Shell Learning Notes

1. Shell Script Parameters

• The $n represents the nth parameter passed to the script, for example, $ $ = 1th argument, and $ = 2nd parameter ....
• $# number of command-line arguments
• The name of the current script
• $* returns the values of all parameters in the form "parameter 1, parameter 2, Parameter 3 ..."
• [Email protected] with "parameter 1" "Parameter 2" "Parameter 3" ... Returns the value of all parameters in the form
• $_ last parameter of the command executed before saving
• $? The return status code of the previous command or function
• $$ returns the process ID (PID) of this program

Note: variables [email protected] can be "parameter 1" "Parameter 2" "Parameter 3". The form returns all parameter values, therefore, [email protected] and "$" "$ $" ... is equivalent, if the user passes a parameter that contains spaces or other special characters, you need to use [email protected] to get all the values of the parameters, and you cannot use $*.

2. Variables and references

• In the shell, the case of variable names is sensitive, so the two variable names that differ in capitalization do not represent the same variable.
• M=${X/ACD/1}---To change the ACD in X to 1 and assign it to M.

  # x=acdqw12# m=${x/acd/1echo  $m #m=1qw12

• 3 kinds of quotation marks are different: the single quotation mark (") is enclosed in a character nonalphanumeric as a normal character-full reference; the characters enclosed in double quotation marks (" ") except for" $ "" \ "" "" "" "these characters still retain their special functions, the remaining characters are treated as ordinary characters-part references; The string is interpreted by the shell as a command, and at execution time, the shell executes the command first and replaces the entire anti-quote part with its standard output.
• The backslash "\" is called an escape character and is used to convert special characters in some shells to ordinary characters, such as "$" or "'".

  echo/bin/echo  \ $SHELL # $SHELL.

• By convention, environment variables in the shell are all capitalized.
• Command substitution: The inverse quotation marks and the parentheses are equivalent;

  ' Shell_command ' ==$ (Shell_command)

• Variable assignment Syntax: variable_name=value;variable_name represents the variable name; value represents the values that will be assigned to the variable; the middle equals "=" is called an assignment symbol, and the left and right sides of the assignment symbol cannot be directly followed by a space, or the shell treats it as a command ; The correct assignment statement:

  # v1=linux# v2='redhet linux'# v3="Redhat Linux $HOSTTYPE  "# v4=12345.

• To clear a variable:

  Unset variable_name

3. Condition Testing

• There are two types of conditional test syntax: test and [; test expression;[expression]; expression and left and right square brackets must also have a space between them.
• The left and right sides of the symbol for the test operator =,! =, and z must contain a space.
• Integer test Syntax:

  Test Number1 op number2 or [Number1 op number2]

  OP expression operator:-eq (equals),-ne (Not Equal),-GT (greater than),-lt (less than),-ge (greater than or equal),-le (less than equals); [12-gt]-->echo $?] 1.
• File test Syntax:

  file ]

  The-a file is present, the-B file is present and is a block file, the-c file is present and is a character file, and the-D file exists and is the directory;-e file is the same as-A; the-s file is longer than 0 or the file is not empty;-F fil e files exist and are regular files; the-w file file exists and is writable; the-x file file exists and is executable; the-r file is present and readable; the-u file file does not have a suid bit, and the-l file is present and symbolic.
• Logical operators: String tests, Integer tests, and files all provide a test condition, whereas in shell programming, it is often encountered that multiple conditions are judged simultaneously, and logical operators can combine many different conditions to form a complex conditional expression;

  !  -a expression2 logic with expression1 o expression2 logic or

• ":" The colon is a special command that refers to an empty command, which does nothing, but its exit status is always 0, and if we change the command as a condition in the IF statement, then the then clause will be executed forever.
• Conditional Judgment statement: if

  if expression  Then    statement1   statement2    ... fi
  Or
  
  If Expression;then
      Statement1
      Statement2
  Fi

  If Else

  #!/bin/BashEcho "Please enter a score"#输入提示信息read Score #读取用户输入数据 # If the user does not enter a number, prompt the user to re-enterif[-Z"$score"]; Then#-Z is an empty stringEcho "You enter nothing. Please enter a score:"Read ScoreElse#如果用户输入的数据不对, re-enterif["$score"-lt0-O"$score"-gt -]; Then      Echo "The score should be between 0 and 100.Please enter again:"Read ScoreElse#输出Aif["$score"-ge -]; Then           Echo "The grade is A."        Else#输出Bif["$score"-ge the]; Then                Echo "The grade is B."            Else#输出Cif["$score"-ge -]; Then                      Echo "The grade is C."                 Else#输出Dif["$score"-ge -]; Then                          Echo "The grade is D."                      Else#输出EEcho "The grade is E."                       fi                  fi             fi         fi     fifi

  If Elif

  #!/bin/BashEcho "Please enter a score"#输入提示信息read Score #读取用户输入数据 # If the user does not enter a number, prompt the user to re-enterif[-Z"$score"]; Then#-Z is an empty stringEcho "You enter nothing. Please enter a score:"Read ScoreElse#如果用户输入的数据不对, re-enterif["$score"-lt0-O"$score"-gt -]; Then      Echo "The score should be between 0 and 100.Please enter again:"Read ScoreElse#输出Aif["$score"-ge -]; Then           Echo "The grade is A."#输出Belif["$score"-ge the]; Then            Echo "The grade is B."#输出Celif["$score"-ge -]; Then            Echo "The grade is C."#输出Delif["$score"-ge -]; Then            Echo "The grade is D."        Else#输出EEcho "The grade is E."         fi     fifi

  Case

   Case inch value1)    Statement1    statement2 ...    STATEMENTN;; value2)     statement1     statement2 ...     STATEMENTN;; ... valuen)     statement1 statement2 ...     STATEMENTN;; *)     statement1     statement2     ... STATEMENTN;; Esac

  In the above syntax, variable is a variable, and the case statement compares the value of the variable to each value in Value1~valuen, and if it is equal to the value of a value, the set of statements corresponding to that value are executed. When encountering ";;" Symbol, it jumps out of the case statement, executes the statement following the ESAC statement, and executes a set of statements after the * If no value matches the value of variable.

Note: For the case statement above, the user should be aware of the following points:

I for variable name variable, can use double quotation marks, also can not use;

II The conditional test part of each case clause ends with a closing parenthesis ")";

III Each CASE clause is a pair of semicolons ";;" As a terminator, during the execution of a script, when a pair of semicolons is encountered, all the case clauses following the current clause clause are skipped, including the corresponding clauses of *, executing the other statements following the ESAC clause.

The IV case statement structure ends with ESAC.

4. Calculation of the expression of an operation

• Expr expression computes the value of an expression when using expr, the operation symbol needs to be transferred, \* \ (\).

  #!/bin/Bashresult=`Expr 2- -`Echo "$result"result=`Expr 2\* -`Echo "$result"result=`Expr\(2-6\) \*2`Echo "$result"

• Use $ (()) for arithmetic operations without the need to transfer operators and parentheses.

  #!/bin/bashresult=$ ((3+6))echo"$result"  result=$ ((3*6))echo"$result "

• Use $[...] with the same usage as $ (())
• You can use the Let command to execute one or more arithmetic expressions where the variable name does not need to use the $ symbol and must be referenced if the expression contains spaces or other special characters.

  #!/bin/Bashn=tenletn=n+1echo'$n'let n=n*echo"$n"

5. Bitwise operators

Operator	Description	Example
<<	Move left	4<<2, move 4 to the left 2 bits, the result is 16
>>	Move right	8>>2, move 8 to the right 2 bits, the result is 2
&	Bitwise-AND	8&&4, the bitwise AND operation of 8 and 4, with a result of 0
|	Bitwise OR	8|4, 8 and 4 are bitwise OR arithmetic, the result is 12
~	Bitwise non-	8 bitwise non-operation with a result of-9
^	Bitwise XOR OR	10^6, 10 and 6 are bits by XOR, and the result is 12

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Shell Learning Notes