Essential Linux Commands and Scripts for System Administration

1. Mount a Windows share on Linux Use

mount.cifs //IP_ADDRESS/server /mnt/server -o user=administrator,password=123456

after creating the mount point.

2. View HTTP concurrent requests and TCP connection states Run

netstat -n | awk '/^tcp/ {++b[$NF]} END {for(a in b) print a,b[a]}'

. Use ulimit -n to check the max open file descriptors and adjust /etc/security/limits.conf if needed.

3. Capture top IPs accessing port 80 with tcpdump

tcpdump -i eth0 -tnn dst port 80 -c 1000 | awk -F"." '{print $1"."$2"."$3"."$4"."}' | sort | uniq -c | sort -nr | head -5

.

4. Count files in /var/log ls /var/log/ -1R | grep "-" | wc -l.

5. Show per‑IP connection counts

netstat -n | awk '/^tcp/ {print $5}' | awk -F: '{print $1}' | sort | uniq -c | sort -rn

.

6. Generate a 32‑character random password

cat /dev/urandom | head -1 | md5sum | head -c 32 >> /pass

.

7. Find top 5 IPs in Apache access.log

cat access.log | awk '{print $1}' | sort | uniq -c | sort -n -r | head -5

.

8. View binary file contents Use hexdump -C filename with options -c, -b, -o, -d, -x for different formats.

9. Meaning of VSZ and RSS in ps aux VSZ = virtual memory size, RSS = resident set size (physical memory used).

10. Check and repair a filesystem Run fsck /dev/hda5 to detect and fix inconsistencies.

11. Linux boot sequence BIOS → MBR → Bootloader → Kernel → init (runlevel) → rc scripts → local services → login.

12. Symbolic vs. hard links Symbolic links act like Windows shortcuts ( ln -s source link); hard links duplicate the inode ( ln source hardlink) and cannot cross filesystems.

13. Save current partition table dd if=/dev/sda of=./mbr.txt bs=1 count=512.

14. Basic vi/vim line operations Copy line: yy, paste: P; delete line: dd; delete all: dG; go to line 90: :90; search text: /pattern.

15. Install GRUB manually grub-install /dev/sda.

16. Modify kernel parameters Edit /etc/sysctl.conf then apply with sysctl -p.

17. Generate a random number between 1 and 39 expr $[RANDOM%39] + 1.

18. Limit Apache new connections per second Use firewall rule, e.g.,

iptables -A INPUT -d 172.16.100.1 -p tcp --dport 80 -m limit --limit 1/second -j ACCEPT

.

19. FTP active vs. passive mode Active (PORT) – client opens data port; Passive (PASV) – server opens data port.

20. Show commented lines in /etc/inittab grep "^#\{1,\}[^ ]" /etc/inittab.

21. Show lines containing a single digit surrounded by colons in /etc/inittab grep "\:[0-9]\{1\}:" /etc/inittab.

22. Add a script as a system service Include shebang, chkconfig header, then run chkconfig httpd --add and control with service httpd start|restart.

#!/bin/bash
#description: useradd
for i in `seq -f"%02g" 1 20`; do
  useradd user$i
  echo "user$i-`echo $RANDOM|md5sum|cut -c 1-5`"|passwd --stdin user$i >/dev/null 2>&1
done

23. Scan a /24 network for online hosts

#!/bin/bash
for ip in `seq 1 255`; do
  ping -c 1 192.168.1.$ip >/dev/null 2>&1
  if [ $? -eq 0 ]; then
    echo 192.168.1.$ip UP
  else
    echo 192.168.1.$ip DOWN
  fi
done
wait

24. Check a script for syntax errors and edit if needed

#!/bin/bash
read -p "please input check script-> " file
if [ -f $file ]; then
  sh -n $file >/dev/null 2>&1
  if [ $? -ne 0 ]; then
    read -p "You input $file syntax error,[Type q to exit or Type vim to edit]" answer
    case $answer in
      q|Q) exit 0 ;;
      vim) vim $file ;;
      *) exit 0 ;;
    esac
  fi
else
  echo "$file not exist"
  exit 1
fi

25. Download a file to a directory with error handling

#!/bin/bash
url=$1
dir=$2
download() {
  cd $dir >/dev/null 2>&1 || {
    read -p "$dir No such file or directory,create?(y/n)" answer
    if [ "$answer" == "y" ]; then
      mkdir -p $dir && cd $dir
    else
      return 51
    fi
  }
  wget $url >/dev/null 2>&1 || return 52
}
download $url $dir
echo $?

26. Script to wipe a disk, create partitions, and format them

# Function to clean and partition /dev/sdb
Disk_Mod() {
  Sd=$1
  while :; do
    read -p "Warning!!! This operation will clean $Sd data. Next=y, Quit=n [y|n]:" Choice
    case $Choice in
      y) dd if=/dev/zero of=$Sd bs=512 count=1 &> /dev/null && break || return 67 ;;
      n) exit 88 ;;
      *) echo "Invalid choice, please choose again." ;;
    esac
  done
  echo -e "n
p
1

+100M
n
p
2

+1024M
w
" | fdisk $Sd &> /dev/null || return 68
  partprobe
  Part=$(fdisk -l $Sd | tail -2 | awk '{print $1}')
  for M in $Part; do
    mke2fs -j $M &> /dev/null || { ErrorPart=$M; return 69; }
  done
  return 0
}
Disk_Mod /dev/sdb
Res=$?
[ $Res -eq 0 ] && exit 0
[ $Res -eq 66 ] && echo "Error! Invalid input."
[ $Res -eq 67 ] && echo "Error! Command -> dd fdisk mke2fs"

27. Show command timestamps in history Add to profile: HISTTIMEFORMAT="%Y-%m-%d %H:%M:%S"; export HISTTIMEFORMAT.