Comprehensive Bash Scripting Guide: Conditionals, Loops, and Signal Traps

1. Conditional Selection and Judgment

1.1 if statement

Usage format:

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

Example: age validation script that checks for non‑numeric input, out‑of‑range values, and age thresholds.

#判断年纪
#!/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 validates that the input contains only digits, then checks whether the age is less than 150 and greater than 18.

Another example validates a score with similar logic.

#判断分数
#!/bin/bash
read -p "Please input your score: " score
if [[ $score =~ [^0-9] ]]; then
  echo "please input a int"
  exit 10
elif [ $score -gt 100 ]; then
  echo "Your score is wrong"
  exit 20
elif [ $score -ge 85 ]; then
  echo "Your score is very good"
elif [ $score -ge 60 ]; then
  echo "Your score is soso"
else
  echo "You are loser"
fi

Analysis: the script checks for non‑numeric characters, then validates the score range.

1.2 case statement

Usage format:

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

The case command supports glob‑style patterns such as * , ? , [] , and a|b .

#判断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

Analysis: the script treats any case‑insensitive form of "yes" as yes, any form of "no" as no, and all other inputs as false.

2. Loops

2.1 for loop

Two common forms:

# form 1
for name in list; do
  # loop body
 done
# form 2 (C‑style)
for (( exp1; exp2; exp3 )); do
  # loop body
 done

Example: calculate the sum of numbers from 1 to n.

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

Analysis: after validating the input, the script iterates from 1 to the entered number, accumulating the total.

Another example uses a C‑style for loop to sum only odd numbers up to 100.

#求出（1+3+...+100）的总和
for (( i=1, sum=0; i<=100; i++ )); do
  [ $[i%2] -eq 1 ] && let sum+=i
 done
echo sum=$sum

2.2 while loop

Usage:

while condition; do
  # loop body
 done

Special form for reading a file line by line:

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

Example: sum of all odd numbers less than 100.

#100以内所有正奇数之和
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"

2.3 until loop

Syntax is the opposite of while :

until condition; do
  # loop body
 done

Example: monitor a user and kill the process when the user logs in.

#监控xiaoming用户，登录就杀死
until pgrep -u xiaoming &> /dev/null; do
  sleep 0.5
 done
pkill -9 -u xiaoming

2.4 select loop (menu)

Creates an interactive menu:

select variable in list; do
  # commands
 done

Typical usage with case to handle the choice.

#生成菜单，并显示选中的价钱
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

Use continue [N] to skip to the next iteration of the N‑th outer loop, and break [N] to exit the N‑th outer loop.

#①求（1+3+...+49+53+...+100）的和
#!/bin/bash
sum=0
for i in {1..100}; do
  [ $i -eq 51 ] && continue
  [ $[ $i % 2 ] -eq 1 ] && { let sum+=i; let i++; }
done
echo sum=$sum

Analysis: skips the iteration when i equals 51, otherwise adds odd numbers to the sum.

4. Signal Trapping with trap

Basic syntax:

trap 'command' SIGNAL   # execute command when SIGNAL is received
trap '' SIGNAL          # ignore SIGNAL
trap '-' SIGNAL          # restore default handling
trap -p                 # list current traps

Common signals: SIGHUP, SIGINT (Ctrl+C), SIGQUIT, SIGKILL, SIGTERM, SIGCONT, SIGSTOP.

Example: prevent Ctrl+C from terminating a loop.

#①打印0-9，ctrl+c不能终止
#!/bin/bash
trap 'echo press ctrl+c' 2
for ((i=0;i<10;i++)); do
  sleep 1
  echo $i
done

Analysis: the script catches signal 2 (SIGINT) and prints a message instead of exiting.

5. Miscellaneous Bash Snippets

Generating random characters:

#生成8个随机大小写字母或数字
cat /dev/urandom | tr -dc [:alnum:] | head -c 8

Generating random numbers:

echo $RANDOM               # raw random number
echo $[RANDOM%7]           # 0‑6
echo $[$[RANDOM%7]+31]     # 31‑37

Printing colored text:

echo -e "\033[31malong\033[0m"   # red text
echo -e "\033[1;31malong\033[0m" # bold red
echo -e "\033[41malong\033[0m"   # red background

5.1 9×9 Multiplication Table

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

5.2 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 $num`; do
    xing=$[2*$i-1]
    for j in `seq 1 $[ $num-$i ]`; do echo -ne " "; done
    for k in `seq 1 $xing`; do
      color=$[$[RANDOM%7]+31]
      echo -ne "\033[1;${color};5m*\033[0m"
    done
    echo
  done
fi

5.3 Chessboard Pattern

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

These snippets demonstrate practical Bash techniques for automation, user interaction, and visual output.