Master Essential Linux Commands: Port Checks, Disk Usage, File Management & More

Practical Commands

1. Check Port Usage

# Check if port 5000 is in use
netstat -lnp | grep 5000
# Alternative method
lsof -i:'5000'
# Kill the process occupying the port
kill -9 28533
# Install missing tools if needed
yum -y install net-tools
# Install firewalld if firewall is required
yum install firewalld

2. Disk and CPU Information

# Show disk usage
df -h
# Show directory usage
du -lh --max-depth=1
du -sh *
# Show CPU information
lscpu

Key parameters extracted from lscpu output include CPU(s): 4, On-line CPU(s) list: 0‑3, Thread(s) per core: 2, Core(s) per socket: 2, Socket(s): 1, and the model name indicating Intel, AMD, etc.

3. Delete Files by Time

find /home/lifeccp/dicom/studies -mtime +21 -name "*.*" -exec rm -Rf {} \;

Brief explanation of the shell command

/home/lifeccp/dicom/studies – target directory

-mtime – modification time selector

+21 – files older than 21 days

"*.*" – file pattern

-exec rm -Rf {} \; – force delete

Note: In practice the command may delete files 1‑2 days older than expected; verify after execution.

4. scp Remote Copy Details

Syntax: scp [options] source destination Key options include:

-1 force SSH1 protocol

-2 force SSH2 protocol

-4 use IPv4 only

-6 use IPv6 only

-B batch mode (no password prompt)

-C enable compression

-p preserve timestamps and permissions

-q suppress progress bar

-r recursive copy

-v verbose output

-c cipher for encryption

-F alternative ssh config file

-i identity file

-l limit bandwidth (Kbit/s)

-o ssh option

-P port (uppercase P)

-S program for data transfer

Examples:

scp -r /root/lk [email protected]:/home/lk/cpfile
scp -r [email protected]:/home/lk /root

5. Find Files by Date

# List files in /recordings/ for a specific date
ls --full-time /recordings/ | sed -n '/2018-03-21/p'
# Find files between two dates
find images/ -newermt '2021-01-01' ! -newermt '2021-01-31'
# Copy files within a date range to another server
scp -p <file_path> <user>@<IP>:/target_path

6. File and Directory Statistics

6.1 Count Files

# Count files in current directory
ls -l | grep "^-" | wc -l
# Count files recursively
find ./ -type f | wc -l
# Count directories recursively
ls -lR | grep "^d" | wc -l

6.2 Disk, Inode and Memory Statistics

Use du, df and free for detailed size and usage information. Example commands:

# du detailed options
du -h *
du -sh *
# df usage
df -h
# free memory
free -m

7. System Information

# Kernel version
uname -r
# Distribution info
lsb_release -a
cat /etc/*-release
# Full system info
uname -a
# Real‑time bandwidth
nload

7.1 Process Details with top

top
top -d 1 -p <pid>

Key fields displayed by top include PID, USER, PR, NI, VIRT, RES, SHR, S, %CPU, %MEM, TIME+, and COMMAND.

7.2 Timezone Settings

timedatectl
timedatectl set-time "YYYY-MM-DD HH:MM:SS"
timedatectl list-timezones
timedatectl set-timezone Asia/Shanghai
timedatectl set-ntp yes
timedatectl set-local-rtc 1
hwclock --systohc --localtime

8. Open and View Ports

# List open ports
firewall-cmd --list-ports
# Open a single port
firewall-cmd --zone=public --add-port=8080/tcp --permanent
# Open a range of ports
firewall-cmd --zone=public --add-port=20000-29999/tcp --permanent
# Close a port
firewall-cmd --zone=public --remove-port=8080/tcp --permanent
# Reload firewall
firewall-cmd --reload

9. Change Ownership, Group and Permissions

# Change group
chgrp [-R] group file
# Change owner
chown [-R] owner file
chown [-R] owner:group file
# Add execute permission
chmod +x file.sh
# Symbolic permission changes
chmod g+w,o+x filename
chmod g=rwx,o=rx filename

10. Extended Commands

10.1 Periodic Execution (watch)

watch -n 1 'ls'

10.2 Command History

history
history -c

10.3 Routing Information

route
traceroute -T -p [port] [target_ip]

10.4 crontab (Scheduled Tasks)

# Edit crontab
crontab -e
# Example: run daily at 02:00
00 2 * * * command
# Start/stop cron service
service crond start
service crond restart
service crond stop
# List crontab
crontab -l
service crond status

10.5 Mounting Filesystems

# Mount ISO read‑only
mount -t iso9660 -o ro /dev/cdrom /mnt/cdrom/
# Unmount
umount /mnt/cdrom

10.6 Quick Replacement in Large Files

# Replace @ with |
:%s/@/|/g

10.7 User Password Expiration

# Show password info
chage -l username
# Set max age
chage -M 99999 username
# Set explicit expiration date
usermod -e 'YYYY-MM-DD' username