CKA: Essential Shell commands

1. Redirection operators(>> vs >) : > and >> are redirection operators used to send output to files instead of the terminal. Here's what they mean:

   1. > : Redirect and Overwrite

      • Redirects stdout (standard output) to a file.

      • Overwrites the file if it exists.

   2. >> : Redirect and Append

      • Redirects stdout to a file.

      • Appends to the file if it exists.

      • Creates the file if it doesn't exist.

2. tail utility : The tail utility is a command-line tool used to display the end (tail) of a file, usually text files like logs, defaults to displaying the last 10 lines of a file.

   • -f flag in tail -f stands for "follow", which means: shows the last 10 lines and then keep the file open and continuously output new lines as they are appended to the file.

3. dpkg : A low-level tool that Installs .deb files without dependency resolution.

4. apt-get : A package manager used to install/remove/update packages which can resolve dependencies and fetch packages from online repositories (which dpkg cannot).

5. systemctl : The command-line tool used to interact with systemd, the modern init system and service manager used in most major Linux distributions (like Ubuntu, CentOS, Debian, RHEL, etc.)

   • Used to manage systemd services (start/stop services, enable at boot, etc.).

       # Start, stop, restart, enable, disable, and check the status of services.
       systemctl start nginx          # Start nginx service
       systemctl stop nginx           # Stop it
       systemctl restart nginx        # Restart
       systemctl status nginx         # Show current status
       systemctl enable nginx         # Start nginx automatically at boot
       systemctl disable nginx        # Disable it from auto-starting
     

6. grep : Global Regular Expression Print, go-to utility for searching text in files, logs, or command output using patterns or regular expressions.

   • -i flag for case-insensitive search

   • -r or -R for recursive search in all files in a directory

   • -n shows line numbers of matching lines

   • -w will match whole words only

   • --color for highlighting the match in the output

   •           # eg, how to combine it with kubectl for debugging
               kubectl get pods -A | grep CrashLoopBackOff
     

7. sysctl : Used to view or modify kernel parameters (like net.ipv4.ip_forward).

   •       sysctl net.ipv4.ip_forward          # Check value
           sysctl -w net.ipv4.ip_forward=1     # Set temporarily
     

8. sudo −i: sudo utility in Linux systems lets you run commands with elevated privileges, typically as the root (superuser) account, without having to log in as root directly

   • Although it stands for “superuser do“, now it can be configured to let you run commands as any user, not just root

   • sudo -i is a special form of sudo that gives you an interactive root login shell, almost as if you logged in directly as the root user

9. curl & wget: Both curl and wget are command-line utilities for transferring data over networks

   • curl: Transfers data to/from a server using many supported protocols (HTTP, HTTPS, FTP, SFTP, SCP, LDAP, IMAP, POP3, SMTP, etc)

   • It’s actually a general-purpose HTTP/HTTPS client for making requests to any endpoint.

   • wget: Primarily for downloading files from the web (HTTP, HTTPS, FTP).

   • While curl is very flexible for both download and upload, it is mostly used for making http request in CKA; wget is designed mainly for retrieving content, especially useful for big files or recursive site downloads.

10. nslookup: command-line tool used to query DNS (Domain Name System) servers to get information about domain names, IP addresses, and other DNS records.

    • Looks up the IP address for a given domain name and vice-versa (reverse lookup).

    • Retrieves specific DNS record types (A, AAAA, MX, TXT, NS, etc.).

    • Useful for testing or troubleshooting DNS configuration issues.

11. netstat : Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, IPv4 statistics.

    • Shows active TCP/UDP connections (both incoming and outgoing).

    • Lists ports the system is listening on.

    • Displays routing tables.

    • Shows network interface statistics (packets sent/received, errors).

    • Helps identify which process is using which port.

    • netstat -tulnp is used to display all actively listening TCP and UDP ports, along with the processes using them.

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Good to know

1. kubectl --help: shows the you the built-in usage guide for kubectl including all the subcommands and options

   • My favourite and super helpful during the exam, saves a lot of time wasted searching through docs.

2. yq: command-line utility for reading, writing, and transforming YAML file

   • yq can be a big time-saver when working with YAML manifests for Kubernetes

   • For example: use yq '.spec.containers[].image' pod.yaml for grabbing image of pod

3. man: displays the manual pages for commands, basically the built-in, offline reference guide for most CLI tools

   • Useful for instantly check syntax for CLI tools right in the terminal

4. kubectl explain: shows the API schema for any Kubernetes object

   • Helpful when working with custom CRDs as CRDs docs are not available in the official exam

   • Using explain subcommand we can see the details of our API schema such as field names and descriptions

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Most of the utilities listed here are used by SysAdmins and DevOps folks on day to day basis, however having it listed out like this could be a good reference during preparation.

This list is not exhaustive, let me know if I’ve missed any commands so that I can keep the list updated as much as possible!! 🤞