How to Use the Screen Utility in Linux

Many programming languages have conditional statements, such as if-else. These statements are also present in Bash, the default shell used in almost all modern Linux distributions. The if-else statements are used to check conditions — they allow the execution of specific commands depending on whether the condition is true or false. The if-else statements work exactly the same way as in any programming language. In this article, we will discuss how to use if-else statements in the Bash shell through practical examples. The if Statement in Bash The if statement in Bash allows you to execute specific commands depending on the truth value of the given condition. Two logical statements are used to check for truth: True and False. The if statement is used when you need to check a condition. It controls the flow of script execution, allowing decisions to be made based on variable values, command results, and other conditions. The if statement works as follows: First, the program checks the condition (the condition can be a command or a mathematical expression) contained in the if statement. If the condition is true, the program executes the commands listed after the then keyword. If the condition is false, the program executes the commands listed after the else statement. The syntax of the if statement in Bash is as follows: if [condition]; then # commands to execute if the condition is true fi Let's break down the operation of the if statement with a simple practical example. We will create a script that asks the user for a number, and if the number entered is greater than 10, the system will return the message "The number is greater than 10." Create a new file with a .sh extension, for example, using the nano editor: nano greater_than_10.sh Insert the following code: #!/bin/bash read -p "Enter a number: " number if [ $number -gt 10 ]; then echo "The number is greater than 10." fi Provide the file with execute permissions: chmod +x greater_than_10.sh Now, run the script: ./greater_than_10.sh Output: Enter a number: Enter any number, for example, 32, and press Enter. Since 32 is greater than 10, and this condition returns True, the program will execute the echo command. Enter a number: 32The number is greater than 10 Let’s break down the script in more detail: The conditions are written in square brackets. In this example, the -gt operator is used (greater than, equivalent to the > symbol). Next, we check the condition. If it’s True, the program executes the command after the then keyword. The script ends with the fi keyword, signaling the end of the if block. However, this script has one major drawback: it does not handle the case when the entered number is less than 10. The script will not return anything because there is no condition for that case. To address this issue, we will use the else statement, which we will discuss in the next chapter. The if-else Statement in Bash In the previous section, we ran a script with only one condition in the if statement — True. We didn’t specify any action for the False condition. As a result, if we entered a value leading to False, there was no response. If we want the script to perform specific actions for the false condition False, we need to use the else statement, which follows the if statement. The if-else statement in Bash is used to perform conditional operations. It allows the execution of specific commands depending on whether the condition is true or false. The syntax for if-else is as follows: if [condition]; then # commands executed if the condition is true else # commands executed if the condition is false fi Remember that keywords, including if and else, in Bash shell scripts are case-sensitive. Be careful when using keywords in script files. Let's consider using the if-else statements in a practical example. In this case, we will create a Bash script that asks the user for a number, and the system will display whether the number is greater than or less than 10. Create a new file with a .sh extension: nano check.sh Insert the following code: #!/bin/bash read -p "Enter a number: " number if [ $number -gt 10 ]; then echo "The number is greater than 10." else echo "The number is less than or equal to 10." fi Grant the file execute permissions: chmod +x check.sh Now, run the script: ./check.sh The algorithm for the script works as follows: After the if keyword, we specify the condition in square brackets. In this example, we use the -gt operator (greater than, equivalent to the > symbol). The condition is checked. If the condition is true, the program executes the command after the then keyword— in this case, it prints the message "The number is greater than 10". If the condition is false, the program executes the command after the else keyword, printing the message "The number is less than or equal to 10". Once one of the conditions is met, the program will end, as indicated by the fi keyword at the end. Output if the number is greater than 10: Enter a number: 56The number is greater than 10. Output if the number is less than 10: Enter a number: 6The number is less than or equal to 10. Practical Use of if-else in Bash Let's look at the practical application of the if-else statement in Bash, which can be used when writing scripts. Script Example 1. Checking if Run as root First, we will create a script that checks whether the script file is run as the root user. This can be useful when writing scripts that require root privileges, such as installing packages as the root user. Create a file named check-for-root.sh: nano check-for-root.sh Use the following code to check for root user: #!/bin/bash if [[ $EUID -ne 0 ]]; then /usr/bin/printf "${R}>>>>${NC} Please run as root\n" exit 1 fi Grant the file execute permissions: chmod +x check-for-root.sh And run it: ./check-for-root.sh If the script is run as a regular user, the console will print the message "Please run as root". The check for the root user uses the condition $EUID -ne 0, where: $EUID is an environment variable that holds the numeric user ID. In Linux systems, the root user always has the ID 0, while other user accounts start at 1000. -ne is a comparison operator meaning "not equal". Instead of ne, you can also use !=. Script Example 2. Checking the Linux distribution Next, let's create another script that checks which Linux distribution is being used. If the script is run on Ubuntu, it will print the message "This is Ubuntu". If the script is run on any other Linux distribution, it will print "Not Ubuntu. You can run this script only on Ubuntu distributions". Create a file named check-for-distribution.sh: nano check-for-distribution.sh Use the following code: #!/bin/bash dist=`grep DISTRIB_ID /etc/*-release | awk -F '=' '{print $2}'` if [ "$dist" == "Ubuntu" ]; then echo "This is Ubuntu" else echo "Not Ubuntu. You can run this script only on Ubuntu distributions" fi Make the file executable: chmod +x check-for-distribution.sh And run it: ./check-for-distribution.sh If the script is run on an Ubuntu system, it will print "This is Ubuntu". On any other distribution, it will print "Not Ubuntu. You can run this script only on Ubuntu distributions". Script Example 3. Checking if File Exists Now, let’s look at another practical example. We will create a Bash script that checks if a file named file1.txt exists. If it doesn't exist, the script will create it. The script checks for the file in the same directory it is run. If the file already exists, the script will print a message without creating the file. Create a file named check-file.sh: nano check-file.sh Use the following script code: #!/bin/bash FILE="file1.txt" if [ ! -f "$FILE" ]; then touch "$FILE" echo "$FILE has been created." else echo "$FILE already exists." fi Grant execute permissions for the script: chmod +x check-file.sh Run the script: ./check-file.sh If the file1.txt file already exists in the directory from which the script is run, you will see the message "file1.txt already exists.". The file will not be created. Conclusion In this article, we reviewed the principles of logical statements such as if-else in the Bash shell and provided practical examples of using these statements. These examples are useful when writing scripts to automate system tasks or checks.

Effective system administration in Linux requires constant awareness of running processes. Whether diagnosing performance bottlenecks, identifying unauthorized tasks, or ensuring critical services remain operational, the ps aux command is an indispensable tool.  This guide provides a comprehensive exploration of ps aux, from foundational concepts to advanced filtering techniques, equipping you to extract actionable insights from process data. Prerequisites To follow the tutorial: Deploy a Linux cloud server instance at Hostman SSH into the server instance Understanding Processes in Linux Before we explore the ps aux command, let's take a moment to understand what processes are in the context of a Linux system. What are Processes? A process represents an active program or service running on your Linux system. Each time you execute a command, launch an application, or initiate a background service, you create a process. Linux assigns a unique identifier, called a Process ID (PID), to each process. This PID allows the system to track and manage individual processes effectively. Why are Processes Grouped in Linux? Linux employs a hierarchical structure to organize processes. This structure resembles a family tree, where the initial process, init (or systemd), acts as the parent or ancestor. All other processes descend from this initial process, forming a parent-child relationship. This hierarchy facilitates efficient process management and resource allocation. The ps Command The ps (process status) command provides a static snapshot of active processes at the moment of execution. Unlike dynamic tools such as top or htop, which update in real-time, ps is ideal for scripting, logging, or analyzing processes at a specific point in time. The ps aux syntax merges three key options: a: Displays processes from all users, not just the current user. u: Formats output with user-oriented details like CPU and memory usage. x: Includes processes without an attached terminal, such as daemons and background services. This combination offers unparalleled visibility into system activity, making it a go-to tool for troubleshooting and analysis. Decoding the ps aux Output Executing ps aux generates a table with 11 columns, each providing critical insights into process behavior. Below is a detailed explanation of these columns: USER This column identifies the process owner. Entries range from standard users to system accounts like root, mysql, or www-data. Monitoring this field helps detect unauthorized processes or identify which users consume excessive resources. PID The Process ID (PID) is a unique numerical identifier assigned to each task. Administrators use PIDs to manage processes—for example, terminating a misbehaving application with kill [PID] or adjusting its priority using renice. %CPU and %MEM These columns display the percentage of CPU and RAM resources consumed by the process. Values above 50% in either column often indicate performance bottlenecks. For instance, a database process consuming 80% CPU might signal inefficient queries or insufficient hardware capacity. VSZ and RSS VSZ (Virtual Memory Size) denotes the total virtual memory allocated to the process, including memory swapped to disk. On the other hand, RSS (Resident Set Size) represents the physical memory actively used by the process. A process with a high VSZ but low RSS might reserve memory without actively utilizing it, which is common in applications that preallocate resources. TTY This field shows the terminal associated with the process. A ? indicates no terminal linkage, which is typical for background services like cron or systemd-managed tasks. STAT The STAT column reveals process states through a primary character + optional attributes: Primary States: R: Running or ready to execute. S: Sleeping, waiting for an event or signal. I: Idle kernel thread D: Uninterruptible sleep (usually tied to I/O operations). Z: Zombie—a terminated process awaiting removal by its parent. Key Attributes: s: Session leader N: Low priority <: High priority For example, a STAT value of Ss denotes a sleeping session leader, while l< indicates an idle kernel thread with high priority. START and TIME START indicates the time or date the process began. Useful for identifying long-running tasks. TIME represents the cumulative CPU time consumed since launch. A process running for days with minimal TIME is likely idle. COMMAND This column displays the command or application that initiated the process. It helps identify the purpose of a task—for example, /usr/bin/python3 for a Python script or /usr/sbin/nginx for an Nginx web server. Advanced Process Filtering Techniques While ps aux provides a wealth of data, its output can be overwhelming on busy systems. Below are methods to refine and analyze results effectively. Isolating Specific Processes To focus on a particular service—such as SSH—pipe the output to grep: ps aux | grep sshd Example output: root 579 0.0 0.5 15436 5512 ? Ss 2024 9:35 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups root 2090997 0.0 0.8 17456 8788 ? Ss 11:26 0:00 sshd: root@pts/0 root 2092718 0.0 0.1 4024 1960 pts/0 S+ 12:19 0:00 grep --color=auto sshd This filters lines containing sshd, revealing all SSH-related processes. To exclude the grep command itself from results, use a regular expression: ps aux | grep "[s]shd"  Example output: root 579 0.0 0.5 15436 5512 ? Ss 2024 9:35 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups root 2090997 0.0 0.8 17456 8788 ? Ss 11:26 0:00 sshd: root@pts/0 Sorting by Resource Consumption Identify CPU-intensive processes by sorting the output in descending order: ps aux --sort=-%cpu | head -n 10 Example output: USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND mysql 1734280 0.4 36.4 1325172 357284 ? Ssl Jan30 87:39 /usr/sbin/mysqld redis 1424968 0.3 0.6 136648 6240 ? Ssl Jan18 112:25 /usr/bin/redis-server 127.0.0.1:6379 root 1 0.0 0.6 165832 6824 ? Ss 2024 5:51 /lib/systemd/systemd --system --deserialize 45 root 2 0.0 0.0 0 0 ? S 2024 0:00 [kthreadd] root 3 0.0 0.0 0 0 ? I< 2024 0:00 [rcu_gp] root 4 0.0 0.0 0 0 ? I< 2024 0:00 [rcu_par_gp] root 5 0.0 0.0 0 0 ? I< 2024 0:00 [slub_flushwq] root 6 0.0 0.0 0 0 ? I< 2024 0:00 [netns] root 8 0.0 0.0 0 0 ? I< 2024 0:00 [kworker/0:0H-events_highpri] Similarly, you can sort by memory usage to detect potential leaks: ps aux --sort=-%mem | head -n 10 Example output: USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND mysql 1734280 0.4 36.4 1325172 357284 ? Ssl Jan30 87:39 /usr/sbin/mysqld root 330 0.0 4.4 269016 43900 ? S<s 2024 22:43 /lib/systemd/systemd-journald root 368 0.0 2.7 289316 27100 ? SLsl 2024 8:19 /sbin/multipathd -d -s root 1548462 0.0 2.5 1914688 25488 ? Ssl Jan23 2:08 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock root 1317247 0.0 1.8 1801036 17760 ? Ssl Jan14 22:24 /usr/bin/containerd root 556 0.0 1.2 30104 11956 ? Ss 2024 0:00 /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers root 635 0.0 1.1 107224 11092 ? Ssl 2024 0:00 /usr/bin/python3 /usr/share/unattended-upgrades/unattended-upgrade-shutdown --wait-for-signal root 2090997 0.0 0.8 17456 8788 ? Ss 11:26 0:00 sshd: root@pts/0 root 2091033 0.0 0.8 9936 8480 pts/0 Ss 11:26 0:00 bash --rcfile /dev/fd/63 Real-Time Monitoring Combine ps aux with the watch command to refresh output every 2 seconds: watch -n 2 "ps aux --sort=-%cpu" This provides a dynamic view of CPU usage trends. Zombie Process Detection Zombie processes, though largely harmless, clutter the process list. Locate them with: ps aux | grep 'Z' Persistent zombies often indicate issues with parent processes failing to clean up child tasks. Practical Use Cases Now, let’s explore some common use cases of the ps aux command in Linux: Diagnosing High CPU Usage Follow the below steps: Execute this command to list processes by CPU consumption. ps aux --sort=-%cpu Identify the culprit—for example, a malfunctioning script using 95% CPU. If unresponsive, terminate the process gracefully with: kill [PID] Or forcibly with: kill -9 [PID] Detecting Memory Leaks Simply do the following: Sort processes by memory usage: ps aux --sort=-%mem Investigate tasks with abnormally high %MEM values. Restart the offending service or escalate to developers for code optimization. Auditing User Activity List all processes owned by a specific user (e.g., Jenkins): ps aux | grep ^jenkins This helps enforce resource quotas or investigate suspicious activity. Best Practices for Process Management Let’s now take a quick look at some best practices to keep in mind when managing Linux processes: Graceful Termination: Prefer kill [PID] over kill -9 to allow processes to clean up resources. Log Snapshots: Periodically save process lists for audits: ps aux > /var/log/process_audit_$(date +%F).log Contextual Analysis: A high %CPU value might be normal for a video encoder but alarming for a text editor. Hence, it’s essential to consider the context when making an analysis. Common Pitfalls to Avoid Here are some pitfalls to look out for when using ps aux in Linux: Misinterpreting VSZ: High virtual memory usage doesn’t always indicate a problem—it includes swapped-out data. Overlooking Zombies: While mostly benign, recurring zombies warrant investigating parent processes. Terminating Critical Services: Always verify the COMMAND field before using kill to avoid disrupting essential services. Conclusion The ps aux command is a cornerstone of Linux system administration, offering deep insights into process behavior and resource utilization. You can diagnose performance issues, optimize resource allocation, and maintain system stability by mastering its output interpretation, filtering techniques, and real-world applications.  For further exploration, consult the ps manual (man ps) or integrate process monitoring into automated scripts for proactive system management.

When working with networks in Linux, you may need to open or close a network port. Port management is essential for security — the fewer open ports in a system, the fewer potential attack vectors it has. Furthermore, if a port is closed, an attacker cannot gather information about the service running on that specific port. This guide will explain how to open or close ports as well as how to check open ports in Linux distributions such as Ubuntu/Debian and CentOS/RHEL using firewalls like ufw, firewalld, and iptables. It will also  We will demonstrate this process on two Linux distributions: Ubuntu 22.04 and CentOS 9, run on Hostman VPS. All commands provided here will work on any Debian-based or RHEL-based distributions. What is a Network Port? Ports are used to access specific applications and protocols. For example, a server can host both a web server and a database—ports direct traffic to the appropriate service. Technically, a network port is a non-negative integer ranging from 0 to 65535. Reserved Ports (0-1023): Used by popular protocols and network services like SSH (port 22), FTP (port 21), HTTP (port 80), and HTTPS (port 443). Registered Ports (1024-49151): These ports can be used by specific applications for communication. Dynamic Ports (49151-65535): These are used for temporary connections and can be dynamically assigned to applications. How to Open Ports in Debian-Based Linux Distributions On Debian-based systems (Ubuntu, Debian, Linux Mint, etc.), you can use ufw (Uncomplicated Firewall). ufw comes pre-installed on most popular APT-based distributions. To check if ufw is installed, run: ufw version If the version is displayed, ufw is installed. Otherwise, install it with: apt update && apt -y install ufw By default, ufw is inactive, meaning all ports are open. You can check its status with: ufw status To activate it, use: ufw enable You will need to confirm by entering y. Note that enabling ufw may interrupt current SSH connections. By default, ufw blocks all incoming traffic and allows all outgoing traffic. To check the default policy, use: cat /etc/default/ufw Opening Ports in ufw To open a port, use the command: ufw allow <port_number> For example, to open port 22 for SSH, run: ufw allow 22 You can list multiple port numbers separated by commas, followed by the protocol (tcp or udp): ufw allow 80,443,8081,8443/tcpufw allow 80,443,8081,8443/udp Instead of specifying port numbers, you can use the service name as defined in /etc/services. For example, to open the Telnet service, which uses port 23 by default: ufw allow telnet Note: You cannot specify multiple service names at once; ufw will return an error: To open a port range, use the following syntax: ufw allow <start_port>:<end_port>/<protocol> Example: ufw allow 8000:8080/tcp Closing Ports in ufw To close a port using ufw, use the command: ufw deny <port_number> For example, to close port 80, run: ufw deny 80 You can also use the service name instead of the port number. For example, to close port 21 used by the FTP protocol: ufw deny ftp Checking Open Ports in ufw To list all open and closed ports in the Linux system, use: ufw status Another option to view open ports in Linux is: ufw status verbose How to Open a Port in RHEL-Based Linux Distributions Linux RHEL-based distributions (CentOS 7+, RHEL 7+, Fedora 18+, OpenSUSE 15+) use firewalld by default. Opening Ports in firewalld To check if firewalld is installed, run: firewall-offline-cmd -V If the version is displayed, firewalld is installed. Otherwise, install it manually: dnf install firewalld By default, firewalld is disabled. Check its status with: firewall-cmd --state To enable firewalld, run: systemctl start firewalld To open port 8080 for the TCP protocol, use: firewall-cmd --zone=public --add-port=8080/tcp --permanent --zone=public: Specifies the zone for the rule. --add-port=8080/tcp: Specifies the port and protocol (TCP or UDP). --permanent: Saves the rule to persist after a system reboot. Without this parameter, the change will only last until the next reboot. Alternatively, you can open a port in Linux by specifying a service name instead of a port number. For example, to open the HTTP (port 80) protocol: firewall-cmd --zone=public --add-service=http --permanent Reload firewalld to apply the changes: firewall-cmd --reload Closing Ports in firewalld You can close a port using either its number or service name. To close a port using its number, run: firewall-cmd --zone=public --remove-port=8080/tcp --permanent To close a port using the service name, run: firewall-cmd --zone=public --remove-service=http --permanent After opening or closing a port, always reload firewalld to apply the changes: firewall-cmd --reload Listing Open Ports in firewalld To list all open ports in your Linux system, you can use: firewall-cmd --list-ports Managing Ports in iptables Unlike ufw and firewalld, iptables comes pre-installed in many Linux distributions, including Ubuntu, Debian, RHEL, Rocky Linux, and AlmaLinux. Opening Ports in iptables To open port 8182 for incoming connections, use: iptables -A INPUT -p tcp --dport 8182 -j ACCEPT -A INPUT: The -A flag is used to add one or more rules. INPUT specifies the chain to which the rule will be added (in this case, incoming connections). -p tcp: Specifies the protocol. Supported values include tcp, udp, udplite, icmp, esp, ah, and sctp. --dport 8182: Specifies the port to be opened or closed. -j ACCEPT: Defines the action for the port. ACCEPT allows traffic through the port. To open a port for outgoing connections, use the OUTPUT chain instead: iptables -A OUTPUT -p tcp --dport 8182 -j ACCEPT To open a range of ports, use the --match multiport option: iptables -A INPUT -p tcp --match multiport --dports 1024:2000 -j ACCEPT Closing Ports in iptables To close a port, use the -D option and set the action to DROP. For example, to close port 8182 for incoming connections: iptables -A INPUT -p tcp --dport 8182 -j DROP To close a range of ports, use the same syntax as for opening a range, but replace ACCEPT with DROP: iptables -A INPUT -p tcp --match multiport --dports 1024:2000 -j DROP Saving iptables Rules By default, iptables rules are only effective until you restart the server. To save the rules permanently, install the iptables-persistent utility. For APT-based distributions: apt update && apt -y install iptables-persistent For DNF-based distributions: dnf -y install iptables-persistent To save the current rules, run: iptables-save After the next server reboot, the rules will be automatically reloaded. Viewing Open Ports in iptables To list all current rules and opened ports on the Linux machine, use: iptables -L -v -n To list rules specifically for IPv4, use: iptables -S To list rules for IPv6, use: ip6tables -S Conclusion In this guide, we demonstrated how to open and close network ports in Linux and check currently open ports using three different utilities: ufw, firewalld, and iptables. Proper port management reduces the risk of potential network attacks and helps obscure information about the services using those ports.