While and until loops

While loops and until: the number of cycles is variable.

One, while loop

1. Format:

While condition test; do

Circulation body;

Done

Example: The and of all positive integers in 1-100 with while.

#!/bin/bash

Declare-i Sum=0,i=1

While [$i-le];d o

Let sum+= $i

Let i++

Done

Echo $sum

Example: Use while to find all even numbers within 1-100 and.

#!/bin/bash

Declare-i sum=0,i=2

While [$i-le];d o

Let sum+= $i

Let i+=2

Done

Echo $sum

Example: The keyboard prompts the user to enter characters, converts the user input lowercase letters to uppercase,

Once the conversion is done, remind the user to enter again and convert again until quit is entered.

Quit says quit.

#!/bin/bash

Read-p "Enter a word:" Word

while [["$word"! = "Quit"]];d o

echo $word |tr ' A-Z ' A-Z

Read-p "Enter A word again:" Word

Done

Example: Prompt the user to enter a user name, if present, displays the user UID and shell information, otherwise, the user is not displayed;

When the display is complete, prompts the user to enter again, or quit if quit.

#!/bin/bash

Read-p "Enter a username:" username

while [["$userName"! = "Quit"]];d o

If ID $userName &>/dev/null;then

grep "^ $userName \>"/etc/passwd |cut-d:-f3,7

Else

echo "$userName is not exist."

Fi

Read-p "Enter a username again:" username

Done

Example: Prompt the user to enter a user name,

If it does not exist, prompt the user to enter it again.

If present, determine if the user is logged in,

If you are not logged in, you are prompted to "$userName is isn't here.",

Then stop for 5 seconds and again determine if the user is logged in,

Until the user logs on to the system, "$userName is on" appears.

#!/bin/bash

Read-p "Enter A user name:" UserName

While! ID $userName &>/dev/null;do

Read-p "Enter a user name again:" UserName

Done

While! W.H.O. | grep "^ $userName" &>/dev/null;d o

echo "$userName is isn't here."

Sleep 5

Done

echo "$userName is on."

Second, until cycle: The condition is not satisfied, then the loop, otherwise exit (visible in the while opposite).

Example: With Untile cycle, the sum of all positive integers within 100 is obtained.

#!/bin/bash

Declare-i sum=0

Declare-i I=1

Untile [$i-gt];d o

Let sum+= $i

Let i++

Done

Echo $sum

Three, the combination condition test:

1. Logic and:

[Condition1] && [Condition2]

[Condition1-a Condition2]

[[Condition1 && Condition2]]

Note that:&& can only be used in double brackets [[]].

2. Logic or:

[Condition1] | | [Condition2]

[Condition1-o Condition2]

[[Condition1 | | condition2]]

Note: | | Can only be used in double brackets [[]].

3. Use the while loop to read the file one line at a time until the end of the file;

While read line;d o

Loop body

Done </path/to/somefile

Example: Removing a user with a default shell of bash on the current system

#!/bin/bash

While Read Line;do

[[' Echo $line |cut-d:-f7 ' = = '/bin/bash ']] && echo $line |cut-d:-F1

Done </ETC/PASSWD

While and until loops