Linux - Basic

Processes Basics

Every running program on Linux is a process, identified by a numeric PID.

ps aux              # list all running processes
top                  # live, updating view of processes and resource usage
kill 1234            # ask process 1234 to stop (gracefully)
kill -9 1234          # force-kill process 1234, no cleanup allowed

kill is a slightly misleading name - by default it doesn't violently kill anything, it asks the process to shut down (technically sending a signal called SIGTERM) and trusts it to clean up and exit on its own. kill -9 is the actual force-kill, skipping cleanup entirely. Reach for plain kill first; only use -9 when a process is genuinely stuck and ignoring the polite request.

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Package Management Basics

You rarely download and manually install software on a Linux server. Instead, your distribution has a package manager that fetches software, installs it, tracks its dependencies, and can cleanly remove it later.

# Debian/Ubuntu-based systems
sudo apt update              # refresh the list of available packages
sudo apt install nginx       # install nginx and anything it depends on

# RHEL/CentOS/Fedora-based systems
sudo yum install nginx       # (or 'dnf' on newer versions)

Which one you use depends entirely on which Linux distribution you're on - Ubuntu and Debian use apt, RHEL/CentOS/Fedora use yum or dnf. They do the same job with different commands, so knowing which family you're on matters before you go searching for install instructions.

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Environment Variables and PATH

An environment variable is a named value available to processes running in your shell - configuration that lives outside any file.

echo $HOME            # print the value of the HOME variable
export API_KEY=abc123   # set a variable for this session and anything launched from it

The most important one to understand is PATH. When you type a command like ls, the shell doesn't magically know where the ls program lives - it searches through a list of directories stored in PATH, in order, until it finds a matching program name. This is why "command not found" often just means the program isn't installed anywhere listed in PATH, even if it exists somewhere on disk.

echo $PATH
# /usr/local/bin:/usr/bin:/bin

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Piping and Redirection

Small commands become powerful when you chain them together. This is one of the defining ideas of Linux: build tools that do one thing well, then combine them.

ls -la | grep ".txt"        # pipe: feed ls's output into grep, which filters for lines containing ".txt"
echo "hello" > file.txt      # redirect: write "hello" into file.txt, overwriting it
echo "world" >> file.txt     # append "world" to the end of file.txt instead of overwriting
cat < file.txt                # redirect a file in as input

> versus >> is a common beginner mix-up with real consequences: > overwrites the file's entire contents, >> appends to the end. Using > when you meant >> is a fast way to accidentally erase a log file you wanted to add to.

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Basic Networking Commands

Even at the basic level, you'll routinely need to check whether something is reachable.

ping google.com        # send packets and see if/how fast something responds (Ctrl+C to stop)
curl https://example.com   # fetch a URL and print the response - great for testing APIs
curl -I https://example.com   # -I fetches only the response headers, not the full body

ping tells you whether a host is reachable at the network level at all. curl goes further - it actually speaks the application protocol (usually HTTP), so it can tell you not just "is the server up" but "is the server up and answering correctly." Getting a ping response but a failed curl is a very common and useful diagnostic signal: the machine is alive, but whatever's supposed to be listening on that port isn't behaving.