For/while/untill Loop of shell script programming loops

Loop execution: Repeats a piece of code 0, 1, or more times;

Enter the condition: when the condition satisfies, only then enters the circulation;

Exit conditions: Each cycle should have an exit condition to have the opportunity to exit the loop;

Bash script:

For loop

While loop

Until cycle

For loop:

Two types of formats:

(1) Traversing the list

(2) Control variables

(1) Traversing the list:

For varaible in LIST; Do

Loop body

Done

Entry conditions: As long as the list has elements, you can enter the loop;

Exit Criteria: The element traversal in the list is completed;

How the LISTT is generated:

(1) give directly;

(2) List of integers

(a) {start: End

(b) SEQ [start [incremtal]] Last

(3) command to return a list

(4) Glob

(5) Variable reference

[email protected], $*

Example: adding three users

#!/bin/bash#for username in User21 user22 user23; Do if ID $username &>/dev/null;    Then echo "$username exists."    else useradd $username && echo "Add user $username finished." Fidone

Example: The sum of all positive integers within 100 is obtained;

#!/bin/bash#declare-i sum=0for i in {1..100}; Do echo "\ $sum are $sum, \ $i is $i" sum=$[$sum + $i]doneecho $sum

Example: Determine the content type of each file under the/var/log directory

#!/bin/bash#for filename in /var/log/*; do    if [ -f $ filename ]; then        echo  "Common file."     elif [ -d  $filename  ]; then         echo  "Directory."     elif [ -L  $filename  ]; then         echo  "Symbolic link."     elif [ -b  $filename  ]; then         echo  "Block special file."     elif [ -c  $filename  ]; then         echo  "Character special file."     elif [ -S  $filename  ]; then         echo  "Socket file."    else        echo " unkown. "     fidone

For loop format:

For varaible in LIST; Do

Loop body

Done

For loop implementation 9*9 multiplication table

#!/bin/bash#for j in {1..9}; Do for I in $ (seq 1 $j); Do Echo-n-E "${i}x${j}=$[${i}*${j}]\t" done echo-Done

While loop:

While CONDITION; Do

Loop body

Cyclic control variable correction expression

Done

Entry condition: Condition test is true

Exit Condition: Condition test is "false"

Example: The sum of all positive integers within 100 is obtained;

#!/bin/bash#declare-i sum=0declare-i i=1while [$i-le 100]; Do let sum+= $i let I++doneecho $sum

Until cycle:

Until CONDITION; Do

Loop body

Cyclic control variable correction expression

Done

Entry condition: Condition test for "false"

Exit Condition: Condition test is true

Example: The sum of all positive integers within 100 is obtained;

#!/bin/bash#declare-i sum=0declare-i i=1until [$i-gt 100]; Do let sum+= $i let I++doneecho $sum

Special usage of the while loop (the line that traverses the file):

while read VARIABLE; Do

Circulation body;

Done </path/from/somefile

Read each line in the/path/from/somefile file sequentially and assign the base value to the variable variable;

Example: Find the user whose ID number is even, display its user name, ID and default shell;

#!/bin/bash#while read line;    Do userid=$ (echo $line | cut-d:-F3) username=$ (echo $line | cut-d:-F1) usershell=$ (echo $line | cut-d:-F7) If [$[$userid%2]-eq 0];    Then echo "$username, $userid, $usershell." Fidone </etc/passwd

Special usage for the FOR loop:

For (control variable initialization; conditional judgment expression; control variable correction statement)); Do

Loop body

Done

Control variable initialization: Executes only once when the loop code begins to run;

The correction of the control variables: each round of the end of the cycle will be the control variable correction operation, and then make the conditional judgment;

Example: Sum of all positive integers within 100

#!/bin/bash#declare-i sum=0for ((i=1;i<=100;i++)); Do let sum+= $idoneecho "sum: $sum."

Example: printing a 99 multiplication table

#!/bin/bash#for ((j=1;j<=9;j++)); Do for ((i=1;i<=j;i++)); Do echo-e-n "${i}x${j}=$[${i}*${j}]\t" done Echodone

This article is from the "Wang Liming" blog, make sure to keep this source http://afterdawn.blog.51cto.com/7503144/1916005

For/while/untill Loop of shell script programming loops