The shell script of Linux learning-------control flow structure

[This is my own study notes, welcome reprint, but please specify the source:http://blog.csdn.net/jesson20121020]

Start learning some shell flow control structures like other high-level languages today

Flow control Statements:1. If statement

Statement format:

If Condition1      then   command1else condition2then   command2else   Command3fi

NOTE: The If statement must be terminated with FI.

If there is no condition2, the IF statement can be simplified as follows:

If Conditionthen       command1else       Command2fi

Or:

If Conditionthen       Command1fi

Example:

if_test.sh

#!/bin/bash#if_testecho-n "Please input number (A and B):" Read a bif [$a-lt $b]then   echo "$a was less than $b" El If [$a-gt $b]then   echo "$a is greater than $b" else   echoes "$a ia equal to $b" fi

Give the executable permission to execute the script:

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx if_test.sh[email protected]:~/develop/worksapce/ shell_workspace$./if_test.sh Please input the number (A and B): 1010 IA equal to 10[email protected]:~/develop/worksapce /shell_workspace$./if_test.sh Please input the number (A and B): 1210 is less than 12[email PROTECTED]:~/DEVELOP/WORKSAP ce/shell_workspace$./if_test.sh Please input the number (A and B): 8 is greater than 2

Note that this is used for comparison operators and more detailed comparison operators, please refer to man test.

2. Case Statements

Statement format:

Case value in

Mode 1)

Command1

;;

Mode 2)

Command2

;;

Esac

Attention:

The 1case value must be followed by the word in, each pattern must end with a closing parenthesis, and the value can be a variable or constant.

2 pattern Match * denotes any character;? means any single characters; [...] Represents any character in a class or range.

Example:

case_test.sh

#!/bin/bash#case_testecho-n "Enter a number from 1 to 3:" Read ncase $n in1)    echo "you select 1"    ;; 2)    echo "you select 2"    ;; 3)    echo "you select 3"    ;; *)  echo "you select *"    ;; Esac

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx case_test.sh [email protected]:~/develop/worksapce /shell_workspace$./case_test.sh Enter A number from 1 to 3:1you select 1[email protected]:~/develop/worksapce/shell_work space$./case_test.sh Enter A number from 1 to 3:2you select 2[email protected]:~/develop/worksapce/shell_workspace$./cas E_test.sh Enter A number from 1 to 3:3you select 3[email protected]:~/develop/worksapce/shell_workspace$./case_test.sh En  ter a number from 1 to 3:*you select *[email protected]:~/develop/worksapce/shell_workspace$./case_test.sh Enter A number From 1 to 3:20you SELECT *

3. For Loop

Format:

For variable name in list

Do

Command 1

Command 2

Done

Example:

for_test1.sh

#!/bin/bash#for_testfor Loop in 1 2 3 4 5do    echo $loopdone

Operation Result:

[Email protected]:~/develop/worksapce/shell_workspace$./for_test1.sh 12345

for_test2.sh

#!/bin/bash#for_test2for Loop in orange red Blue Graydo   echo $loopdone

Operation Result:

[Email protected]:~/develop/worksapce/shell_workspace$./for_test2.sh Orangeredbluegray

for_test3.sh

#!/bin/bash#for_test3for loop in ' Cat name.txt ' do    echo $loopdone

Operation Result:

[Email protected]:~/develop/worksapce/shell_workspace$ cat Name.txt Jessoncherry[email protected]:~/develop/ worksapce/shell_workspace$./for_test3.sh Jessoncherry

4. Until Cycle

Format:

Until conditions

Do

Command

Done

until_test.sh

#!/bin/bash#until_testi=10until [$i-lt 0]do    echo $i let    i=i-1   # ((i= $i-1))   #i =$[i-1]done

Give permission to execute the result as follows:

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx until_test.sh [email protected]:~/develop/ worksapce/shell_workspace$./until_test.sh 109876543210

　　

5. While Loop

Format:

While command

Do

Command

.......

Done

Example:

while_test1.sh

#!/bin/bash#while_test1while echo-n "Enter your favorite star:" Read Mingxingdo    echo "$Mingxing is your favorite star" done

Give permission to execute:

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx while_test1.sh [email protected]:~/develop/ worksapce/shell_workspace$./while_test1.sh Enter your favorite star: Zhou Jie Zhou Jie is your favorite star enter your favorite star: Sun Nan Sun Nan is the star you like

while_test2.sh

#!/bin/bash#while_testwhile Read Namedo   echo $NAMEdone <name.txt

Give permission to execute:

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx while_test2.sh [email protected]:~/develop/ worksapce/shell_workspace$./while_test2.sh Jessoncherry

6. Break and Continue

Break [n]

-Exit Loop

-If you are in an embedded loop, you can specify N to jump out of the number of loops.

Continue

-Skip Cycle Step

Note that, as in other high-level languages, the Continue command is similar to break, with only a little difference, and it does not jump out of the loop, but is passing through the current loop step.

Example:

break_test.sh

#!/bin/bash#break_test.shwhile:d o    echo-n "Enter any number (1,..., 5):"    read ANS case    $ANS in    1|2|3|4| 5)        echo "You enter anumber between 1 and 5"        ;;    *)        echo "Wrong Number,bye" break        ;;        ;    Esacdone

Give permission to do the following:

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx break_test.sh [email protected]:~/develop/ worksapce/shell_workspace$./break_test.sh Enter any number (1,..., 5): 1You Enter anumber between 1 and 5Enter any number (1, ..., 5): 2You Enter anumber between 1 and 5Enter any number (1,..., 5): 3You Enter anumber between 1 and 5Enter any number (1,.. ., 5): 4You Enter anumber between 1 and 5Enter any number (1,..., 5): 5You Enter anumber between 1 and 5Enter any number (1,..., 5): 6Wrong Number,bye

continue_test.sh

#!/bin/bash#continue_break_testwhile:d o     echo-n "Enter any number (1,..., 5):"     read ANS case     $ANS in     1| 2|3|4|5)         echo "You enter a number between 1 and 5"         ;;     *)         echo-n "wrong number,continue (y/n)?:"         read is_continue case         $IS _continue in         y|yes| y| Yes)                continue                ;;         *)                break                ;;         Esac     Esacdone

Similarly, give executable permission to execute

[Email protected]:~/develop/worksapce/shell_workspace$ chmod a+rx continue_test.sh [email protected]:~/develop/ worksapce/shell_workspace$./continue_test.sh Enter any number (1,..., 5): 2You Enter A number between 1 and 5Enter any Numbe R (1,..., 5): 3You Enter a number between 1 and 5Enter any number (1,..., 5): 4You Enter a number between 1 and 5Enter any Numbe R (1,..., 5): 5You Enter a number between 1 and 5Enter any number (1,..., 5): 8Wrong number,continue (y/n)?: Yenter any number (1,. .., 5): 1You Enter a number between 1 and 5Enter any number (1,..., 5): 79Wrong number,continue (y/n)?: N

The shell script of Linux learning-------control flow structure