While loop and advanced control loop commands
In Bash, you can use a while loop to execute a block of code repeatedly as long as a certain condition is met. The basic syntax for a while loop is as follows:
while [ condition ] do # code to be executed doneHere’s an example of how you might use a while loop in a Bash script:
# Set the initial value of the counter counter=1 # Set the maximum value of the counter max=10 # Run the loop until the counter exceeds the maximum value while [ $counter -le $max ] do # Print the current value of the counter echo $counter # Increment the counter by 1 counter=$((counter+1)) doneThis script will print the numbers 1 through 10, one number per line. The condition in the while loop is [ $counter -le $max ], which means “continue looping as long as the value of $counter is less than or equal to the value of $max.”
Inside the loop, the script prints the current value of $counter using the echo command and then increments the value of $counter by 1 using the counter=$((counter+1)) expression. When the value of $counter becomes greater than $max, the loop will exit and the script will continue executing the code after the done statement.
Control flow of while loop using continue and break
In a while loop in Bash, the continue and break statements are used to control the flow of the loop.
The continue statement causes the loop to skip the rest of the current iteration and move on to the next one. For example:
i=1 while [ $i -le 10 ]; do if [ $i -eq 5 ]; then i=$((i+1)) continue
fi echo $i i=$((i+1)) doneThis script will print the numbers 1 through 10, but it will skip the number 5.
The break statement causes the loop to exit immediately. For example:
i=1 while [ $i -le 10 ]; do if [ $i -eq 5 ]; then
break
fi echo $i i=$((i+1)) doneThis script will print the numbers 1 through 4 and then exit the loop.
You can use the break and continue statements in any type of loop, including while, for, and until loops.
Combination of read command and while
You can use the read command in a while loop to continue reading input from the terminal until the user enters the letter ‘q’. Here is an example of how you might do this:
while true; do read -p "Enter a number or 'q' to quit: " input if [ "$input" = "q" ]; then break fi # Process the input doneThis script will prompt the user to enter a number or ‘q’ to quit. It will continue reading input until the user enters ‘q’, at which point it will exit the loop and terminate the script.
You can use the read command to read input from the terminal and store it in a variable. In this case, the input is stored in the input variable. You can then use an if statement to check whether the input is ‘q’ and use the break statement to exit the loop if it is.
You can modify this script to do whatever processing you want with the input. For example, you could use an if statement to check whether the input is a valid number and perform some operation on it, or you could use a case statement to perform different actions based on the input.
Error checking in while loop
Here is an example of a while loop that checks for an error code after running a command:
# Run the command and store the exit code in a variable command_exit_code=$? # Set a flag to indicate if an error has occurred error_occurred=0 # Loop until the command succeeds or the maximum number of attempts is reached while [ $command_exit_code -ne 0 ] && [ $error_occurred -lt 10 ] do
# Run the command again
command # Update the exit code variable command_exit_code=$? # Increment the error count error_occurred=$((error_occurred+1)) done # Check if an error occurred if [ $error_occurred -eq 10 ] then echo "Error: The command failed after 10 attempts."
else
echo "The command was successful."
fiThis loop will run the command repeatedly until it succeeds or the maximum number of attempts (10 in this example) is reached. If the command succeeds, the loop will exit and the script will print a message indicating that the command was successful. If the command fails after 10 attempts, the script will print an error message.