Shell branching and looping

1. Common environment variables

variable name	description	example
path	finds a collection of paths for all external commands. If you do not have a PATH environment variable, all external commands you execute need to state their file path If the PATH environment variable contains more than 2 paths that correspond to the same name, select the file that was found first when executing this command	Echo $PATH path= $PATH:/usr/bin Export PATH
shell	current default shell environment (shell program)
?
home	user home directory

A special environment variable? : It saves an integer returned to the kernel after the execution of the last command, and we usually agree that if a program executes successfully, it returns 0 to the kernel, otherwise it returns non 0, and the value is 0~255,-128~127.

2. Test Condition Testing Command

(1) Numeric comparison test $ variable Option number

Options	Meaning
-lt	Less than
-le	Less than or equal
-gt	Greater than
-ge	Greater than or equal
-eq	Judging if they are equal

(2) string problem Test $ variable Option string

"": This represents an empty string;

Options	Meaning
=	Represents equality
!=	Range
-Z	is empty
-nz	is not empty

(3) File problem Test option file name

Commonly used is the test-e file name (to determine whether the file exists);

[]: Also the conditional Test command, [: Represents an external command,]: Indicates the option, so the two front and back must be separated by a space;

3. Sequential execution

Executes in row order, consistent with the order in which the contents of the file are written;

To scan and interpret verbatim characters;

4. Branch structure

(1), if branch

if arbitrary command; then

#若干命令

Fi

#其它若干命令

#!/bin/bashread-p "Please Input userage:" Userageif test $userAge-lt 20-a $userAge-gt 18; Then echo "18<userage<20" fi

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If arbitrary command

Then

#若干命令

Fi

#!/bin/bashread-p "Please Input userage:" Userageif test $userAge-lt 20-a $userAge-gt and then echo "18<userag E<20 "fi

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if arbitrary command; then # several commands; fi

If [$userAge-gt 10-a $userAge-lt 18]; Then echo the "age too small"; Fi

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If then fi belongs to the internal syntax command of Bash and is syntactically controlled. The function of FI is to end the IF statement block; then it can be written after if, but it must be preceded by a semicolon, separated from the preceding command, or it can be written under if.

If arbitrary command

Then

#若干命令

elif arbitrary command

Then

#若干命令

Fi

#!/bin/bashread-p "Please Input userage:" Userageif test $userAge-lt 20-a $userAge-gt and then echo "18<userag E<20 "elif test $userAge-gt 30then echo" Userage > "fi

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If arbitrary command

Then

#若干命令

Else

#若干命令

Fi

#!/bin/bashread-p "Please Input userage:" Userageif test $userAge-lt 20-a $userAge-gt and then echo "18<userag E<20 "Else echo" no range "fi

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If the condition is judged by $?, this is the internal mechanism of the IF.

(2), Case branch

The function and structure of the case branch is similar to the switch...case statement of the C language.

Case $ variable in

An expression)

Several statements

;;

An expression)

Several statements

;;

......

Esac

A case expression refers to a number of constants that can be written together (using the ' | ' Representation ', provides a wildcard asterisk number that represents the case block for the default semantics.

The string in the case expression is equal to the value of the variable (string equality judgment);

#!/bin/bashread-p "Please Input your choose:" Choosecase $choose in 1|2) echo "Choose 1 or 2";;      3) echo "Choose 3";;  *) echo "no choose";; Esac

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5. Cyclic execution

(1), while

while arbitrary command;

#若干命令

Done

Reverse single quotation marks: Executes the contents of the inverted single quotation marks as a command, and then replaces the entire inverted single-quote content with the standard output of the command!!!

' Echo Hello '------->hello

Expr is the command to perform various operations:

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#!/bin/bashindex=0while test $index-lt 10do echo $index index= ' expr $index + 1 ' done

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While arbitrary command

Do

#若干命令

Done

Note: The shell script does not have a do ... while loop;

(2), for

The For loop is used to traverse a string (traversed by rows), which includes a carriage return line;

For traversal name in several line strings

Do

#若干命令

Done

#!/bin/bashfor a In ' Cat./abc.txt ' #将abc a line of content in a. txt file is given to A;do Echo $adone

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6. Special Circumstances

(1), break: Jump out of the current layer of break loop;

(2), when the while encounters a pipe character

Command | While read variable name

Do

#若干命令

Done

The above loop allows you to traverse the standard output produced by the command before the pipe character, where the variable name holds the contents of a row.

#!/bin/bashfind/usr/include-name "W*.h" |    While read Filenamedo cmd= ' echo cp $filename./' #将 ' in the standard output to CMD; $cmd #每次将一个文件拷贝到当前目录下;d One #ls-1 is a vertical print file name

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(3), when the while encounters the redirect input

While read variable name

Do

#若干命令

Done < files

is to read the contents of the file from one line of the file;

#!/bin/bashwhile Read Filenamedo echo $filenamedone <./abc.txt

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Equivalent to the following notation:

For variable name in ' Cat file name '

Do

#若干命令

Done

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Shell branching and looping