Shell loop syntax

While do done, until do done (indefinite loop)

In general, the most common status of an indefinite loop is the following two states:

 
While[Condition] <=The status in the brackets is the limit type.Do<=DoIt is the beginning of the loop!ProgramSectionDone<=DoneIs the end of the loop
 

While Chinese is "when...", so this method is 『When the condition is set, a loop is performed until the condition of the condition is not set..

 

There is another way to change the loop:

Until[Condition]DoProcedure SectionDone
 

This method is the opposite of while 『When the condition is set, the loop is terminated. Otherwise, the loop is continued.Is it the opposite?

 

Let's make a simple exercise with while. If you want the user to enter yes or yes to end the program running, otherwise, the user will be notified of the input string.

[Root @ WWW Scripts] # vi sh13. Sh  # ! /Bin/ Bash # program: # repeat question  Until  User input correct answer. # History :# 2005 / 08 / 29  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport path  While [ "  $ YN  " ! = "  Yes  " - "  $ YN " ! = "  Yes  "  ]  Do  Read -P "  Please input Yes/yes to stop this program:  "  YN  Done  Echo   "  OK! You input the correct answer.  " 

 

The above example shows that "when the $ YN variable is not" yes "and $ YN is not" yes ", the program in the circle will be carried out .』 If $ YN is "yes" or "yes", it will leave the loop ~ What if until is used? Haha interesting Luo ~ His condition will become like this:

[Root @ WWW Scripts] # vi sh13- 2 . Sh  # ! /Bin/ Bash # program: # repeat question  Until  User input correct answer. # History :#  2005 / 08 / 29  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ /Binexport path  Until [ "  $ YN  " = "  Yes  " -O "  $ YN  " = "  Yes  "  ]  Do  Read -P "  Please input Yes/yes to stop this program:  "  YN  Done  Echo   "  OK! You input the correct answer.  " 

 

What are the differences between the two items! ^_^ Again. What if I want to calculate the data of 1 + 2 + 3 +... + 100? Use the loop ~ He is like this:

[Root @ WWW Scripts] # vi sh14. Sh  # ! /Bin/ Bash # program: # Use loop to calculate "  1 + 2 + 3 +... + 100  "  Result. # History :#  2005 / 08 / 29  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport paths = 0  # This is the total numeric variable I = 0 # This is a cumulative value, that is 1 ,2 , 3  ....  While [ "  $ I  " ! = "  100  "  ]  Do  I = $ ($ I + 1 ) # Every time I is added 1  S = $ ($ S +$ I) # Add the total number once every time!  Done  Echo   "  The result of '1 + 2 + 3 +... + 100 'is => $ s  " 

 

For... do... done (fixed loop)

His syntax is:

 
ForVaRInCon1 con2 con3...DoProgram SectionDone
 

In the preceding example, when the variable content of $ VaR is working in the Loop:

1. For the first round, the content of $ VaR is con1;
2. For the second round, the content of $ VaR is con2;
3. For the third round, the content of $ VaR is con3;
4. ....

We can make a simple exercise. Suppose I have three animals: dog, cat, and elephant. If I want to output the following words in each row: "There are dogs...", you can:

[Root @ WWW Scripts] # vi sh15. Sh  # ! /Bin/ Bash # program: # using  For ... Loop to print 3  Animals # History :#  2005 / 08 / 29  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport path  For Animal In Dog Cat  Elephant  Do      Echo   "  There are $ {animal} s ....  "  Done 

 

The domain I want to detect is the host's 192.168.1.1 ~ 192.168.1.100, because there are 100 hosts, I will not enter 1 to 100 after? You can do this now!

[Root @ WWW Scripts] # vi sh17. Sh # ! /Bin/ Bash # Program # Use  Ping Command to check the network '  S PC state.  # History #  2009 / 02 / 18  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport pathnetwork = "  192.168.1 "  # First define the first part of a domain!  For Sitenu In $ ( Seq   1   100 )# Seq  Abbreviation of Sequence  Do  # Obtain  Ping  Is the return value correct or failed!  Ping -C 1 -W   1 $ {Network}. $ {sitenu} &>/dev/ Null & Result = 0 | Result = 1  # Start to show whether the result is correct start (up) or wrong no connection (down)  If [ "  $ Result  " = 0 ]; Then                  Echo   " Server $ {network}. $ {sitenu} is up.  "          Else                  Echo   "  Server $ {network}. $ {sitenu} is down.  "          Fi  Done 

 

What should I do if I want the user to enter a directory file name and find out the permission for the file name in a directory?

[Root @ WWW Scripts] # vi sh18. Sh  # ! /Bin/ Bash # program: # user input  Dir Name, IFind  The permission of files. # History :#  2005 / 08 / 29  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport path #  1  . Check whether the directory exists first? Read -P "  Please input a directory:  "   Dir  If [ "  $ Dir  " = "" -O! -D "  $ Dir  " ]; Then      Echo   "  The $ DIR is not exist in your system.  "  Exit  1  Fi  # 2  . Start the test file ~ Filelist = $ ( Ls $ Dir  ) # List all file names in this directory  For Filename In  $ Filelist  Do  Perm = ""  Test -R "  $ DIR/$ filename  " & Perm ="  $ Perm readable  "  Test - W   "  $ DIR/$ filename  " & Perm = "  $ Perm writable  "  Test -X "  $ DIR/$ filename  " & Perm = " $ Perm executable  "      Echo   "  The file $ DIR/$ filename's permission is $ perm  "  Done 

 

For... do... done numerical processing

In addition to the above method, there is another way to write for loop! Syntax:

 
For(Initial value; Restriction value; running step ))DoProgram SectionDone
 

This syntax is suitable for numeric operations. The content of the three strings following the for clause indicates:

• Initial Value: the initial value of a variable in the loop, which is directly configured with something similar to I = 1;
• Limit Value: when the value of a variable is within the limit value, the loop continues. For example, I <= 100;
• Running steps: the variable changes every time a loop is made. For example, I = I + 1.

It is worth noting that in the configuration of "Running step", if you add 1 at a time, you can use a method similar to "I ++, that is to say, I adds one for every loop. Okay. In this way, we can accumulate 1 to the loop of user input!

[Root @ WWW Scripts] # vi sh19. Sh  # ! /Bin/ Bash # program: # Try  Do Calculate 1 + 2 +... + $ {Your_input} # History :#  2005 / 08 /29  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport pathread -P "  Please input a number, I will count for 1 + 2 +... + your_input:  "  Nus = 0  For (I = 1 ; I <= $ Nu; I = I + 1  ))  Do S = $ ($ S + $ I ))  Done  Echo   "  The result of '1 + 2 + 3 +... + $ nu 'is ==> $ s  " 

 

From http://vbird.dic.ksu.edu.tw/linux_basic/0340bashshell-scripts_5.php#loop