Master Bash Loops: Sum, Even Numbers, Multiplication Table, and System Checks

Calculate the sum from 1 to 100

Using a for loop:

#!/bin/bash
declare -i sum=0
for ((i=1;i<=100;i++)); do
  let sum+=$i
done
echo "sum=$sum"

Using an until loop:

#!/bin/bash
i=1
sum=0
until [ $i -gt 100 ]; do
  let sum+=$i
  let i++
done
echo "sum is:$sum"

Using a while loop:

#!/bin/bash
i=1
sum=0
while [ $i -le 100 ]; do
  let sum+=$i
  let i++
done
echo "sum is:$sum"

Sum of even numbers within 100

Using a for loop:

#!/bin/bash
i=0
sum=0
for i in `seq 100`; do
  if [ $[$i%2] -eq 1 ]; then
    continue
  fi
  let sum+=$i
done
echo "sum is:$sum"

Using a while loop:

#!/bin/bash
i=0
sum=0
while [ $i -le 100 ]; do
  let i++
  if [ $[$i%2] -eq 1 ]; then
    continue
  fi
  let sum+=$i
done
echo "sum is: $sum"

Using an until loop:

#!/bin/bash
i=0
sum=0
until [ $i -gt 100 ]; do
  let i++
  if [ $[ $i%2 ] -eq 1 ]; then
    continue
  fi
  let sum+=$i
done
echo "sum is: $sum"

Generate a 9×9 multiplication table

Using a for loop:

#!/bin/bash
for ((j=1;j<=9;j++)); do
  for ((i=1;i<=j;i++)); do
    echo -ne "$i*$j=$(($i*$j))\t"
  done
  echo
 done

Using a while loop:

#!/bin/bash
i=1
j=1
while [ $j -le 9 ]; do
  while [ $i -le $j ]; do
    echo -ne "$i*$j=$(($i*$j))\t"
    let i++
  done
  echo
  let i=1
  let j++
 done

Using an until loop:

#!/bin/bash
i=1
j=1
until [ $j -gt 9 ]; do
  until [ $i -gt $j ]; do
    echo -ne "$i*$j=$(($i*$j))\t"
    let i++
  done
  echo
  let i=1
  let j++
 done

Check if a user is logged in and log the event

Using a while loop to poll every 10 seconds:

#!/bin/bash
read -p "pls input a username: " username
while ! `who | grep "^$username" > /dev/null`; do
  sleep 10
done
echo "$(date +%F-%H:%M:%S) $username logged on" >> /tmp/user.log

Retrieve system information via a case menu

#!/bin/bash
cat <<EOF
1) show cpu information;
2) show memory information;
3) show disk information;
4) quit
EOF

cpu_info(){
  lscpu
}

mem_info(){
  cat /proc/meminfo
}

disk_info(){
  fdisk -l
}

quit(){
  echo "quit"
  exit 0
}

read -p "pls input a num: " num
if [ $num -ne 1 -a $num -ne 2 -a $num -ne 3 -a $num -ne 4 ]; then
  read -p "pls input a num again: " num
fi

case "$num" in
  1) cpu_info ;;
  2) mem_info ;;
  3) disk_info ;;
  4) quit ;;
esac