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🌀 Loops in Bash (Complete Guide)

🔹 1. for Loops

Basic form

for var in item1 item2 item3; do
    echo "$var"
done

Over ranges

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

Output:

Number 1
Number 2
Number 3
Number 4
Number 5

With step

for i in {0..20..5}; do
    echo "$i"
done

Output: 0 5 10 15 20

C-style for loop

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

Loop over files

for file in *.txt; do
    echo "Processing $file"
done

Loop over command output

for user in $(cut -d: -f1 /etc/passwd); do
    echo "User: $user"
done

Nested for loops

for i in {1..3}; do
    for j in {a..c}; do
        echo "$i$j"
    done
done

Infinite for loop

for (( ; ; )); do
    echo "Infinite loop, press Ctrl+C to exit"
    sleep 1
done

Terminal one-liners

for i in {1..5}; do echo "Hello $i"; done

🔹 2. while Loops

Basic

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

Reading a file line by line

while read line; do
    echo "Line: $line"
done < file.txt

Until condition is met

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

While true (infinite loop)

while true; do
    echo "Running..."
    sleep 1
done

While with command output

while read user; do
    echo "User: $user"
done < <(cut -d: -f1 /etc/passwd)

Interactive while loop

while read -p "Enter something (or 'quit'): " input; do
    [[ $input == "quit" ]] && break
    echo "You typed: $input"
done

Terminal one-liners

count=1; while [[ $count -le 3 ]]; do echo "Loop $count"; ((count++)); done

🔹 3. Control Statements (inside both for and while)

Statement Meaning
break Exit the loop immediately
continue Skip current iteration
exit Exit the script completely

Examples:

for i in {1..5}; do
    if [[ $i -eq 3 ]]; then
        echo "Breaking at $i"
        break
    fi
    echo "$i"
done
for i in {1..5}; do
    if [[ $i -eq 3 ]]; then
        echo "Skipping $i"
        continue
    fi
    echo "$i"
done

🔹 4. Loop Tricks

  • Loop over arguments
for arg in "$@"; do
    echo "Arg: $arg"
done
  • Parallel commands
for site in google.com github.com; do
    ping -c1 $site &   # run in background
done
wait
  • Arithmetic loop with while
i=0
while (( i < 5 )); do
    echo $i
    ((i++))
done
  • Colorized output in loops
for i in {1..5}; do
    echo -e "\e[32mIteration $i\e[0m"
done

🔹 5. Advanced Loop Usage

  • Process substitution
while read line; do
    echo ">> $line"
done < <(ls -1)
  • Loop with timeout
end=$((SECONDS+5))
while [ $SECONDS -lt $end ]; do
    echo "Looping..."
    sleep 1
done
  • Nested while with file + counter
count=1
while read line; do
    echo "$count: $line"
    ((count++))
done < /etc/passwd

🔹 6. Interactive Terminal Use

  • Simple loop
for i in {1..5}; do echo "hello $i"; done
  • Infinite loop in terminal
while true; do date; sleep 1; done
  • Loop until user presses Ctrl+C
for (( ; ; )); do echo "Running"; sleep 2; done

✅ Summary: In Bash, loops (for, while, until) let you:

  • Iterate over numbers, files, strings, arguments
  • Read input from files or commands
  • Run infinitely (until stopped)
  • Control flow with break, continue, exit
  • Use both in scripts and one-liners in terminal

🎯 Bash until Loop

🔹 1. Basic Syntax

until [ condition ]
do
    commands
done
  • Runs until the condition becomes true
  • Opposite of while loop (which runs while condition is true)

🔹 2. Simple Example

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

👉 Output:

Count = 1
Count = 2
Count = 3
Count = 4
Count = 5

🔹 3. Infinite Until Loop

until false
do
    echo "This runs forever (press Ctrl+C to stop)"
done

🔹 4. Reverse Counting

n=5
until [ $n -eq 0 ]
do
    echo "Countdown: $n"
    ((n--))
done
echo "Blastoff!"

🔹 5. Wait for a File to Exist

file="/tmp/myfile.txt"

until [ -f "$file" ]
do
    echo "Waiting for $file ..."
    sleep 2
done

echo "$file is ready!"

🔹 6. With Break & Continue

x=0
until [ $x -ge 10 ]
do
    ((x++))
    if (( x == 5 )); then
        echo "Skipping 5"
        continue
    fi
    if (( x == 8 )); then
        echo "Stopping at 8"
        break
    fi
    echo "x = $x"
done

🔹 7. One-Liner in Terminal

until ping -c1 google.com &>/dev/null; do echo "No internet... retrying"; sleep 2; done
echo "Connected!"

🔹 8. Comparison with while

Feature while loop until loop
Condition Runs while condition is true Runs until condition is true
Usage style "Do this as long as..." "Do this until..."
Infinite by default while true; do ...; done until false; do ...; done

✅ Summary: Use while when you know what condition must stay true. Use until when you want to wait for a condition to become true (e.g., file exists, process ends, internet available).

🔹 break and continue in Bash

🛑 1. break – Exit the Loop Immediately

  • Ends the current loop entirely.
  • Control moves to the next statement after the loop.

Example: stop when number is 5

for i in {1..10}
do
    if [ $i -eq 5 ]; then
        echo "Breaking at $i"
        break
    fi
    echo "i = $i"
done

👉 Output:

i = 1
i = 2
i = 3
i = 4
Breaking at 5

🔄 2. continue – Skip Current Iteration

  • Skips the rest of the loop body for the current iteration.
  • Moves to the next loop cycle.

Example: skip number 5

for i in {1..7}
do
    if [ $i -eq 5 ]; then
        echo "Skipping $i"
        continue
    fi
    echo "i = $i"
done

👉 Output:

i = 1
i = 2
i = 3
i = 4
Skipping 5
i = 6
i = 7

🔹 3. With while Loop

n=0
while [ $n -lt 7 ]
do
    ((n++))
    if [ $n -eq 3 ]; then
        echo "Continue at $n"
        continue
    fi
    if [ $n -eq 6 ]; then
        echo "Break at $n"
        break
    fi
    echo "n = $n"
done

🔹 4. With until Loop

x=0
until [ $x -ge 10 ]
do
    ((x++))
    if (( x == 4 )); then
        echo "Skipping $x"
        continue
    fi
    if (( x == 8 )); then
        echo "Breaking at $x"
        break
    fi
    echo "x = $x"
done

🔹 5. Nested Loops with break n

  • You can specify a numeric argument to break out of n levels of loops.
for i in {1..3}; do
    for j in {1..3}; do
        if [ $j -eq 2 ]; then
            echo "Breaking both loops at i=$i, j=$j"
            break 2   # exit both loops
        fi
        echo "i=$i, j=$j"
    done
done

👉 Without break 2, only the inner loop would stop.

🔹 6. Real-Life Example (skip hidden files)

for file in *; do
    if [[ $file == .* ]]; then
        continue   # skip hidden files
    fi
    echo "Processing: $file"
done

✅ Summary:

  • break → exit the loop completely
  • continue → skip current iteration, go to next
  • break N → exit multiple nested loops
init=0
while [[ $init -lt 10 ]];do
    ((init++))
    if [[ $init -eq 5 ]];then
        continue
    fi
    echo ${init}
done

init=0; while [[ $init -lt 10 ]]; do ((init++)); if [[ $init -eq 5 ]]; then continue; fi; echo $init; done

Here is the exact **Bash code** from the image provided:

```bash
#!/bin/bash
initNumber=1
while [[ ${initNumber} -lt 3 ]]
do
    for i in item1 item2 item3
    do
        echo "${initNumber} - ${i}"
        if [[ $i == item2 ]]
        then
            break 2
        fi
    done
    ((initNumber++))
done

#!/bin/bash
initNumber=1
while [[ ${initNumber} -lt 3 ]]
do
    for i in item1 item2 item3
    do
        echo "${initNumber} - ${i}"
        if [[ $i == item2 ]]
        then
            break 2
        fi
    done
    ((initNumber++))
done
  • Break normally breaks only one loop (the innermost one).
  • Break N breaks out of N levels of loops.
  • So break 2 breaks out of both the for loop and the outer while loop.