Checking the System

Below is a list of common operations to check the current status of a linux system

Hardware

Some general information about the machine can be obtained by using the uname command. With the -a flag it will print:

• Kernel Name
• Node hostname
• Kernel release and version
• Processor type
• Operating system

uname -a

Checking uptime

The uptime command can be used to determine how long has the server been “up”

uptime --pretty

Memory

To check the physical available memory use:

grep MemTotal /proc/meminfo

However, running free gives a more complete overview on memory usage:

free -h

CPU

The command lscpu displays information about the CPU architecture:

lscpu

With the extended flag, the information can be compressed in a table

lscpu --extended

For more level of detail, the respective /proc file can be read and grepped

cat /proc/cpuinfo

Disks and filesystems

The lsblk command is used to display information about block devices (i.e., hard drives, SSDs, etc.) on the system. It displays information in the “lower” level of devices which may or not be mounted.

lsblk

To look into disk space and utilization, the df command is used. This displays information about the amount of space used and avaiable in filesystems that are mounted in the system.

df -h

Some of the most commonly used options for df include:

• h: Display disk space in a human-readable format.
• T: Display the filesystem type.
• i: Display the number of inodes (i.e., the number of files and directories) on the filesystem.

To go more into detail about the space used by particular files or directories, du is used.

du [options] [file|directory]

Some frequently used commands are:

• du -h dir Shows the size of all files in the current directory in a human readable format
• du -sh dir Same as above, but limits the results to a single directory (instead of recursive)
• du -ah dir includes hidden files
• du -h --max-depth=N dir List the human-readable sizes of a directory and any subdirectories, up to N levels deep.

Networking

Some tools can be used for general networking checking:

ip is a tool that allows to show / manipulate routing, devices, policy routing and tunnels. ip address or ip a allows to show, remove and manage the ips of the different interfaces. THe following command summarizes the interfaces in the system and their assigned IPs

ip -brief a 

Checking for open ports

To check (or verify) open ports can be useful in determining if a service is running or not (e.g. web server).

The first option to do this is with the ss (sockets statistics) tool. This tool allows to not only display information but kill socket connections. To display open ports the following command can be used

sudo ss -tulpnw

This will show all TCP (t) and UDP (u) ports that are open and listening (l). It will display the process (p) using them and their ports in a numeric way n. Finally it will display everything in wide format.

An alternative is to use nmap. This a very powerful tool used, among other things, to scan a network and discover hosts and services. To use it for finding out open ports, is enough to point it to the localhost ip:

sudo nmap 127.0.0.1

This will scan the machine and summarize the ports open and the services listening

Checking firewalls

ufw (Uncomplicated Firewall) and iptables are both firewall tools that can be used to secure Linux systems. UFW is a frontend for iptables that simplifies the process of configuring and managing a firewall.

To enable the firewall:

sudo ufw enable

Then, adding rules can be done with either allow or deny followed by the port and the protocol. If no protocol is specified it will apply for both

sudo ufw allow 80/tcp #Allow TCP connections on port 80
sudo ufw deny 22 #Deny TCP and UDP connections on port 22
sudo ufw allow out 3306/tcp #Allow only outbound TCP traffic from port 3306

The status and the rules can be checked at any time with

sudo ufw status

Software

Distribution

Two reliable ways of checking which distribution is a machine running are:

First, the /etc/os-release file. This provides information about the OS in the form of variables that can be sourced. An example would be:

#!/bin/bash
# source the os-release file
source /etc/os-release
# check if the distribution is Ubuntu
if [ "$NAME" == "Ubuntu" ]; then
    echo "This is Ubuntu"
fi
# check if the distribution is CentOS
if [ "$NAME" == "CentOS Linux" ]; then
    echo "This is CentOS"
fi

Second, the lsb_release command can be used to obtain distribution information. An example below:

#!/bin/bash
# use lsb_release to get distribution information
DISTRIB_ID=$(lsb_release -si)
DISTRIB_RELEASE=$(lsb_release -sr)
# check if the distribution is Ubuntu
if [ "$DISTRIB_ID" == "Ubuntu" ]; then
    echo "This is Ubuntu"
fi
# check if the distribution is CentOS
if [ "$DISTRIB_ID" == "CentOS" ]; then
    echo "This is CentOS"
fi

Installed packages

Checking the packages installed in the system depends on the family of the installed distribution.

Debian based (APT)

In debian, ubuntu and related distributions, the list of installed packages can be generated in two ways:

First using the low level dpkg command

dpkg -l

Second using the higher level apt:

apt list --installed

In addition, apt show can be used to obtain information about installed (or not installed) packages

Red Hat Based (YUM)

In RedHat, CentOS and related distributions, the list of installed packages can be generated in two ways:

First using the low level rpm command

rpm -qa

Second using the higher level yum:

yum list installed

In addition, yum info can be used to obtain information about installed (or not installed) packages

Processes

To see the processes running in the system, the main tool is ps. This non-interactive tool, has several options to show information about running processes. The syntax of the arguments can be either standard or BSD.

Some useful commands are presented below:

ps #Shows processes in current shell
ps -ef #Shows all processes in the system with full format
ps aux #BSD Argument style, shows all processes of all users
ps auxww #List all running processes including the full command string
ps aox pid,ppcpu #BSD Argument style, shows only PID and %CPU
ps -o ppid= -p 0000 #Get the parent PID of a process
ps --sort size #Sort processes by memory consumption

There is also ways to check processes interactively mainly with top and htop. These are real-time tools that show the status of the system. Just running them executes them and menus inside allow to configure the output.

Services (Daemons)

A system service or daemon is a program that runs in the background on a Linux system to provide functionality or perform a task, such as managing network connections or serving web pages. In order to monitor them and manage them, most distributions use systemctl

To check the status of a service run

systemctl status service

In a similar way, systemctl is-active, systemctl is-failed and systemctl is-enabled can be used to check a service.

To list all services active in the system run

systemctl list-units

Filters can be applied to this command. A very common one is to filter just services:

systemctl list-units --type=service

Finally to read a service definition can be done with:

systemctl cat service

More detailed information can be found in this article