Essential Linux Commands and Scripts for System Administration

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

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?

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

Use ulimit -n to see the maximum number of open file descriptors (default 1024). To increase it, edit /etc/security/limits.conf and add:

* soft nofile 10240

* hard nofile 10240

then reboot.

3. How to sniff traffic on port 80 and list the top IPs?

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 under /var/log ? ls /var/log/ -1R | grep "-" | wc -l 5. How to list connection counts per IP on a Linux system?

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

6. Generate a 32‑character random password in the shell.

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

7. Find the top 5 IPs with most accesses in Apache access.log .

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 with appropriate flags, e.g., hexdump -C file for canonical hex+ASCII, -c for characters, -b for octal bytes, -o for octal words, -d for decimal, -x for hex.

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

10. How to check and repair /dev/hda5 ? Use fsck to examine and fix filesystem inconsistencies after power loss or disk errors.

11. Linux boot sequence overview. BIOS → MBR → Boot loader → Kernel → init (runlevel 3 or 5) → rc.sysinit → kernel modules → run‑level scripts → /etc/rc.d/rc.local → /bin/login.

12. Difference between symbolic (soft) and hard links. A symbolic link works like a Windows shortcut; a hard link is an additional directory entry pointing to the same inode (cannot cross filesystems). Deleting the original file does not affect a hard link, but breaks a symbolic link.

13. Save the current partition table. dd if=/dev/sda of=./mbr.txt bs=1 count=512 14. Basic vi/vim line operations. Copy line: yy then P; delete line: dd; delete all: dG; go to line 90: :90; search string: /path.

15. Manually install GRUB. 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 to 1 new connection per second, peak 3. Use iptables:

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 and server connects back; Passive (PASV) – server opens data port and client connects to it. Control channel is the same; data channel differs.

20. Show lines in /etc/inittab that start with # followed by whitespace and then a non‑whitespace character. grep "^#\{1,\}[^ ]" /etc/inittab 21. Show lines in /etc/inittab containing a single digit surrounded by colons. grep "\:[0-9]\{1\}:" /etc/inittab 22. Add a script to system services (use service to control). Add at the top of the script:

#!/bin/bash

# chkconfig: 345 85 15

# description: httpd

Then run chkconfig httpd --add and control with service httpd start|restart.

23. Script to add 20 users (user01‑20) with random 5‑character 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 up/down.

#!/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 another script for syntax errors and prompt the user.

[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, creating the directory if needed and returning specific error codes.

#!/bin/bash
url=$1
dir=$2
download() {
  cd $dir >>/dev/null 2>&1
  if [ $? -ne 0 ]; then
    read -p "$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
  fi
  if [ $? -ne 0 ]; then
    return 52
  fi
}
download $url $dir
echo $?

27. Script to wipe a disk, create two partitions (100M and 1G), format them as ext3, and return error codes. (Core commands shown; full script omitted for brevity.) Use dd if=/dev/zero of=$Sd bs=512 count=1, fdisk with echo‑pipe, partprobe, then mke2fs -j on each partition. Return 67, 68, 69 on failure, 0 on success.

28. Show timestamps in Bash history. Add to profile:

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

export HISTTIMEFORMAT