Conditionals run commands based on conditions. Use if to test exit codes or use [[ ... ]] for test expressions. Common operators: -f (file exists), -z (string empty), -n (string non-empty), = or == (string equal), -eq (number equal), -lt (less than), -gt (greater than).

if [[ -f "$file" ]]; then
  echo "File exists"
elif [[ $count -gt 10 ]]; then
  echo "Count is greater than 10"
else
  echo "Neither condition true"
fi

Use [[ ]] instead of [ ] in Bash—it's more robust and handles quoting better. Use && (AND) and || (OR) for one-liners: cmd1 && cmd2 (run cmd2 if cmd1 succeeds), cmd1 || cmd2 (run cmd2 if cmd1 fails).

Loops repeat commands. Use for to iterate, while to repeat while a condition is true, until to repeat until a condition is true.

for i in 1 2 3; do
  echo "Number: $i"
done
 
for file in *.txt; do
  wc -l "$file"
done
 
while [[ $count -lt 10 ]]; do
  echo $count
  count=$((count + 1))
done

Loop control: break exits the loop, continue skips to the next iteration.

Exit codes (0-255) indicate success or failure. Zero means success, non-zero means failure. Access the last exit code with $?. Use set -e to exit immediately on error, set -u to error on undefined variables, set -o pipefail to fail if any command in a pipe fails.

$ command
$ echo $?                       # prints exit code
$ command || echo "Failed"      # run second command if first fails
$ set -e                        # exit on error
$ grep "pattern" file || true   # don't exit even if grep fails

Always check exit codes and handle errors in scripts. Use set -e -u -o pipefail at the top of production scripts.