Bash While Loop

The short answer

In Bash, the while statement is a control structure used to repeat a set of instructions for as long as a specified condition remains true.

To declare a while loop, you can use the following syntax:

while [condition];
do
  # instructions
done

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Where:

• condition is an expression used to keep the loop running for as long as it evaluates to true.
• do and done are used to delimit the instructions executed by the while loop.
• instructions is a list of one or more commands that will be executed by the while loop.

For example, let's consider this script used to print every digit from 1 to 9:

#!/bin/bash

x=1

while [ $x -le 9 ];
do
  echo $x
  (( x++ ))
done

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Where:

• $x -le 9 is used to evaluate whether the x variable is lesser than or equal to 9.
• echo $x is used to output the value of the x variable.
• (( x++ )) is used to increment the value of the x variable by 1 at each loop iteration.

Terminating the loop with the break statement

The break statement is used to immediately terminate the current loop, regardless of whether the loop's condition evaluates or not to false.

For example, let's consider this script that determines whether a number is prime:

#!/bin/bash

is_prime=true
number=3
i=2

while [ $i -lt $number ];
do
  if [[ $number -le 1 || $(($number % $i)) -eq 0 ]]; then
    is_prime=false
    break
  fi
  ((i++))
done

echo $is_prime

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Where:

• $i -lt $number checks whether the i variable is lesser than the number variable.
• $number -le 1 checks whether the number variable is lesser than or equal to 1.
• $(($number % $i)) -eq 0 checks whether the number variable is divisible by the i variable.
• break immediately terminates the while loop if either one of the conditions above are true.

Skipping a loop iteration with the continue statement

The continue statement is used to skip the current iteration of a loop and directly proceed to the next iteration, without executing the remaining instructions.

For example, let's consider this script that prints all the numbers between 1 and 15 that are divisible by 3:

#!/bin/bash

number=1
max=15

while [ $number -le $max ];
do
  if [ $((number % 3)) -ne 0 ]; then
    ((number++))
    continue
  fi
  echo "$number"
  ((number++))
done

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Where:

• $number -le $max checks if the number variable is lesser than or equal to the max variable.
• $((number % 3)) -ne 0 checks if the number variable is divisible by 3.
• ((number++)) increments the number variable by 1.
• continue skips the remaining instructions and immediately starts a new loop iteration from the beginning.

Reading from a file line by line

To read and process a text file line by line, you can combine the while loop with the read command as follows:

#!/bin/bash

file="/path/to/file"

while read -r line
do
  # instructions
done < "$file"

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Where:

• The file variable is used to store the absolute or relative path to the file.
• The read -r command is used to read the file line by line.
• The line variable is used to store the current line returned by the read command.
• The < operator is used to specify the source the read command should read its input from.

For example, this script reads each line of the "/var/log/apache2/access.log" file and only output the lines that contain the 192.168.2.101 IP address:

#!/bin/bash

log_file="/var/log/apache2/access.log"
ip_address="192.168.2.101"

while IFS= read -r line
do
  if [[ $line == *"$ip_address"* ]]; then
    echo "$line"
  fi
done < "$log_file"

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Reading from the standard input

To read and process text from the standard input, you can combine the while loop with the read command as follows:

bash
#!/bin/bash

while true;
do
  read input
done

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Where:

• input is a variable that stores the input captured by the read command.

For example, let's consider this script that makes a player enter a number and a second player guess that number:

#!/bin/bash

tries=0

read -p "Please enter a number: " number
echo "Got it! Clearing the screen in 3 seconds..."
sleep 3
clear

while true;
do
  read -p "Guess the number: " guess
  if [[ number -eq guess ]]; then
    echo "Great job! You guess in $tries tries."
    break
  else
    echo "Wrong number. Try again..."
    ((tries++))
  fi
done

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Where:

• read -p prompts the user with a string and reads its input.
• sleep pauses the execution of the program for a certain amount of seconds.
• clear clears all the text from the terminal window.

Iterating on array elements

To iterate over the elements of an array, you can initialize an index variable to zero and use the loop condition to check if the index is less than the array's length, incrementing the index within the loop to access and process each element sequentially.

#!/bin/bash

array=(element1, ..., elementN)

index=0

while [ $index -lt ${#array[@]} ];
do
  # do something with element: ${array[$index]}
  ((index++))
done

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Where:

• element1, …, elementN are a list of comma-separated array values, such as strings or numbers.
• index is a positional index used to access the current array element.
• ${#array[@]} is used to get the number of elements (i.e. the length) of the array.
• ${array[$index]} is used to access the value specified at index.
• ((index++)) is used to increment the index value by one.

For example, let's consider this script that outputs all the files in the current directory with a .js file extension:

#!/bin/bash

files=($(ls))
index=0

while [ $index -lt ${#files[@]} ];
do
  if [[ ${files[$index]} =~ .js$ ]]; then
    echo "${files[$index]}"
  fi
  ((index++))
done

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Where:

• ($(ls)) is used to declare the list of files returned by the ls command as an array of strings.
• ${files[$index]} =~ .js$ checks if the current filename ends with the string ".js".

Creating an infinite while loop

To create an infinite while loop in Bash, you must define a condition that never evaluates to false.

The most common way of doing so is to use the built-in true value as follows:

#!/bin/bash

while true;
do
  # instructions
done

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Alternatively, you can also use a semicolon character (:) as follows:

bash
#!/bin/bash

while :
do
  # instructions
done

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For example, let's consider this script that simply "echoes" back into the terminal the user input until it reads the "exit" string:

#!/bin/bash

echo "Enter text (type 'exit' to quit):"

while true;
do
  read -p "> " input
  if [ "$input" == "exit" ]; then
    echo "Goodbye!"
    break
  fi
  echo "You entered: $input"
done

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