Control Flow and LogicLesson 2.2

Bash for loops and while loops with examples

C-style for loop, for-in loop, while loop, until loop, break and continue, loop over files, loop over command output, IFS variable

Looping in Bash

Bash has multiple loop types. Choose based on what you're iterating: a list, a range, or a condition.

# For-in: iterate a list
for fruit in apple banana cherry; do
  echo "Processing: $fruit"
done

# C-style: numeric range
for (( i=1; i<=5; i++ )); do
  echo "Step $i"
done

# Brace expansion shortcut
for i in {1..10}; do
  echo $i
done

While Loop

count=0
while [[ $count -lt 5 ]]; do
  echo "Count: $count"
  (( count++ ))
done

# Read file line by line — the correct way
while IFS= read -r line; do
  echo "Line: $line"
done < /etc/hosts

Looping Over Files

# Glob expansion — handles spaces in filenames correctly
for file in /var/log/*.log; do
  [[ -f "$file" ]] || continue
  echo "Processing: $file"
done

# Loop over command output — avoid this pattern:
# for f in $(ls); do  ← breaks on spaces
# Use glob instead

IFS= read -r line is the canonical way to read files. IFS= prevents leading/trailing whitespace stripping. -r prevents backslash interpretation. Skipping either causes subtle data corruption.