LFCS Course Notes

Runtime kernel tuning starts with sysctl. Use sysctl to inspect or change kernel parameters while the system is running, and recognize that the same tunables appear under /proc/sys as writable kernel interfaces. A change made directly under /proc/sys or with sysctl -w affects the running kernel immediately but does not survive reboot unless it is written to persistent sysctl configuration.

• Changing a sysctl value at runtime does not make it persistent after reboot unless it is written to sysctl configuration.

• If the task says after reboot, look for persistent config or systemctl enable rather than only a runtime command.

Interface addressing begins with ip address show. Use the broad form to inspect all configured addresses, and add dev when the task asks for one interface only. The command reports IPv4 and IPv6 addresses, link state, and interface names, which makes it the starting point before changing routes or services.

• Do not mix server-side sshd_config settings with client-side ssh_config Host settings.

• If one interface is named, use ip address show dev rather than inspecting every interface.

LVM starts with physical volumes. pvcreate marks a block device for LVM use, vgcreate combines one or more physical volumes into a volume group, and lvcreate allocates logical volumes from that group. The exam often checks the order of those commands because each layer depends on the previous one.

• pvcreate, vgcreate, and lvcreate happen in that order because LVM layers depend on each other.

• If the task asks for persistent mounts, edit /etc/fstab; if it asks for now, use mount or umount.

Git tasks on LFCS usually test basic repository handling rather than advanced branching. git clone copies a repository, git status shows tracked and untracked changes, and git commit records staged changes with a message. If a task asks whether work is ready to commit, status is the safer command before committing.

• Use du for file-tree usage and df for filesystem capacity; they answer different disk questions.

• If unmounting fails, lsof can identify processes holding files open.

grep searches text using patterns, and LFCS tasks often include regular expression details. A regex can match character classes, anchors, or repeated text, while carriage-return characters from Windows-style files can make output look correct but fail a pattern. If a line ends with CRLF, account for the carriage return or normalize the file before matching.

• setuid on an executable and the sticky bit on a shared directory solve different permission problems.

• If grep fails on copied text, consider hidden carriage returns or CRLF line endings.