--- title: "Linux Foundation Certified System Administrator Flashcards (LFCS)" description: "Spaced-repetition deck for the LFCS exam: essential commands, users and groups, storage, networking, services, and security." author: "Mohammad Abu Mattar" canonical: https://mkabumattar.com/flashcards/post/lfcs-linux-system-administrator-flashcards --- # Linux Foundation Certified System Administrator Flashcards (LFCS) ## Flashcards ### 1. How do you find files by name under a directory tree? Use find with a path and -name pattern. ``` find /var/log -name "*.log" -type f ``` ### 2. What does grep -r do, and how do you show line numbers? grep -r searches recursively through a directory; add -n for line numbers and -i for case-insensitive. ``` grep -rin "error" /var/log ``` ### 3. How do you replace text in a stream or file with sed? Use sed with the s/old/new/g substitution. Add -i to edit the file in place. ``` sed -i 's/localhost/127.0.0.1/g' config.ini ``` ### 4. How do you print the second column of whitespace-separated text? Use awk and reference the field by number. ``` awk '{print $2}' access.log ``` ### 5. What is the difference between > and >> in the shell? > redirects stdout and truncates (overwrites) the file; >> appends to it. Use 2> to redirect stderr. ### 6. How do you make a script executable and run it? Add the execute bit with chmod +x, then run it by path. ``` chmod +x deploy.sh ./deploy.sh ``` ### 7. What do the digits in chmod 640 mean? Owner read+write (6), group read (4), others none (0). The three digits are owner, group, others; 4=read, 2=write, 1=execute summed. ### 8. How do you change a file's owner and group? Use chown with user:group. Add -R to recurse into a directory. ``` chown -R www-data:www-data /var/www ``` ### 9. What does the setuid bit do, and how do you spot it? A setuid binary runs with the file owner's privileges rather than the caller's. It shows as an s in the owner execute position (e.g. -rwsr-xr-x). ### 10. How do you create a user with a home directory and a login shell? useradd -m creates the home directory; -s sets the shell. ``` useradd -m -s /bin/bash alice ``` ### 11. How do you add an existing user to a group without removing other groups? Use usermod -aG (append). Forgetting -a replaces all supplementary groups. ``` usermod -aG docker alice ``` ### 12. Which file should you edit to grant sudo access, and how? Edit the sudoers config with visudo (never a plain editor), or drop a file in /etc/sudoers.d/. visudo validates syntax before saving. ### 13. How do you set password aging for a user? Use chage: -M sets max days, -m min days, -W the warning window. ``` chage -M 90 -W 7 alice ``` ### 14. What are the three layers of LVM? Physical Volumes (PVs) on disks/partitions, grouped into a Volume Group (VG), carved into Logical Volumes (LVs) you format and mount. ### 15. How do you create and mount an ext4 filesystem? Format with mkfs.ext4, then mount it at a mount point. ``` mkfs.ext4 /dev/vg0/data mount /dev/vg0/data /mnt/data ``` ### 16. How do you make a mount persist across reboots? Add an entry to /etc/fstab (ideally by UUID from blkid), then test with mount -a before rebooting. ``` UUID=xxxx /mnt/data ext4 defaults 0 2 ``` ### 17. How do you extend a logical volume and grow its filesystem? Extend the LV with lvextend, then grow the filesystem (resize2fs for ext4, xfs_growfs for XFS). ``` lvextend -L +5G /dev/vg0/data resize2fs /dev/vg0/data ``` ### 18. How do you check disk usage vs free space? df -h shows free space per mounted filesystem; du -sh shows the size of a directory tree. ### 19. What replaced ifconfig and route on modern Linux? The ip command from iproute2: ip addr for interfaces/addresses, ip route for the routing table, ip link for link state. ### 20. How do you see listening ports and the processes behind them? Use ss with -tulpn (TCP, UDP, listening, process, numeric). ``` ss -tulpn ``` ### 21. How do you open a port with firewalld permanently? Add the port to a zone with --permanent, then reload. ``` firewall-cmd --permanent --add-port=8080/tcp firewall-cmd --reload ``` ### 22. Where is static hostname-to-IP resolution configured? In /etc/hosts. The order of resolution sources is set in /etc/nsswitch.conf, and DNS servers in /etc/resolv.conf. ### 23. How do you start a service now and enable it at boot? systemctl start runs it now; systemctl enable sets it to start on boot. --now does both. ``` systemctl enable --now nginx ``` ### 24. How do you view logs for a specific unit? Use journalctl -u . Add -f to follow, -b to limit to the current boot. ``` journalctl -u nginx -f ``` ### 25. What is a systemd target, and what replaced runlevels? A target groups units into a system state (e.g. multi-user.target, graphical.target). Targets replaced the old numeric runlevels. ### 26. How do you schedule a recurring job with cron? Edit the crontab with crontab -e; each line is `min hour day month weekday command`. ``` 0 2 * * * /usr/local/bin/backup.sh ``` ### 27. How do you see why a service failed to start? Check systemctl status for the exit code and recent log lines, then journalctl -u -b for the full boot log. ### 28. How do you harden SSH against password brute-forcing? In /etc/ssh/sshd_config set PasswordAuthentication no and PermitRootLogin no, use key-based auth, then reload sshd. ### 29. What are POSIX ACLs and how do you set one? ACLs grant permissions beyond the owner/group/other model. Set with setfacl and view with getfacl. ``` setfacl -m u:alice:rw report.txt getfacl report.txt ``` ### 30. What does SELinux enforcing vs permissive mean? Enforcing blocks actions that violate policy and logs them; permissive only logs violations without blocking. Check and set with getenforce / setenforce. ### 31. How do you find files with the setuid bit set (a security audit)? Use find with -perm to match the setuid bit across the filesystem. ``` find / -perm -4000 -type f 2>/dev/null ``` ### 32. How do you check and rotate logs to avoid filling the disk? logrotate (configured in /etc/logrotate.d/) rotates, compresses, and prunes logs on a schedule. Test a config with logrotate -d.