Essential Bash Scripts for Linux Operations: Sync, Monitoring, and Automation

1. Detect file consistency between two servers

<code>#!/bin/bash
######################################
# Detect file consistency between two servers
#####################################
# Compare md5 values of files in a directory on two servers

dir=/data/web
b_ip=192.168.88.10
# Generate md5 list for local server
find $dir -type f | xargs md5sum > /tmp/md5_a.txt
# Generate md5 list for remote server
ssh $b_ip "find $dir -type f | xargs md5sum > /tmp/md5_b.txt"
scp $b_ip:/tmp/md5_b.txt /tmp
# Compare files
for f in `awk '{print $2}' /tmp/md5_a.txt`; do
  if grep -qw "$f" /tmp/md5_b.txt; then
    md5_a=`grep -w "$f" /tmp/md5_a.txt | awk '{print $1}'`
    md5_b=`grep -w "$f" /tmp/md5_b.txt | awk '{print $1}'`
    if [ $md5_a != $md5_b ]; then
      echo "$f changed."
    fi
  else
    echo "$f deleted."
  fi
done</code>

2. Periodically clear file contents and log file sizes

<code>#!/bin/bash
#################################################################
# Run hourly; at 00:00 or 12:00 clear file contents, otherwise log sizes
################################################################
logfile=/tmp/$(date +%H-%F).log
n=$(date +%H)
if [ $n -eq 00 ] || [ $n -eq 12 ]; then
  for i in $(find /data/log/ -type f); do
    true > $i
  done
else
  for i in $(find /data/log/ -type f); do
    du -sh $i >> $logfile
  done
fi</code>

3. Monitor network interface traffic and log

<code>#!/bin/bash
#######################################################
# Record network traffic every minute in bps
#######################################################
LANG=en
logfile=/tmp/$(date +%d).log
exec >> $logfile
date +"%F %H:%M"
sar -n DEV 1 59 | grep Average | grep ens33 | awk '{print $2,"\t","input:\t",$5*1000*8,"bps","\n",$2,"\t","output:\t",$6*1000*8,"bps"}'
echo "####################"
# Loop continues without sleep because sar runs 59 seconds
while :; do :; done</code>

4. Count numbers per line and total numbers in a document

<code>#!/bin/bash
#########################################################
# Count numeric characters per line and total in a file
#########################################################
n=$(wc -l a.txt | awk '{print $1}')
sum=0
for i in $(seq 1 $n); do
  line=$(sed -n "${i}p" a.txt)
  n_n=$(echo $line | sed s'/[^0-9]//'g | wc -L)
  sum=$((sum + n_n))
  echo "$n_n"
done
echo "sum:$sum"</code>

5. Kill all scripts

<code>#!/bin/bash
################################################################
# Kill processes matching a given name to reduce load
################################################################
ps aux | grep "指定进程名" | grep -v grep | awk '{print $2}' | xargs kill -9</code>

6. Download a file from an FTP server

<code>#!/bin/bash
if [ $# -ne 1 ]; then
  echo "Usage: $0 filename"
fi
dir=$(dirname $1)
file=$(basename $1)
ftp -n -v <<EOF
open 192.168.1.10
user admin password
binary
cd $dir
get "$file"
EOF</code>

7. Input five numbers (0‑100) and compute sum, min, max

<code>#!/bin/bash
COUNT=1
SUM=0
MIN=0
MAX=100
while [ $COUNT -le 5 ]; do
  read -p "请输入1-10个整数：" INT
  if [[ ! $INT =~ ^[0-9]+$ ]]; then
    echo "输入必须是整数！"
    exit 1
  elif [[ $INT -gt 100 ]]; then
    echo "输入必须是100以内！"
    exit 1
  fi
  SUM=$((SUM+INT))
  [ $MIN -lt $INT ] && MIN=$INT
  [ $MAX -gt $INT ] && MAX=$INT
  let COUNT++
done
echo "SUM: $SUM"
echo "MIN: $MIN"
echo "MAX: $MAX"</code>

8. Number guessing game

<code>#!/bin/bash
# Generate a random number between 1 and 100
num=$[RANDOM%100+1]
while :; do
  read -p "计算机生成了一个 1‑100 的随机数,你猜: " cai
  if [ $cai -eq $num ]; then
    echo "恭喜,猜对了"
    exit
  elif [ $cai -gt $num ]; then
    echo "Oops,猜大了"
  else
    echo "Oops,猜小了"
  fi
done</code>

9. Monitor Nginx access log for 502 errors and restart php‑fpm

<code>#!/bin/bash
# Scenario: LNMP environment, 502 errors disappear after php‑fpm restart
log=/data/log/access.log
N=30   # threshold: 30 occurrences in last 300 lines (~10% of 300)
while :; do
  err=$(tail -n 300 $log | grep -c '502')
  if [ $err -ge $N ]; then
    /etc/init.d/php-fpm restart 2>/dev/null
    sleep 60   # prevent rapid restart loops
  fi
  sleep 10
done</code>

10. Delete lines with letters in first five lines and strip letters from lines 6‑10

<code># Delete lines 1‑5 that contain letters, then remove all letters from lines 6‑10
sed -n '1,5p' 2.txt | sed '/[a-zA-Z]/d'
sed -n '6,10p' 2.txt | sed 's/[a-zA-Z]//g'
sed -n '11,$p' 2.txt</code>

11. Calculate total size of .html files in current directory

<code># Method 1 (du)
find . -name "*.html" -exec du -k {} \; | awk '{sum+=$1} END {print sum}'
# Method 2 (ls)
sum=0
for size in $(ls -l *.html | awk '{print $5}'); do
  sum=$((sum+size))
done
echo $sum</code>

12. Scan host ports

<code>#!/bin/bash
HOST=$1
PORTS="22 25 80 8080"
for PORT in $PORTS; do
  if echo >/dev/tcp/$HOST/$PORT 2>/dev/null; then
    echo "$PORT open"
  else
    echo "$PORT close"
  fi
done</code>

13. Print words with fewer than six letters from a sentence

<code># Example sentence
for s in Bash also interprets a number of multi-character options.; do
  n=$(echo $s | wc -c)
  if [ $n -lt 6 ]; then
    echo $s
  fi
done</code>

14. Menu‑driven command execution based on numeric input

<code>#!/bin/bash
echo "*cmd menu* 1-date 2-ls 3-who 4-pwd 0-exit"
while :; do
  read -p "please input number :" n
  if [ -z "$n" ]; then continue; fi
  if ! [[ $n =~ ^[0-9]$ ]]; then exit 0; fi
  case $n in
    1) date ;;
    2) ls ;;
    3) who ;;
    4) pwd ;;
    0) break ;;
    *) echo "please input number is [1-4]" ;;
  esac
done</code>

15. Expect script for SSH non‑interactive command execution

<code># login.exp (Expect script)
#!/usr/bin/expect
set ip [lindex $argv 0]
set user [lindex $argv 1]
set passwd [lindex $argv 2]
set cmd [lindex $argv 3]
if { $argc != 4 } { puts "Usage: expect login.exp ip user passwd cmd"; exit 1 }
set timeout 30
spawn ssh $user@$ip
expect {
  "(yes/no)" { send "yes\r"; exp_continue }
  "password:" { send "$passwd\r" }
}
expect "$user@*" { send "$cmd\r" }
expect "$user@*" { send "exit\r" }
expect eof</code>

16. Create 10 users with random passwords and log them

<code>#!/bin/bash
for u in $(seq -w 0 09); do
  useradd user_$u
  p=$(mkpasswd -s 0 -l 10)
  echo $p | passwd --stdin user_$u
  echo "$p" | passwd user_$u
  echo "user_$u $p" >> /tmp/userpassword
done</code>

17. Monitor httpd process count and restart Apache if needed

<code>#!/bin/bash
check_service() {
  j=0
  for i in $(seq 1 5); do
    /usr/local/apache2/bin/apachectl restart 2>/var/log/httpderr.log
    if [ $? -eq 0 ]; then break; else j=$((j+1)); fi
    if [ $j -eq 5 ]; then mail.py; exit; fi
  done
}
while :; do
  n=$(pgrep -l httpd | wc -l)
  if [ $n -gt 500 ]; then
    /usr/local/apache2/bin/apachectl restart
    if [ $? -ne 0 ]; then
      check_service
    else
      sleep 60
      n2=$(pgrep -l httpd | wc -l)
      if [ $n2 -gt 500 ]; then mail.py; exit; fi
    fi
  fi
  sleep 10
done</code>

18. Batch change passwords on multiple servers via SSH

<code>#!/bin/bash
OLD_INFO=old_pass.txt
NEW_INFO=new_pass.txt
for IP in $(awk '/^[^#]/{print $1}' $OLD_INFO); do
  USER=$(awk -v I=$IP 'I==$1{print $2}' $OLD_INFO)
  PASS=$(awk -v I=$IP 'I==$1{print $3}' $OLD_INFO)
  PORT=$(awk -v I=$IP 'I==$1{print $4}' $OLD_INFO)
  NEW_PASS=$(mkpasswd -l 8)
  echo "$IP $USER $NEW_PASS $PORT" >> $NEW_INFO
  expect -c "
    spawn ssh -p$PORT $USER@$IP
    set timeout 2
    expect {\n      \"(yes/no)\" {send \"yes\r\";exp_continue}\n      \"password:\" {send \"$PASS\r\";exp_continue}\n      \"$USER@*\" {send \"echo '$NEW_PASS' |passwd --stdin $USER\r exit\r";exp_continue}\n    }"
done</code>

19. iptables auto‑block IPs with excessive request rates

<code># Block IPs with >200 requests per minute (Nginx example)
#!/bin/bash
DATE=$(date +%d/%b/%Y:%H:%M)
ABNORMAL_IP=$(tail -n5000 access.log | grep $DATE | awk '{a[$1]++} END {for(i in a) if(a[i]>100) print i}')
for IP in $ABNORMAL_IP; do
  if [ $(iptables -vnL | grep -c "$IP") -eq 0 ]; then
    iptables -I INPUT -s $IP -j DROP
  fi
done
# Block IPs with >10 failed SSH attempts (auth.log example)
DATE=$(date +"%b %d %H")
ABNORMAL_IP=$(tail -n10000 /var/log/auth.log | grep "$DATE" | awk '/Failed/{a[$(NF-3)]++} END {for(i in a) if(a[i]>5) print i}')
for IP in $ABNORMAL_IP; do
  if [ $(iptables -vnL | grep -c "$IP") -eq 0 ]; then
    iptables -A INPUT -s $IP -j DROP
    echo "$(date +"%F %T") - iptables -A INPUT -s $IP -j DROP" >> ~/ssh-login-limit.log
  fi
done</code>

20. Block/Unblock IPs based on web access logs

<code>#!/bin/bash
logfile=/data/log/access.log
ipt=/sbin/iptables
ips=/tmp/ips.txt
block() {
  d1=$(date -d "-1 minute" +%H%M)
  grep "$d1:" $logfile | awk '{print $1}' | sort -n | uniq -c | sort -n > $ips
  for i in $(awk '$1>100 {print $2}' $ips); do
    $ipt -I INPUT -p tcp --dport 80 -s $i -j REJECT
    echo "$(date +%F-%T) $i" >> /tmp/badip.log
  done
}
unblock() {
  for a in $($ipt -nvL INPUT --line-numbers | grep '0.0.0.0/0' | awk '$2<10 {print $1}' | sort -nr); do
    $ipt -D INPUT $a
done
  $ipt -Z
}
minute=$(date +%M)
if [ $minute -eq 00 ] || [ $minute -eq 30 ]; then
  unblock
  block
else
  block
fi</code>

21. Validate user‑entered IP address

<code>#!/bin/bash
function check_ip(){
  local IP=$1
  if echo $IP | grep -E "^[0-9]{1,3}(\.[0-9]{1,3}){3}$" >/dev/null; then
    if [[ $(echo $IP | awk -F. '{print ($1<=255 && $2<=255 && $3<=255 && $4<=255)}') == 1 ]]; then
      echo "$IP available."
      return 0
    else
      echo "$IP not available!"
      return 1
    fi
  else
    echo "Format error! Please input again."
    return 1
  fi
}
while true; do
  read -p "Please enter IP: " IP
  check_ip $IP && break
done</code>