LWM-Linux/07 - Shell Scripting Basics/Bash Loops.md

Loops in Bash

Loops are fundamental constructs in programming that allow you to execute a set of commands repeatedly. In Bash scripting, loops are essential for automating repetitive tasks, processing multiple files, and working with data structures. This tutorial will cover the three main types of loops in Bash: for loops, while loops, and until loops.

1. For Loops

The 'for' loop is used to iterate over a list of items or a range of values.

Syntax:

for variable in list
do
    commands
done

Examples:

• Iterating over a list of items:

for fruit in apple banana orange
do
    echo "I like $fruit"
done

• Iterating over a range of numbers:

for i in {1..5}
do
    echo "Number: $i"
done

• C-style for loop:

for ((i=0; i<5; i++))
do
    echo "Count: $i"
done

• Iterating over files in a directory:

for file in *.txt
do
    echo "Processing $file"
    # Add commands to process each file
done

2. While Loops

The 'while' loop executes a set of commands as long as a given condition is true.

Syntax:

while condition
do
    commands
done

Examples:

• Basic while loop:

count=1
while [ $count -le 5 ]
do
    echo "Count: $count"
    ((count++))
done

• Reading input until a condition is met:

while read -p "Enter a number (0 to exit): " num
do
    if [ "$num" -eq 0 ]; then
        echo "Exiting..."
        break
    fi
    echo "You entered: $num"
done

3. Until Loops

The 'until' loop is similar to the while loop, but it executes commands until a condition becomes true.

Syntax:

until condition
do
    commands
done

Example:

count=1
until [ $count -gt 5 ]
do
    echo "Count: $count"
    ((count++))
done

4. Loop Control Statements

Bash provides two main loop control statements:

• break: Exits the loop immediately
• continue: Skips the rest of the current iteration and moves to the next one

Example using both:

for i in {1..10}
do
    if [ $i -eq 5 ]; then
        continue
    fi
    if [ $i -eq 8 ]; then
        break
    fi
    echo "Number: $i"
done

5. Advanced Loop Techniques

• Nested loops:

for i in {1..3}
do
    for j in {1..3}
    do
        echo "i=$i, j=$j"
    done
done

• Looping through arrays:

declare -a fruits=("apple" "banana" "orange" "grape")
for fruit in "${fruits[@]}"
do
    echo "I like $fruit"
done

• Infinite loops (use with caution):

while true
do
    echo "This will run forever unless interrupted"
    sleep 1
done

• Looping with command substitution:

for line in $(cat file.txt)
do
    echo "Line: $line"
done

• Parallel execution of loops:

for job in job1 job2 job3
do
    (
        echo "Starting $job"
        sleep 2
        echo "Finished $job"
    ) &
done
wait
echo "All jobs completed"