ループ：`for`、`while`、`until`

Imagine you need to perform the same action many times, like saying 'Hello' to everyone in a list, or counting from 1 to 100. Doing this manually would be tedious and error-prone.

This is where loops come in! Loops allow your script to repeat a set of commands multiple times, saving you effort and making your scripts more efficient.

The for loop is perfect when you know exactly what items you want to iterate over. It goes through a list, processing each item one by one.

Its basic syntax looks like this:

• for item in list; do
• # commands to run for each item
• done

The item variable will hold each value from the list in turn.

Let's see the for loop iterate through a list of names. Each name is assigned to the name variable, and then printed.

Try running this example:

What if you need to loop a specific number of times, like counting from 1 to 5? You can generate sequences for the for loop:

• Brace Expansion: {1..5} creates "1 2 3 4 5".
• seq command: $(seq 1 5) also creates "1 2 3 4 5".

Both are handy for numerical iterations.

Here's how you can use brace expansion to count up to a specific number. The loop variable i takes on each number in the sequence.

Try running this example:

The while loop is used when you want to repeat commands as long as a certain condition remains true. It continuously checks the condition before each iteration.

Its basic structure is:

• while condition; do
• # commands to run
• done

The loop stops as soon as the condition becomes false.

Let's use a while loop to count. We initialize a variable count, and the loop continues as long as count is less than 3.

Remember to update the variable inside the loop, or it might run forever (an 'infinite loop')!

The until loop is similar to while, but with an inverse condition. It executes commands until a specified condition becomes true.

Its syntax is:

• until condition; do
• # commands to run
• done

The loop continues as long as the condition is false.

Here's an until loop waiting for a variable num to reach 3. It will keep running as long as num is not greater than or equal to 3.

Try running this example:

Choosing the right loop makes your scripts clearer and more efficient:

• for loop: Best for iterating over a known list of items (files, names, numbers in a range).
• while loop: Ideal when you need to repeat commands as long as a condition holds true (e.g., reading lines from a file, waiting for user input, counting).
• until loop: Useful when you want to repeat commands until a specific condition becomes true (e.g., waiting for a process to finish, retrying an operation).

Let's test your understanding of shell loops.

Well done! You've learned the fundamental looping constructs in shell scripting:

• The for loop iterates over a fixed list of items or a range.
• The while loop repeats commands as long as a condition is true.
• The until loop repeats commands until a condition becomes true.

These loops are essential for automating repetitive tasks and making your scripts dynamic and powerful. Keep practicing!