Master 18 Essential Bash Scripts for Linux System Automation

1. Detect file consistency between two servers

#!/bin/bash
#####################################
#检测两台服务器指定目录下的文件一致性
#####################################
#通过对比两台服务器上文件的md5值，达到检测一致性的目的

dir=/data/web
b_ip=192.168.88.10
#将指定目录下的文件全部遍历出来并作为md5sum命令的参数，进而得到所有文件的md5值，并写入到指定文件中
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
#将文件名作为遍历对象进行一一比对
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
################################################################
#每小时执行一次脚本（任务计划），当时间为0点或12点时，将目标目录下的所有文件内容清空，但不删除文件，其他时间则只统计各个文件的大小
################################################################
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
#######################################################
#检测网卡流量，并按规定格式记录在日志中（每分钟记录一次）
#######################################################
while :
do
    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","
",$2,"\t","output:\t",$6*1000*8,"bps"}'
    echo "####################"
    # sar runs 59 seconds, no extra sleep needed
    done

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

#!/bin/bash
#########################################################
#计算文档每行出现的数字个数，并计算整个文档的数字总数
########################################################

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)
    echo $n_n
    sum=$((sum + n_n))
done
echo "sum:$sum"

5. Download a file from an FTP server

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

6. Read five numbers (0‑100) and report 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"

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

#!/bin/bash
###########################################################
#监测Nginx访问日志502情况，并做相应动作
###########################################################
log=/data/log/access.log
N=30   #阈值：30 条 502 错误（约 10%）
while :
do
    err=$(tail -n 300 $log | grep -c '502')
    if [ $err -ge $N ]; then
        /etc/init.d/php-fpm restart 2>/dev/null
        # 防止无限重启，延迟 60 秒
        sleep 60
    fi
    sleep 10
done

8. Scan host ports

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

9. Print words with fewer than six letters from a sample sentence

#!/bin/bash
# 示例语句：Bash also interprets a number of multi-character options.
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

10. Expect script for password‑less SSH command execution

#!/bin/bash
USER=root
PASS=123.com
IP=192.168.1.120
expect <<EOF
set timeout 30
spawn ssh $USER@$IP
expect {
    "(yes/no)" {send "yes\r"; exp_continue}
    "password:" {send "$PASS\r"}
}
expect "$USER@*" {send "$1\r"}
expect "$USER@*" {send "exit\r"}
expect eof
EOF

11. Create 10 users with random 10‑character passwords and log them

#!/bin/bash
##############################################################
#创建10个用户，并分别设置密码，密码要求10位且包含大小写字母以及数字
##############################################################
for u in $(seq -w 0 9); do
    useradd user_$u
    p=$(mkpasswd -s 0 -l 10)
    echo $p | passwd --stdin user_$u
    echo "user_$u $p" >> /tmp/userpassword
done

12. Monitor httpd process count and restart when exceeding 500

#!/bin/bash
check_service(){
    for i in $(seq 1 5); do
        /usr/local/apache2/bin/apachectl restart 2>/var/log/httpderr.log
        if [ $? -eq 0 ]; then
            break
        fi
    done
}
while :
do
    n=$(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 0
            fi
        fi
    fi
    sleep 10
done

13. iptables auto‑block IPs with 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" >/dev/null; then
        iptables -I INPUT -s $IP -j DROP
    fi
done

14. Block IPs with too many SSH login failures

#!/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" >/dev/null; then
        iptables -I INPUT -s $IP -j DROP
    fi
done

15. Block IPs that exceed request thresholds in web logs and unblock later

#!/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
    $ipt -Z
}
# Run block every minute; run unblock at minute 00 and 30
while :; do
    minute=$(date +%M)
    if [ $minute -eq 00 ] || [ $minute -eq 30 ]; then
        unblock
    fi
    block
    sleep 60
done

16. Validate user‑entered IP address

#!/bin/bash
function check_ip(){
    IP=$1
    if echo $IP | grep -E "^[0-9]{1,3}(\.[0-9]{1,3}){3}$" >/dev/null; then
        VALID=$(echo $IP | awk -F. '$1<=255 && $2<=255 && $3<=255 && $4<=255 {print "yes"}')
        if [ "$VALID" = "yes" ]; then
            echo "$IP available."
            return 0
        else
            echo "$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
    echo "Please try again."
done