Operations 14 min read

Essential Bash Scripts for System Monitoring, User Management, and Automation

This article provides a collection of practical Bash scripts for generating random strings, colorized output, batch user creation, package and service checks, host ping monitoring, CPU/memory/disk utilization alerts, website availability testing, and MySQL replication status verification, all useful for Linux system administration and interview preparation.

Efficient Ops

Jul 21, 2019

Essential Bash Scripts for System Monitoring, User Management, and Automation

1 Generate Random String or Number

Generate a random 8‑character string:

# Method 1:
# echo $RANDOM | md5sum | cut -c 1-8
# Method 2:
# openssl rand -base64 4
# Method 3:
# cat /proc/sys/kernel/random/uuid | cut -c 1-8

Generate a random 8‑digit number:

# Method 1:
# echo $RANDOM | cksum | cut -c 1-8
# Method 2:
# openssl rand -base64 4 | cksum | cut -c 1-8
# Method 3:
# date +%N | cut -c 1-8

cksum

prints a CRC checksum and byte count.

2 Define a Color Output Function

# Method 1:
function echo_color() {
    if [ $1 == "green" ]; then
        echo -e "[32;40m$2[0m"
    elif [ $1 == "red" ]; then
        echo -e "[31;40m$2[0m"
    fi
}
# Method 2:
function echo_color() {
    case $1 in
        green) echo -e "[32;40m$2[0m" ;;
        red)   echo -e "[31;40m$2[0m" ;;
        *)     echo "Example: echo_color red string" ;;
    esac
}
# Usage example:
# echo_color green "test"

The function keyword can be used with or without the word function.

3 Batch Create Users

#!/bin/bash
DATE=$(date +%F_%T)
USER_FILE=user.txt

echo_color(){
    if [ $1 == "green" ]; then
        echo -e "[32;40m$2[0m"
    elif [ $1 == "red" ]; then
        echo -e "[31;40m$2[0m"
    fi
}

# Backup existing user file
if [ -s $USER_FILE ]; then
    mv $USER_FILE ${USER_FILE}-${DATE}.bak
    echo_color green "$USER_FILE exist, rename ${USER_FILE}-${DATE}.bak"
fi

echo -e "User    Password" >> $USER_FILE
echo "----------------" >> $USER_FILE

for USER in user{1..10}; do
    if ! id $USER &>/dev/null; then
        PASS=$(echo $RANDOM | md5sum | cut -c 1-8)
        useradd $USER
        echo $PASS | passwd --stdin $USER &>/dev/null
        echo -e "$USER    $PASS" >> $USER_FILE
        echo "$USER User create successful."
    else
        echo_color red "$USER User already exists!"
    fi
done

4 Check If a Package Is Installed

#!/bin/bash
if rpm -q sysstat &>/dev/null; then
    echo "sysstat is already installed."
else
    echo "sysstat is not installed!"
fi

5 Check Service Status

#!/bin/bash
PORT_C=$(ss -anu | grep -c 123)
PS_C=$(ps -ef | grep ntpd | grep -vc grep)
if [ $PORT_C -eq 0 -o $PS_C -eq 0 ]; then
    echo "content" | mail -s "subject" [email protected]
fi

6 Verify Host Liveness

Method 1 – store failing IPs in an array and retry three times:

#!/bin/bash
IP_LIST="192.168.18.1 192.168.1.1 192.168.18.2"
for IP in $IP_LIST; do
    NUM=1
    while [ $NUM -le 3 ]; do
        if ping -c 1 $IP > /dev/null; then
            echo "$IP Ping is successful."
            break
        else
            FAIL_COUNT[$NUM]=$IP
            let NUM++
        fi
    done
    if [ ${#FAIL_COUNT[*]} -eq 3 ]; then
        echo "${FAIL_COUNT[1]} Ping is failure!"
        unset FAIL_COUNT[*]
    fi
done

Method 2 – count failures in a variable:

#!/bin/bash
IP_LIST="192.168.18.1 192.168.1.1 192.168.18.2"
for IP in $IP_LIST; do
    FAIL_COUNT=0
    for ((i=1;i<=3;i++)); do
        if ping -c 1 $IP >/dev/null; then
            echo "$IP Ping is successful."
            break
        else
            let FAIL_COUNT++
        fi
    done
    if [ $FAIL_COUNT -eq 3 ]; then
        echo "$IP Ping is failure!"
    fi
done

Method 3 – use a function to break out of the loop when ping succeeds:

#!/bin/bash
ping_success_status() {
    if ping -c 1 $IP >/dev/null; then
        echo "$IP Ping is successful."
        continue
    fi
}
IP_LIST="192.168.18.1 192.168.1.1 192.168.18.2"
for IP in $IP_LIST; do
    ping_success_status
    ping_success_status
    ping_success_status
    echo "$IP Ping is failure!"
 done

7 Monitor CPU, Memory, and Disk Utilization

CPU – use vmstat to get usage and send an email if utilization exceeds 50%:

#!/bin/bash
DATE=$(date +%F" "%H:%M)
IP=$(ifconfig eth0 | awk -F '[ :]+' '/inet addr/{print $4}')  # CentOS6 only
MAIL="[email protected]"
if ! which vmstat &>/dev/null; then
    echo "vmstat command not found, please install procps package."
    exit 1
fi
US=$(vmstat | awk 'NR==3{print $13}')
SY=$(vmstat | awk 'NR==3{print $14}')
USE=$(($US+$SY))
if [ $USE -ge 50 ]; then
    echo -e "
Date: $DATE
Host: $IP
Problem: CPU utilization $USE
" | mail -s "CPU Monitor" $MAIL
fi

Memory – alert when free memory is less than 1 GB:

#!/bin/bash
DATE=$(date +%F" "%H:%M)
IP=$(ifconfig eth0 | awk -F '[ :]+' '/inet addr/{print $4}')
MAIL="[email protected]"
TOTAL=$(free -m | awk '/Mem/{print $2}')
USE=$(free -m | awk '/Mem/{print $3-$6-$7}')
FREE=$(($TOTAL-$USE))
if [ $FREE -lt 1024 ]; then
    echo -e "
Date: $DATE
Host: $IP
Problem: Total=$TOTAL,Use=$USE,Free=$FREE
" | mail -s "Memory Monitor" $MAIL
fi

Disk – send a warning if any partition usage exceeds 80%:

#!/bin/bash
DATE=$(date +%F" "%H:%M)
IP=$(ifconfig eth0 | awk -F '[ :]+' '/inet addr/{print $4}')
MAIL="[email protected]"
TOTAL=$(fdisk -l | awk -F '[: ]+' 'BEGIN{OFS="="}/^Disk /dev/{printf "%s=%sG,",$2,$3}')
PART_USE=$(df -h | awk 'BEGIN{OFS="="}/^\/dev/{print $1,int($5),$6}')
for i in $PART_USE; do
    PART=$(echo $i | cut -d"=" -f1)
    USE=$(echo $i | cut -d"=" -f2)
    MOUNT=$(echo $i | cut -d"=" -f3)
    if [ $USE -gt 80 ]; then
        echo -e "
Date: $DATE
Host: $IP
Total: $TOTAL
Problem: $PART=$USE($MOUNT)
" | mail -s "Disk Monitor" $MAIL
    fi
done

8 Batch Host Disk Utilization Monitoring

Prerequisite: password‑less SSH between monitoring and target hosts. Store host connection info (IP User Port) in a file and iterate:

#!/bin/bash
HOST_INFO=host.info
for IP in $(awk '/^[^#]/{print $1}' $HOST_INFO); do
    USER=$(awk -v ip=$IP '$1==ip{print $2}' $HOST_INFO)
    PORT=$(awk -v ip=$IP '$1==ip{print $3}' $HOST_INFO)
    TMP_FILE=/tmp/disk.tmp
    ssh -p $PORT $USER@$IP df -h > $TMP_FILE
    USE_RATE_LIST=$(awk 'BEGIN{OFS="="}/^\/dev/{print $1,int($5)}' $TMP_FILE)
    for USE_RATE in $USE_RATE_LIST; do
        PART_NAME=${USE_RATE%=*}
        USE_RATE=${USE_RATE#*=}
        if [ $USE_RATE -ge 80 ]; then
            echo "Warning: $PART_NAME Partition usage $USE_RATE%!"
        fi
    done
done

9 Check Website Availability

Method 1 – single request using curl:

check_url() {
    HTTP_CODE=$(curl -o /dev/null --connect-timeout 3 -s -w "%{http_code}" $1)
    if [ $HTTP_CODE -ne 200 ]; then
        echo "Warning: $1 Access failure!"
    fi
}

Method 2 – using wget in spider mode:

check_url() {
    if ! wget -T 10 --tries=1 --spider $1 >/dev/null 2>&1; then
        echo "Warning: $1 Access failure!"
    fi
}

Usage example: check_url www.baidu.com Method 1 for three attempts – loop and break on success:

#!/bin/bash
check_url() {
    HTTP_CODE=$(curl -o /dev/null --connect-timeout 3 -s -w "%{http_code}" $1)
    if [ $HTTP_CODE -eq 200 ]; then
        continue
    fi
}
URL_LIST="www.baidu.com www.agasgf.com"
for URL in $URL_LIST; do
    check_url $URL
    check_url $URL
    check_url $URL
    echo "Warning: $URL Access failure!"
 done

Method 2 – count failures in a variable:

#!/bin/bash
URL_LIST="www.baidu.com www.agasgf.com"
for URL in $URL_LIST; do
    FAIL_COUNT=0
    for ((i=1;i<=3;i++)); do
        HTTP_CODE=$(curl -o /dev/null --connect-timeout 3 -s -w "%{http_code}" $URL)
        if [ $HTTP_CODE -ne 200 ]; then
            let FAIL_COUNT++
        else
            break
        fi
    done
    if [ $FAIL_COUNT -eq 3 ]; then
        echo "Warning: $URL Access failure!"
    fi
done

Method 3 – store failures in an array:

#!/bin/bash
URL_LIST="www.baidu.com www.agasgf.com"
for URL in $URL_LIST; do
    NUM=1
    while [ $NUM -le 3 ]; do
        HTTP_CODE=$(curl -o /dev/null --connect-timeout 3 -s -w "%{http_code}" $URL)
        if [ $HTTP_CODE -ne 200 ]; then
            FAIL_COUNT[$NUM]=$URL
            let NUM++
        else
            break
        fi
    done
    if [ ${#FAIL_COUNT[*]} -eq 3 ]; then
        echo "Warning: $URL Access failure!"
        unset FAIL_COUNT[*]
    fi
done

10 Check MySQL Master‑Slave Synchronization Status

#!/bin/bash
USER=bak
PASSWD=123456
IO_SQL_STATUS=$(mysql -u$USER -p$PASSWD -e "show slave status" | awk -F: '/Slave_.*_Running/{gsub(": ",":");print $0}')
for i in $IO_SQL_STATUS; do
    THREAD_STATUS_NAME=${i%:*}
    THREAD_STATUS=${i#*:}
    if [ "$THREAD_STATUS" != "Yes" ]; then
        echo "Error: MySQL Master‑Slave $THREAD_STATUS_NAME status is $THREAD_STATUS!"
    fi
done

These Bash script examples are practical for interviews and daily operations; hands‑on practice is essential for mastering them.

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Bash system-monitoring Shell scripting

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Efficient Ops

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