To write a Bash while loop, you must first master its basic structure and usage scenarios. 1. The basic format is: while [ condition ] do ... done. As long as the condition is true (the return value is 0), the loop body will be continuously executed; for example, printing numbers 1 to 5 requires increments with variables. 2. Common uses include reading file content line by line or waiting for a certain condition to be fulfilled (if the file appears). 3. Notes include: condition variables must be updated within the loop to avoid dead loops; there must be spaces before and after brackets in the conditional expression; and the sleep time is reasonably set to avoid slow script response. After mastering these key points, you can flexibly use while loops for script development.

When writing Bash scripts, while loops are a very practical tool, especially when you need to repeat a command until a certain condition is met. The usage is actually not difficult, but you have to pay attention to the grammar and usage scenarios.

Basic structure: How to write a while loop?

The basic format of while loop in Bash is as follows:

 While [ condition ]
do
  # Here is the command done to be executed by loop

It means that as long as condition is true (the return value is 0), the content between do and done will be executed all the time.

To give the simplest example, print numbers 1 to 5:

 i=1
while [ $i -le 5 ]
do
  echo $i
  i=$((i 1))
done

As you can see, we first define the variable i=1 , and then determine whether it is less than or equal to 5 in the loop, and output each loop and add one. This enables the function from 1 to 5.

Use scenario: When should I use while?

• Read file contents and process them line by line
  For example, if you want to read the contents of a text file line by line, you can use the following method:

   While read line
  do
    echo "This line is: $line"
  done < filename.txt

  This approach is very common when processing logs, configuration files, or data files.

• Waiting for a certain condition to be fulfilled
  For example, you want the script to wait until a file exists before continuing to execute:

   while [ ! -f /tmp/ready ]
  do
    echo "I'm still waiting for ready file..."
    sleep 1
  done
  echo "The file has appeared, continue execution"

  This can realize a "monitoring" mechanism, suitable for automated process control.

Note: Prone to errors

• Don't forget to update the condition variables

  If you write like this, it will become a dead loop:

   i=1
  while [ $i -le 5 ]
  do
    echo $i
  done

  Because i has not changed, the conditions will always be true, and the program will always be stuck here.

• Pay attention to spaces and symbols

  There must be spaces before and after [ ] in a conditional expression, for example, [ $i -le 5 ] is correct, and [$i -le 5] or [ $i-le 5 ] will report an error.

• Avoid blocking scripts for too long

  If you use sleep in the loop, remember not to be too long, otherwise the script response will slow down. The sleep time can be adjusted according to the actual situation, such as 0.5 seconds or sleep 0.5 .

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Basically that's it. By mastering the basic structure, common usage and precautions, you can flexibly use while loops in most scripts.

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