In Linux, there are several ways to execute commands that continue running even after you close your terminal session:

1. Background execution with &: The ampersand symbol allows a command to run in the background. However, if you close your terminal (like Xshell), the process will terminate.
2. nohup command: This allows a command to keep running after you log out. It closes standard input, redirects output and errors to a file called nohup.out in the current directory, and makes the process immune to terminal disconnection.
3. Combining nohup with &: This runs a command in the background with nohup, allowing both background execution and immunity to terminal disconnection.

Examples:

# Run a Python script in the background
python application.py &

# Execute a Java application that persists after logout
nohup java -server myapp.jar

# Run a Node.js service in background with persistence
nohup node server.js &

Monitoring Output from Persistent Processes

When using nohup, all output is redirected to the nohup.out file. You can monitor this file using the tail command:

# Continuously display the last 50 lines of nohup.out
tail -fn 50 nohup.out

# Display the last 100 lines without following
tail -n 100 nohup.out

Process Management with ps

The ps command displays information about running processes. Here are common options:

Finding Process IDs:

# Find processes related to a specific application
ps -ef | grep nginx

# Display all processes with detailed information
ps aux

Disk Space Management

Managing disk space is essential for system administration:

# Check directory sizes with depth limit
du -h --max-depth=1 /var/www/

# Display disk usage summary
df -h

Background Job Control

The jobs command displays tasks running in the background of the current shell:

Bringing Background Jobs to Foreground

You can move a background job to the foreground using the fg command:

Resuming Stopped Background Jobs

The bg command resumes a stopped background job:

# Resume a specific background job
bg %3

Terminating Processes

You can terminate processes using the kill command with various signals:

System Shutdown and Restart

There are multiple ways to shut down or restart a Linux system:

Using shutdown (recommended):

# Immediate shutdown
shutdown -h now

# Shutdown in 10 minutes
shutdown -h +10

# System restart
shutdown -r now

# Cancel a scheduled shutdown
shutdown -c

Alternative commands:

# Immediate shutdown
halt
poweroff

# System restart
reboot

# Change runlevel (0=halt, 6=reboot)
init 0
init 6

Static IP Configuration (Ubuntu)

To configure a static IP address on Ubuntu:

# Edit network interfaces configuration
sudo vim /etc/network/interfaces

# Add the following configuration:
auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    dns-nameservers 8.8.8.8 8.8.4.4

# Restart networking service
sudo /etc/init.d/networking restart

Firewall Management (Firewalld)

Firewalld is the default firewall management tool on modern Linux distributions:

# Check firewall status
systemctl status firewalld
firewall-cmd --state

# Start, restart, or stop the firewall
sudo systemctl start firewalld
sudo systemctl restart firewalld
sudo systemctl stop firewalld

# List firewall rules
sudo firewall-cmd --list-all

# Manage ports
sudo firewall-cmd --query-port=8080/tcp
sudo firewall-cmd --permanent --add-port=80/tcp
sudo firewall-cmd --permanent --remove-port=8080/tcp
sudo firewall-cmd --reload

# Enable/disable firewall at boot
sudo systemctl enable firewalld
sudo systemctl disable firewalld

Log Analysis with tail and grep

These commands are essential for system monitoring and troubleshooting:

# Follow log file in real-time
tail -f /var/log/syslog

# Display last 100 lines and follow new entries
tail -100f /var/log/application.log

# Search for specific patterns in recent log entries
tail -100f /var/log/app.log | grep "ERROR"

# Find log entries within a time range
tail -1000f /var/log/app.log | grep '2023-05-15 14:[0-5][0-9]'

# Show matching lines plus 5 lines after each match
tail -1000f /var/log/app.log | grep -A 5 "Exception"

SSH Remote Connections

Secure Shell (SSH) allows secure remote server access:

# Basic SSH connection
ssh user@192.168.1.100

# Connect with specific port
ssh -p 2222 user@example.com

# Execute command on remote server
ssh user@server "df -h"

Secure File Transfer with SCP

SCP (Secure Copy) allows secure file transfers between systems:

# Copy directory to remote server
scp -r /local/directory user@remote:/path/to/destination

# Copy file from remote to local
scp user@remote:/remote/file.txt /local/destination/

# Copy with custom port
scp -P 2222 user@remote:/file.txt /local/

# Preserve file attributes
scp -p /local/file user@remote:/destination/

Checking Login History

System administrators can monitor user login activity:

Successful login attempts:

# View recent login history
last -n 10

# View with IP addresses resolved to hostnames
last -a -n 10

Failed login atempts:

# View failed login attempts
lastb -n 10

# Clear failed login logs (requires root)
sudo > /var/log/btmp

Clearing login history:

# Clear successful login logs
sudo > /var/log/wtmp

# Clear failed login logs
sudo > /var/log/btmp