime +7 | xargs rm -f
  
  # 修正后的脚本命令
  # 如果有bug起义，就在当前目录搜索并执行删除
  find ${file_path:=/tmp} -name '*.tar.gz' -type f -mtime +7 | xargs rm -f
  1
  2
  3
  4
  5
  6