20 Essential Linux Commands Every Sysadmin Should Master

1. How to view the current Linux runlevel? Answer: Use who -r or runlevel commands.

2. How to view the default gateway? Answer: Use route -n or netstat -nr, which also display the routing table.

3. How to rebuild the initramfs image on Linux? Answer: On CentOS 5.x / RHEL 5.x use mkinitrd. Example:

Replace uname -r with the desired kernel version for a specific kernel.

On CentOS 6.x / RHEL 6.x use dracut. Example:

To rebuild for a specific kernel:

4. What does the cpio command do? Answer: It copies files into or out of an archive and can extract files from it.

5. What is the patch command and how to use it? Answer: patch applies changes (diff output) to files, converting an old version to a new one. Example workflow: create a diff file, then apply it with patch as shown:

Both old and new files can be single files or directories; -r enables recursive processing.

Applying the patch:

6. What is aspell used for? Answer: It is an interactive spell checker for Linux, a free alternative to ispell, usable both as a library for other programs and as a standalone command‑line tool.

7. How to view a domain's SPF record from the command line? Answer: Use the dig command. Example:

8. How to identify the package that provides a specific file (e.g., /etc/fstab )? Answer:

The command lists the package owning /etc/fstab.

9. Which command shows the status of bond0 ? Answer:

10. What is the purpose of the /proc filesystem? Answer: It is a memory‑based virtual filesystem that exposes kernel and process information such as CPU, memory, partitions, I/O addresses, DMA channels, and running processes. It is maintained automatically by the system.

11. How to find files larger than 10 MB under /usr ? Answer:

12. How to locate files in /home modified more than 120 days ago? Answer:

13. How to find files in /var not accessed in the last 90 days? Answer:

14. How to locate and delete all files named "core" in the entire directory tree? Answer:

15. What does the strings command do? Answer: It extracts and displays printable strings from binary files, useful for analyzing suspicious executables during intrusion investigations.

16. What is the purpose of the tee filter? Answer: It duplicates standard input to multiple destinations, such as writing to a file while also sending output to the screen or another program.

17. What does export PS1="$LOGNAME@hostname:\$PWD:" change? Answer: It modifies the shell prompt to show the username, hostname, and current working directory.

18. What does ll | awk '{print $3,"owns",$9}' do? Answer: It lists files and prints each file's owner and name.

19. What is the purpose of the at command? Answer: It schedules a one‑time task to run at a specified future time, with jobs stored in /var/spool/at and executed by the atd daemon.

20. What does the lspci command show? Answer: It displays information about PCI buses and attached devices; options -v, -vv, or -vvv increase verbosity, and -r improves readability.