Bash Scripts for File Consistency Checks, Log Monitoring, and System Automation

This document provides a series of practical Bash scripts for common system administration and monitoring tasks.

1. Detect file consistency between two servers

#!/bin/bash
# Detect file consistency between two servers
# Compute MD5 sums of all files under /data/web on both servers and compare

dir=/data/web
b_ip=192.168.88.10
find $dir -type f | xargs md5sum > /tmp/md5_a.txt
ssh $b_ip "find $dir -type f | xargs md5sum > /tmp/md5_b.txt"
scp $b_ip:/tmp/md5_b.txt /tmp

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

2. Scheduled file content clearing and size logging

#!/bin/bash
# Run hourly via cron; 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

3. Monitor network interface traffic and log it

#!/bin/bash
# Log traffic every minute for interface ens33
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 forever
while :; do
  # No sleep needed because sar already waits 59 seconds
  :
 done

4. Count numbers per line and total in a document

#!/bin/bash
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 "${n_n}sum=$sum"
done
echo "sum:$sum"

5. Kill all scripts

#!/bin/bash
ps aux | grep 指定进程名 | grep -v grep | awk '{print $2}' | xargs kill -9

6. Download a file from FTP server

#!/bin/bash
if [ $# -ne 1 ]; then
  echo "Usage: $0 filename"
  exit 1
fi
dir=$(dirname $1)
file=$(basename $1)
ftp -n -v <
7. Read five numbers, compute sum, min and max
#!/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: $MAX"
8. Number guessing game
#!/bin/bash
num=$[RANDOM%100+1]
echo $num
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 0
  elif [ $cai -gt $num ]; then
    echo "Oops,猜大了"
  else
    echo "Oops,猜小了"
  fi
done
9. Monitor Nginx access log for 502 errors and restart php-fpm
#!/bin/bash
log=/data/log/access.log
N=30   # threshold (10% of 300 lines)
while :; do
  err=$(tail -n 300 $log | grep -c '502"')
  if [ $err -ge $N ]; then
    /etc/init.d/php-fpm restart 2>/dev/null
    sleep 60   # avoid rapid restarts
  fi
  sleep 10
done
10. Assign command results to variables
# Example 1
for i in $(echo "4 5 6"); do
  eval a$i=$idone
  echo $a4 $a5 $a6
done

# Example 2 (split 192.168.1.1{1,2})
num=0
for i in $(eval echo $*); do
  let num+=1
  eval node${num}="$i"
done
echo $node1 $node2 $node3

# Example 3 using array
arr=(4 5 6)
INDEX1=$(echo ${arr[0]})
INDEX2=$(echo ${arr[1]})
INDEX3=$(echo ${arr[2]})
11. Bulk rename files
# Method 1
for file in $(ls *html); do
  mv $file bbs_${file#*_}
 done

# Method 2
for file in $(find . -maxdepth 1 -name "*html"); do
  mv $file bbs_${file#*_}
 done

# Method 3
rename article bbs *.html
12. Delete lines containing letters in first five lines and remove letters from lines 6‑10
#!/bin/bash
sed -n '1,5'p 2.txt | sed '/[a-zA-Z]/d'
sed -n '6,10'p 2.txt | sed 's/[a-zA-Z]//g'
sed -n '11,$'p 2.txt
13. Count total size of .html files in current directory
# Method 1
find . -name "*.html" -exec du -k {} \; | awk '{sum+=$1} END {print sum}'

# Method 2
for size in $(ls -l *.html | awk '{print $5}'); do
  sum=$((sum+size))
 done
echo $sum
14. Scan host ports
#!/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 "$PORT close"
  fi
done
15. Print words with fewer than six letters from a sentence
#!/bin/bash
for s in Bash also interprets a number of multi-character options.; do
  n=$(echo $s | wc -c)
  if [ $n -lt 6 ]; then
    echo $s
  fi
done
16. Menu‑driven command execution
#!/bin/bash
echo "*cmd menu* 1-date 2-ls 3-who 4-pwd 0-exit"
while :; do
  read -p "please input number :" n
  if [ -z "$n" ]; then continue; fi
  if ! echo $n | grep -E "^[0-9]$" >/dev/null; 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
17. Expect script for non‑interactive SSH
#!/bin/bash
USER=root
PASS=123.com
IP=192.168.1.120
expect <
18. Bulk change user passwords on multiple servers
#!/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 '$1==I{print $2}' $OLD_INFO)
  PASS=$(awk -v I=$IP '$1==I{print $3}' $OLD_INFO)
  PORT=$(awk -v I=$IP '$1==I{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 $USER\r exit\r\"; exp_continue}\n    }"
done
19. iptables auto‑block excessive web requests
#!/bin/bash
DATE=$(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 $IP -j DROP
  fi
done
20. iptables block IPs with too many failed SSH attempts
#!/bin/bash
DATE=$(date +"%a %b %e %H:%M")
ABNORMAL_IP=$(lastb | grep "$DATE" | awk '{a[$3]++} END {for(i in a) if(a[i]>10) print i}')
for IP in $ABNORMAL_IP; do
  if [ $(iptables -vnL | grep -c "$IP") -eq 0 ]; then
    iptables -I INPUT -s $IP -j DROP
  fi
done
21. Block/Unblock IPs based on web traffic patterns
#!/bin/bash
logfile=/data/log/access.log
ipt=/sbin/iptables
ips=/tmp/ips.txt
block(){
  d1=$(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
  $ipt -Z
}
# Run block every minute, unblock at minute 00 and 30
minute=$(date +%M)
if [ "$minute" = "00" ] || [ "$minute" = "30" ]; then
  unblock
  block
else
  block
fi
22. Validate whether a string is a correct IPv4 address
#!/bin/bash
function check_ip(){
  local IP=$1
  local VALID=$(echo $IP | awk -F. '$1<=255 && $2<=255 && $3<=255 && $4<=255 {print "yes"}')
  if echo $IP | grep -E "^[0-9]{1,3}(\.[0-9]{1,3}){3}$" >/dev/null; then
    if [ "$VALID" = "yes" ]; then
      return 0   # valid
    else
      echo "$IP not available!"
      return 1
    fi
  else
    echo "Format error!"
    return 1
  fi
}
while true; do
  read -p "Please enter IP: " IP
  check_ip $IP && break
done
These scripts illustrate typical shell‑based solutions for file verification, log processing, network monitoring, automated security enforcement, and interactive utilities, useful for system administrators and DevOps engineers.