Loop statements commonly used in scripts

The cyclic command is used to run a command or a group of commands for a specified number of times, or until a certain condition is met. Common loop statements in Bash shell include for loop, while loop, and until loop.

I. For Loop statements

1. For Loop syntax

For var in list

Do

Commands

Done

2. For Loop Flowchart

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3. For Loop example

1) enter a file to determine whether the file is directory or file.

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[root@localhost test]# cat 3.sh #!/bin/sh forfile in$1doif[ -d "$file"] then echo "$file is a directory"elif [ -f "$file"] then echo "$file is a file"fi done [root@localhost test]# sh 3.sh /etc/passwd /etc/passwd isa file [root@localhost test]# sh 3.sh /etc /etc isa directory [root@localhost test]#

Note:

Row 3: The location variable $1 is called.

Row 5-11: Use the if statement to determine whether $1 is a file or a directory.

2) calculate the number of files in a directory

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[root@localhost test]# cat 4.sh #!/bin/bash # Count=0forFile in/tmp/*; dofile $File Count=$[$Count+1] done echo "Total files: $Count."

Note:

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Row 7: The Count value of each loop + 1

Ii. While loop statements

The while command allows you to define the command to be tested. If the defined test command returns the 0 state value, execute the statement in the while loop. Otherwise, exit directly.

1) while loop syntax

While test command

Do

Oter command

Done

2) while loop Flowchart

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3) while loop example

Calculate the sum of integers less than 100

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[root@localhost test]# cat 6.sh #!/bin/sh Sum=0Count=1while[ $Count -le 100]; dolet Sum+=$Count let Count++ done echo $Sum

If the user's ID number is an even number, the name and shell are displayed. This operation is performed on all users;

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[root@localhost test]# cat 5.sh #!/bin/sh whileread LINE; doUid=`echo $LINE | cut -d: -f3` if[ $[$Uid%2] -eq 0]; then echo $LINE | cut -d: -f1,7fi done < /etc/passwd

Note: