Essential Linux Command-Line Tricks Every Sysadmin Should Know

1. Mount a Windows share in Linux Use

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

(replace IP and credentials with the Windows host values).

2. View HTTP concurrent requests and TCP connection states

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

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

* soft nofile 10240

* hard nofile 10240

Reboot for changes to take effect.

3. Use tcpdump to capture the top IPs accessing port 80

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. Count connections 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 a shell

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

7. Find the top 5 IPs in Apache access.log

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

8. View the contents of a binary file Use hexdump with appropriate flags, e.g. hexdump -C file for canonical hex+ASCII output. Other useful options: -c (char), -b (octal), -o (two‑byte octal), -d (two‑byte decimal), -x (two‑byte hex).

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

10. Check and repair a filesystem (e.g., /dev/hda5) Use fsck /dev/hda5 to detect and fix inconsistencies.

11. Linux boot sequence BIOS → MBR → Boot loader → Kernel → init (runlevel from inittab, typically 3 for multi‑user CLI or 5 for GUI) → rc.sysinit scripts → /etc/rc.d/rc.local → login prompt.

12. Difference between symbolic (soft) and hard links A symbolic link ( ln -s source link) works like a Windows shortcut and can span filesystems; deleting the target breaks the link. A hard link ( ln source hardlink) creates another directory entry for the same inode, cannot cross filesystems, and both names remain valid if one is removed.

13. Save the current partition table dd if=/dev/sda of=./mbr.txt bs=1 count=512 14. Common vi/vim editing shortcuts Copy line: yy, paste: 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 new connections per second (example using 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 tells server which port to connect back to for data. Passive (PASV): server tells client which port to connect to; data connection is initiated by the client.

20. Show lines in /etc/inittab that start with # followed by whitespace and then non‑whitespace grep "^#\{1,\}[[:space:]]\+[^[:space:]]" /etc/inittab 21. Show lines in /etc/inittab containing a single digit between two colons grep "\:[0-9]\{1\}:" /etc/inittab 22. Add a script to system services (use service ) Include 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‑user20) with random 5‑character passwords

#!/bin/bash
for i in `seq -f "%02g" 1 20`; do
  useradd user$i
  echo "user$i-`echo $RANDOM|md5sum|cut -c1-5`" | passwd --stdin user$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

25. Script to check another script for syntax errors and prompt user

#!/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. 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)" ans
    if [ "$ans" = "y" ]; then
      mkdir -p $dir && cd $dir
    else
      return 51
    fi
  }
  wget $url >/dev/null 2>&1 || return 52
}
download $url $dir
echo $?

27. Script to wipe a disk, create partitions, and format them (error‑coded)

#!/bin/bash
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 &>/dev/null || return 67 ;;
    n) exit 88 ;;
    *) echo "Invalid choice"; return 66 ;;
  esac
  echo -e "n
p
1

+100M
n
p
2

+1024M
w
" | fdisk $dev &>/dev/null || return 68
  partprobe
  for part in $(fdisk -l $dev | tail -2 | awk '{print $1}'); do
    mkfs.ext3 $part &>/dev/null || { ErrorPart=$part; return 69; }
  done
  return 0
}
Disk_Mod /dev/sdb
res=$?
case $res in
  0) echo "Success" ;;
  66) echo "Invalid input" ;;
  67) echo "dd/fdisk/mkfs error" ;;
  68) echo "Partition creation error" ;;
  69) echo "Formatting error on $ErrorPart" ;;
  *) echo "Unknown error" ;;
esac

28. Show timestamps in command history Add to /etc/profile or current shell:

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