Master Bash Conditionals, Loops, and Signal Traps with Real‑World Examples

1. Conditional Selection and Judgment (if)

The if statement evaluates conditions sequentially; the first true condition executes its branch and the whole if block ends. Syntax example:

if condition1; then
  # commands for true branch
elif condition2; then
  # commands for second true branch
else
  # commands when all conditions are false
fi

Example – age validation:

# Determine age
#!/bin/bash
read -p "Please input your age: " age
if [[ $age =~ [^0-9] ]]; then
  echo "please input a int"
  exit 10
elif [ $age -ge 150 ]; then
  echo "your age is wrong"
  exit 20
elif [ $age -gt 18 ]; then
  echo "good good work,day day up"
else
  echo "good good study,day day up"
fi

Analysis: The script first checks for non‑numeric input, then validates the range, and finally distinguishes adults from minors.

Example – score validation follows the same pattern, checking for non‑numeric input, out‑of‑range values, and grading thresholds.

2. Case Statement

The case construct matches a variable against patterns using glob‑style wildcards.

case $name in
  PATTERN1) cmd ;; 
  PATTERN2) cmd ;; 
  *) cmd ;; 
esac

Example – yes/no detection (accepts various capitalizations):

# Determine yes or no
#!/bin/bash
read -p "Please input yes or no: " ans
case $ans in
  [yY][eE][sS]|[yY]) echo yes ;;
  [nN][oO]|[nN]) echo no ;;
  *) echo false ;;
esac

3. Four Loop Types

3.1 for Loop

Two forms are supported:

# List iteration
for var in list; do
  # loop body
 done
# C‑style arithmetic iteration
for (( init; condition; increment )); do
  # loop body
 done

Example – sum of numbers 1 to n:

# Compute sum 1+2+...+n
sum=0
read -p "Please input a positive integer: " num
if [[ $num =~ [^0-9] ]] || [ $num -eq 0 ]; then
  echo "input error"
  exit 1
else
  for i in $(seq 1 $num); do
    sum=$((sum + i))
  done
  echo $sum
fi

3.2 while Loop

Executes as long as the condition is true; the condition is evaluated before each iteration.

while condition; do
  # loop body
 done

Example – sum of odd numbers up to 100:

# Sum of odd numbers ≤ 100
sum=0
i=1
while [ $i -le 100 ]; do
  if [ $((i % 2)) -ne 0 ]; then
    let sum+=i
  fi
  let i++
 done
echo "sum is $sum"

3.3 until Loop

The until loop runs until its condition becomes true (the opposite of while).

until condition; do
  # loop body
 done

Example – monitor a user and kill the session when they log in:

# Monitor user xiaoming
until pgrep -u xiaoming > /dev/null; do
  sleep 0.5
 done
pkill -9 -u xiaoming

3.4 select Loop (Menu)

The select construct creates an interactive numbered menu.

PS3="Please choose the menu: "
select menu in option1 option2 quit; do
  case $REPLY in
    1|2) echo "you chose $menu" ;;
    3) break ;;
    *) echo "invalid option" ;;
  esac
done

4. Signal Trapping with trap

The trap command associates a command with a signal, allowing custom handling.

# Example – ignore Ctrl+C (SIGINT) for the first 3 iterations
#!/bin/bash
trap '' SIGINT   # ignore
for i in {0..2}; do
  sleep 1
  echo $i
 done
trap - SIGINT    # restore default handling
for i in {3..9}; do
  sleep 1
  echo $i
 done

5. Handy Bash One‑Liners

Generate 8 random alphanumeric characters: cat /dev/urandom | tr -dc '[:alnum:]' | head -c 8 Generate a random number: echo $RANDOM (e.g., echo $((RANDOM%7+31)) for 31‑37)

Print colored text using ANSI escape codes, e.g.,

echo -e "\033[31malong\033[0m"

6. Fun Scripts

6.1 9×9 Multiplication Table

#!/bin/bash
for a in {1..9}; do
  for b in $(seq 1 $a); do
    let c=$a*$b
    echo -ne "${a}x${b}=${c}\t\c"
  done
  echo
 done

6.2 Colored Isosceles Triangle

#!/bin/bash
read -p "Please input a num: " num
if [[ $num =~ [^0-9] ]]; then
  echo "input error"
else
  for i in $(seq 1 $num); do
    stars=$((2*i-1))
    for ((j=1; j<=num-i; j++)); do echo -n " "; done
    for ((k=1; k<=stars; k++)); do
      color=$((RANDOM%7+31))
      echo -ne "\033[1;${color};5m*\033[0m"
    done
    echo
  done
fi

6.3 ASCII Chessboard

#!/bin/bash
red="\033[1;41m  \033[0m"
yellow="\033[1;43m  \033[0m"
for i in {1..8}; do
  if (( i % 2 == 0 )); then
    for _ in {1..4}; do echo -ne "${red}${yellow}"; done
  else
    for _ in {1..4}; do echo -ne "${yellow}${red}"; done
  fi
  echo
 done