Loops: for, while, until & Loop Control (break, continue)

In Plain English

Loops repeat code blocks across lists of files, arrays, integer ranges, or streaming inputs. Bash offers standard `for in` loops, C-style three-expression `for (( ; ; ))` loops, condition-driven `while` loops, and inverted `until` loops.

The Mental Model

"Loops are the automated conveyor belts of scripting: instead of manually processing 100 files one by one, you load them onto the loop belt and apply the same operation to every item."

Deep Dive: How It Works

for in Loops: `for file in *.log; do ... done` (note: always quote `"$file"` inside body).

Range Expansion: `for i in {1..5}; do ... done` or `for i in {0..100..10}; do ... done` (with step size 10).

C-Style for Loops: `for (( i=1; i<=10; i++ )); do ... done`.

Loop Control: `break` exits the loop immediately; `continue` skips the rest of the current iteration.

Syntax Blueprint

for item in "${ARRAY[@]}"; do
  echo "$item"
done

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

while [[ $COUNT -gt 0 ]]; do
  (( COUNT-- ))
done

Use for in for collections and while read for streaming lines.

Core Rules to Remember

Range Syntax {1..N}: Generates sequential integer series at parse time without spawning `seq`.

Stream Processing: `while read` processes gigabyte datasets line by line with constant memory.

until Loop: Runs while the condition is FALSE; stops when it becomes TRUE.

Common Beginner Traps & How to Fix Them

Mistake:Using `for file in $(ls *.txt)` to iterate over files.

Why it happens: Command substitution splits filenames containing spaces into multiple broken loop iterations.

How to fix: Use direct globbing instead: `for file in *.txt; do [[ -e "$file" ]] || continue; ... done`.

Pro Tip: Always add `shopt -s nullglob` before glob loops to prevent literal `*.txt` string when no matching files exist.

Live Interactive Example

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Looping Techniques in Action

Git 2.45 / CLI

Output Console

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How it works: for {1..5} iterated through the range and continue skipped odd numbers.

Your Turn: Micro Challenge

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Micro Exercise

Iterate Through a Range with for

Write a for loop `for num in {1..3}; do echo "Item $num"; done`.

Sandbox Output

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