Logic in Bash is built from tests and loops. The modern test syntax is [[ ... ]] (safer than the older [ ... ]). Tests return an exit code, which if, while, and &&/|| use to decide flow.

• String tests: -z empty, -n non-empty, ==, !=
• Numeric tests: -eq -ne -lt -le -gt -ge
• File tests: -f file exists, -d dir exists, -e exists, -r/-w/-x
• Loops: for x in list, while [[ cond ]], and case for multi-branch

#!/usr/bin/env bash
for file in *.log; do
  if [[ -f "$file" && -s "$file" ]]; then   # exists and non-empty
    lines=$(wc -l < "$file")
    echo "$file has $lines lines"
  fi
done

count=0
while [[ $count -lt 3 ]]; do
  echo "tick $count"
  ((count++))
done

case "$1" in
  start) echo "starting" ;;
  stop)  echo "stopping" ;;
  *)     echo "usage: $0 {start|stop}" ;;
esac

1. Loop over all .txt files in a directory and print each name and size.
2. Write an if that checks whether a number argument is greater than 100.
3. Convert a chain of if/elif into a case statement.
4. Use a while loop to read a file line by line.

• ↗ Bash — Conditional constructs