Essential Linux Commands and Scripts for System Administration

1. How to mount a Windows shared directory in Linux?

Use

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

. Create the mount point manually; the user and password correspond to the Windows account.

2. How to view HTTP concurrent request count 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 maximum open file descriptors (default 1024) and adjust via /etc/security/limits.conf if needed.

3. How to sniff traffic on port 80 with tcpdump and list top sources?

Execute

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

.

4. How to count files in /var/log?

Use ls /var/log/ -1R | grep "-" | wc -l.

5. How to list connection counts per IP on a Linux system?

Run

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

.

6. How to generate a 32‑character random password in a shell?

Use

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

.

7. How to find the top 5 IPs in Apache access.log?

Run

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

.

8. How to view the contents of a binary file?

Use hexdump -C filename (or other options: -c for characters, -b for octal, -o for double‑byte octal, -d for decimal, -x for hexadecimal).

9. What do VSZ and RSS represent in ps aux ?

VSZ is the virtual memory size; RSS is the resident set size (physical memory used).

10. How to check and repair a Linux filesystem (e.g., /dev/hda5)?

Use fsck to detect and fix inconsistencies after power loss or disk issues.

11. What is the Linux boot sequence?

BIOS → MBR → boot loader → kernel → init (runlevel 3 or 5) → rc scripts → local services → login.

12. Difference between symbolic (soft) and hard links?

A symbolic link works like a Windows shortcut; a hard link creates another directory entry pointing to the same inode, cannot cross filesystems, and both names reflect changes.

13. How to save the current partition table?

Use dd if=/dev/sda of=./mbr.txt bs=1 count=512.

14. Common vi/vim text‑editing shortcuts in command mode.

Copy line: yy, paste: P Delete line: dd Delete all: dG (note uppercase G)

Go to line 90: :90 Search for a string: /path 15. How to install GRUB manually?

Run grub-install /dev/sda.

16. How to modify kernel parameters?

Edit /etc/sysctl.conf and apply with sysctl -p.

17. How to generate a random number between 1 and 39?

Use expr $[RANDOM%39] + 1.

18. How to limit Apache new connections per second?

Typically done with firewall rules, e.g.,

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

.

19. What are FTP active and passive modes?

Active (PORT) mode: client tells server which port to connect back to for data transfer. Passive (PASV) mode: server tells client which port to connect to for data transfer.

20. Show lines in /etc/inittab starting with # followed by whitespace and non‑whitespace characters.

Use grep "^#\{1,\}[^ ]" /etc/inittab.

21. Show lines in /etc/inittab containing a single digit between two colons.

Use grep "\:[0-9]\{1\}:" /etc/inittab.

22. How to add a script as a system service?

Include a shebang, chkconfig header, and description, then run chkconfig httpd --add to register the service.

23. Script to add 20 users (user01‑20) with random passwords.

#!/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

24. Script to ping all hosts in 192.168.1.0/24 and report status.

#!/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

25. Script to check syntax of another script and offer to edit.

[root@localhost tmp]# cat checksh.sh
#!/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

26. Script function to 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
      wget $url >/dev/null 2>&1
    else
      return 51
    fi
  }
  [ $? -ne 0 ] && return 52
}
download $url $dir
echo $?

27. Script to wipe a disk, create partitions, and format them with ext3.

#!/bin/bash
# Function to clean and partition /dev/sdb
Disk_Mod() {
  local dev=$1
  read -p "Warning!!! This will erase $dev. Continue? [y/n]: " choice
  case $choice in
    y) dd if=/dev/zero of=$dev bs=512 count=1 || return 67 ;;
    n) exit 88 ;;
    *) echo "Invalid choice"; return 27 ;;
  esac
  echo -e "n
p
1

+100M
n
p
2

+1024M
w" | fdisk $dev || return 68
  partprobe
  for part in $(fdisk -l $dev | tail -2 | awk '{print $1}'); do
    mke2fs -j $part >/dev/null || { ErrorPart=$part; 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! dd/fdisk/mke2fs failed."

28. How to make the history command show timestamps?

Add the following to /etc/profile or your shell init file:

HISTTIMEFORMAT="%Y-%m-%d %H:%M:%S"
export HISTTIMEFORMAT