Master Bash Conditionals, Loops, and Signal Traps with Practical Examples

1. Conditional Selection and Judgment

if statement syntax:

if condition1 ; then
  # code when true
elif condition2 ; then
  # code when true
else
  # code when all false
fi

Example: age validation and response based on numeric input.

# 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: Checks for non‑numeric characters, then validates range and age thresholds.

case statement syntax:

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

Example: yes/no input handling with pattern matching.

# 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

2. Four Loop Types

for loop

Two forms:

# List iteration
for name in list; do
  # body
 done
# C‑style iteration
for ((exp1; exp2; exp3)); do
  # body
 done

Example: sum of 1..n.

# Sum 1+2+...+n
#!/bin/bash
read -p "Please input a positive integer: " num
if [[ $num =~ [^0-9] ]] || [ $num -eq 0 ]; then
  echo "input error"
  exit 10
else
  sum=0
  for i in $(seq 1 $num); do
    sum=$((sum + i))
  done
  echo $sum
fi

while loop

Syntax:

while condition; do
  # body
 done

Special form for reading a file line‑by‑line.

while read line; do
  # process $line
 done < /path/to/file

Example: sum of odd numbers up to 100.

# Sum 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"

until loop

Runs until the condition becomes true (inverse of while).

until condition; do
  # body
 done

Example: monitor a user login and kill processes.

# Kill processes of user xiaoming when they log in
until pgrep -u xiaoming &>/dev/null; do
  sleep 0.5
done
pkill -9 -u xiaoming

select loop (menu)

Creates an interactive menu.

PS3="Please choose the menu: "
select menu in mifan huimian jiaozi babaozhou quit; do
  case $REPLY in
    1|4) echo "the price is 15" ;;
    2|3) echo "the price is 20" ;;
    5) break ;;
    *) echo "no the option" ;;
  esac
done

3. Loop Control Statements

continue [N]

skips to the next iteration of the N‑th outer loop; break [N] exits the N‑th outer loop.

# Sum odd numbers 1..100, skip 51
for i in {1..100}; do
  [ $i -eq 51 ] && continue
  [ $((i % 2)) -eq 1 ] && let sum+=i
 done
 echo sum=$sum

4. Shift Command in Loops

Shifts positional parameters left, removing the first argument.

# Create multiple users from arguments
#!/bin/bash
if [ $# -eq 0 ]; then
  echo "Please input a arg (e.g., $0 user1)"
  exit 1
else
  while [ -n "$1" ]; do
    useradd "$1" &>/dev/null
    shift
done
fi

5. Signal Trapping with trap

Define custom actions for signals.

# Prevent Ctrl+C from terminating a loop (signal 2)
#!/bin/bash
trap 'echo press ctrl+c' 2
for ((i=0;i<10;i++)); do
  sleep 1
  echo $i
done

Later restore default handling:

trap '' 2   # ignore SIGINT
# ...
trap '-' SIGINT   # restore default

6. Miscellaneous Bash Tips

Generate random alphanumeric string: cat /dev/urandom | tr -dc '[:alnum:]' | head -c 8 Generate random number: echo $RANDOM or range with arithmetic expansion.

Print colored text using ANSI escape codes.

7. Fun Small Scripts

9×9 Multiplication Table

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

Colorful 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 $num); do
    spaces=$((num-i))
    stars=$((2*i-1))
    printf "%*s" $spaces ""
    for ((k=1;k<=stars;k++)); do
      color=$((RANDOM%7+31))
      echo -ne "\033[1;${color};5m*\033[0m"
    done
    echo
  done
fi

Chessboard Pattern

#!/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

These snippets illustrate core Bash constructs, practical patterns, and visual tricks useful for both learning and daily scripting tasks.