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This article mainly explains "how to repeatedly execute a command in Shell until it runs successfully". The explanation in this article is simple and clear, easy to learn and understand. Please follow the ideas of Xiaobian and study and learn "how to repeatedly execute a command in Shell until it runs successfully" together.

To solve repetitive work, you naturally think of cycles. In Shell, there are three kinds of loops: for, while, and until. In this article, we use the latter two loops: while and until.

About these two kinds of cycles, there is a programming foundation of small partners should be more familiar, here to review.

For a while loop, if the loop condition is true, it repeatedly executes the loop body (contained in do... done struct) until the loop condition becomes false.

For an untile loop, just the opposite of a while loop, it repeatedly executes the loop body (contained in do...) if the loop condition is false. until the loop condition becomes true.

Now that we've reviewed the basics, let's look at how we can use these two loops to achieve our goals.

1. Use the while loop

Again, take the example of repeatedly testing network connectivity. We can use the following commands to free our hands:

$ while ! ping -c 3 baidu.com ; do sleep 2 ; done ; echo succeed

For this command, it will repeat the ping command, if unsuccessful, sleep for 2 seconds and continue trying until successful. After success, the word succeeded will be printed on the terminal.

Brief introduction:

Let's break this command down and briefly describe what each module does.

$ while ! ping -c 3 baidu.com ;

This is the loop condition, our requirement is "when ping fails, execute loop body", so we have to add a before ping!。

$ do sleep 2 ; done ;

This part is the loop. When the loop condition holds, it enters the loop body. To prevent this program from grabbing too many resources, we sleep in the loop for 2 seconds. Once the loop condition does not hold, it jumps out of the loop body.

$ echo succeed ;

This part is the statement executed after exiting the loop body, just printing the word succeed, of course you can do other things.

2. Use the until loop

We use the until loop to do the same thing, which is written as follows:

$ until ping -c 3 baidu.com ; do sleep 2 ; done ; echo succeed

The effect achieved is the same as above, except that when the loop condition is true, the loop exits.

Let's also take a quick break.

$ until ping -c 3 baidu.com ;

This part is the loop condition, which will execute the ping command and the execution result will be used as the basis for judgment. The loop does not exit until the ping command succeeds.

$ do sleep 2 ;

Circulator, sleep for 2 seconds.

$ echo succeed

What to do after success.

3. Repeat the previous command until successful

Here's an additional extension. How do you repeat the previous command until it succeeds?

We know that if you want to repeat the last command, you can use this command:

$ !!

How do you determine if the last command was successful? Can we judge by $? The value is 0 to determine.

# while writing cycle

$ !!; while [ $? -ne 0 ]; do !!; done

# until the cycle is written

$ until !!; do :; done Thank you for reading, the above is "how to repeat the execution of a command in the Shell until the successful operation" of the content, after the study of this article, I believe that everyone on how to repeat the execution of a command in the Shell until the successful operation of this problem has a deeper understanding, the specific use of the situation also needs to be verified by practice. Here is, Xiaobian will push more articles related to knowledge points for everyone, welcome to pay attention!

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