1、for Loop Introduction

for A cycle is a fixed cycle , In other words, you know how many times you need to do the loop , Sometimes I put for A loop is called a counting loop .

stay Shell in for There are two kinds of syntax for loops ：

#  grammar 1：

for  Variable  in  value 1  value 2  value 3 …

    do

         Program 

    done

In this grammar for Number of cycles , Depending on in The number of the following values （ The blank space to separate ）, If there are a few values, loop them a few times , And each loop assigns the value to the variable . in other words , hypothesis in Then there are three values ,for It's going to cycle three times , The first loop will put the value 1 Assign variable , The second loop will put the value 2 Assign variable , And so on .

#  grammar 2：

for(( Initial value ; Cycle control conditions ; Variable change ))

    do

         Program 

    done

Attention should be paid to Grammar 2 ：

  • Initial value ： At the beginning of the loop , You need to give a variable an initial value , Such as i=1;
  • Cycle control conditions ： Used to specify the number of times the variable loops , Such as i<=100, Then as long as i Is less than or equal to 100, The cycle will continue ;
  • Variable change ： After each cycle , How the variable should change , Such as i=i+1, After each cycle , Variable i All the values are added 1.

2、 Example

An example of grammar ：

demand ： Print time .

#  Create script file 

[[email protected] ~]# vim sh/for.sh

#!/bin/bash

for time in morning noon afternoon evening

    do

        echo "This time is $time!"

    done

Executing the script results in ：

[[email protected] tmp]# chmod 755 for1.sh

[[email protected] tmp]# ./for1.sh

This time is morning!

This time is noon!

This time is afternoon!

This time is evening!

Grammar 2 example ：

Grammar two is the same as in other languages for The loop is similar , That is, the number of cycles is fixed in advance .

demand ： from 1 Add to 100.

#!/bin/bash

#  Define a summation variable sum

sum=0

#  Define the cycle 100 Time 

#  stay Shell If you want to carry out mathematical operations , It needs to be enclosed in double parentheses , To recognize that there are numerical operations in brackets .

for((i=1;i<=100;i=i+1))

    do

        #  Each loop gives a variable sum assignment 

        sum=$(($sum+$i))

    done

#  Output 1 Add to 100 And 

echo "The sum of 1+2+...+100 is ：$sum"

3、for Cycle summary

  • The first format for Loops are the most common Shell Circulation mode .
  • The second format for Loops are suitable for mathematical operations , You can easily specify the number of cycles .

4、 practice ： Batch decompress scripts

Mode one ： Batch UnZip 

#  Create script file auto-tar.sh

[[email protected] ~]# vim sh/auto-tar.sh

#  Batch decompress scripts 

#!/bin/bash

#  Enter the compressed package directory .

cd /tmp/sh/tar

#  hold tar File names of all compressed packages in the directory , Save to tar.log In file .

#  single > Is overlay .

#  and tar.log Each file name is a line .

ls *.tar.gz>tar.log

#  hold tar Directory .tgz The name of the compressed package of type is also appended to tar.log In file .

#  double >> It's an addition .

ls *.tgz>>tar.log &>/dev/null

#  Tips ： In the way above , Put the names of all types of compressed files that need to be unzipped , All saved to tar.log In file .

#  Read tar.log The content of the document , How many values are there in the file , How many times will it cycle ,

#  Each loop assigns the file name to the variable i

for i in $(cat tar.log)

    do

        #  decompression , And discard all output 

        tar -zxvf $i &>/dev/null

        #  Note that if there are other compressed packages , It needs to be done here if Judge ,

        #  Decompress the compressed files in different formats .

        #  The same goes for mode 2 .

    done

#  Delete temporary files tar.log, Because the script will not work after execution .

rm -rf /tmp/sh/tar/tar.log

explain ：

In the first way for loop ,in There are several values after , Just cycle a few times , Values should be separated by spaces .

and tar.log What's in the file is 6 The file name of a compressed package , And each file name is on one line ,

[[email protected] tmp]# cat tar.log

apr-1.4.6.tar.gz

apr-util-1.4.1.tar.gz

httpd-2.4.7.tar.gz I

mysq1-5.5.23.tar.gz

php-5.6.15.tar.gz

phpMyAdmin-4.1.4-al1-languages.tar.gz

This format , It is equivalent to calculating one value per line , So you can cycle 6 Time , The value of each time is the file name of a compressed package ,

This completes the batch decompression of the required files .

Mode two ： Batch UnZip

use for The second method of the loop is batch decompression , There are two things to note .

  • First of all ： You need to know the total number of compressed packets , Because I need to use for The second format of the loop is used for batch decompression , You need to know how many times to cycle .

    Solution ： Save all the file names of the files to be unzipped into a file （ The temporary file ）, At this time, the file name of the file to be decompressed becomes a string , And then through wc Command to make statistics .
  • second ： You need to extract the name of each compressed package , Assignment in variables .

    Is the first cycle , The file name of the first package is assigned in the variable , The second cycle , Variable to assign the file name of the second package , Then you can use tar The command unzips the compressed package .
#/bin/bash

#  Enter the compressed package directory .

cd /tmp/sh/tar

#  hold tar File names of all compressed packages in the directory , Save to tar.log In file .

#  single > Is overlay .

#  and tar.log Each file name is a line .

ls *.tar.gz>tar.log

#  hold tar Directory .tgz The name of the compressed package of type is also appended to tar.log In file .

#  double >> It's an addition .

ls *.tgz>>tar.log &>/dev/null

#  Tips ： In the way above , Put the names of all types of compressed files that need to be unzipped , All saved to tar.log In file .

# wc -l Command statistics line number , That is, get the number of files .

num=$(cat /tmp/sh/tar/tar.log | wc -l) # perhaps :wc -l /tmp/sh/tar/tar.log

#  Start traversing the extracted file 

for((i=1;i<="$num";i=i+1))

    do

        #  use awk Command to extract the file name , To get the file name of the extracted file 

        # NR yes awk Built in variables for , At present awk Rows processed , Is the row number of the total data .

        #  Be careful '$i' This place , Still use single quotes , Using double quotation marks will report an error .

        # awk 'NR=='$i' {print $1}  It means to get the column information of the row .

        filename=$(cat tar.log | awk 'NR=='$i' {print $1})

        #  Unzip the file 

        tar -zxvf $filename -C /tmp/sh/tar

    done

#  Delete temporary files tar.log

rm -rf /tmp/sh/tar/tar.log

summary ：

for The first way to cycle , For a Shell Script writing , It's simpler .

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