CELE flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Linux kernel role

The kernel manages CPU scheduling, memory, device drivers, filesystems, and networking, exposing these to user space only through system calls for safety and isolation.

Boot sequence order

Firmware loads the bootloader, which loads the kernel and initramfs; the kernel mounts the root filesystem and starts PID 1 (systemd), which brings up targets.

FHS directories

/etc holds configuration, /var variable data such as logs, /usr read-only user programs, /tmp temporary world-writable files, and /home user home directories.

Standard streams

File descriptors 0, 1, and 2 are stdin, stdout, and stderr. Redirection with >, >>, 2>, and 2>&1 controls where each stream is sent.

UEFI versus BIOS

UEFI boots from an EFI System Partition formatted FAT32 and supports GPT disks over 2 TiB; legacy BIOS uses MBR with a four-primary-partition limit.

LVM layering

Physical volumes are grouped into volume groups, which are divided into logical volumes. lvextend grows an LV, then resize2fs or xfs_growfs grows the filesystem.

Regenerating GRUB config

Edit /etc/default/grub, then run grub2-mkconfig -o /boot/grub2/grub.cfg on RHEL or update-grub on Debian to rebuild the bootloader configuration.

Swap creation

Create a swap file with fallocate or dd, set permissions to 600, initialize with mkswap, activate with swapon, and add an /etc/fstab entry for persistence.

find command usage

find searches a directory tree by name, type, size, or modification time, and can execute a command per result with -exec or delete matches with -delete.

chmod numeric mode

Read, write, and execute map to 4, 2, and 1. Mode 755 gives the owner rwx and group and others r-x; 644 gives owner rw- and others r--.

grep options

grep -r searches recursively, -i ignores case, -n prints line numbers, -v inverts the match, and -E enables extended regular expressions.

tail -f usage

tail -f follows a file as it grows, printing new lines, which makes it the standard way to watch log files in real time.

User account files

/etc/passwd holds usernames, UIDs, GIDs, homes, and shells; /etc/shadow holds password hashes and aging; /etc/group holds group membership.

usermod -aG

usermod -aG group user appends the user to an additional group without removing existing memberships; omitting -a replaces all supplementary groups.

Permission evaluation order

The kernel checks owner permissions first, then group, then other. The first matching class applies, so owner rights override group rights.

Sticky bit

The sticky bit on a directory lets only a file's owner, the directory owner, or root delete it. It is commonly set on /tmp.

SIGTERM versus SIGKILL

SIGTERM (15) requests graceful termination and can be caught; SIGKILL (9) forces immediate termination and cannot be caught or ignored.

Job control keys

Ctrl-Z suspends the foreground job, bg resumes it in the background, fg returns it to the foreground, and jobs lists current jobs.

Nice values

Nice ranges from -20 (highest priority) to 19 (lowest). nice starts a process at a given value; renice changes a running process.

systemctl enable

systemctl enable creates the symlinks that start a unit at boot; start runs it now. enable --now does both in one command.

Private IP ranges

RFC 1918 private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. They are not routed on the public internet.

ss -tuln

ss -tuln lists listening TCP and UDP sockets numerically without resolving names, showing which ports services are bound to.

ssh-copy-id

ssh-copy-id installs a local public key into the remote user's ~/.ssh/authorized_keys, enabling passwordless key-based SSH authentication.

sshd_config hardening

Setting PermitRootLogin no and PasswordAuthentication no in /etc/ssh/sshd_config forces key-based logins and blocks direct root access after sshd restarts.

apt versus dpkg

apt resolves dependencies and fetches packages from repositories; dpkg installs local .deb files and reports missing dependencies without resolving them.

dnf provides

dnf provides /path/to/file identifies which package supplies a file, which is useful when a command is missing.

Repository configuration

Debian-family systems use /etc/apt/sources.list and sources.list.d; RHEL-family systems use .repo files in /etc/yum.repos.d with baseurl or mirrorlist entries.

Source build sequence

Typical source installation runs ./configure to check dependencies, make to compile, and make install to place files, usually requiring -dev or -devel headers.

Shell special variables

$0 is the script name, $1–$9 are positional arguments, $# is the argument count, $? is the last exit status, and $$ is the current PID.

Test file operators

In [ ], -f tests a regular file, -d a directory, -e existence, -z an empty string, and -eq numeric equality between integers.

set -euo pipefail

set -e exits on any error, -u errors on unset variables, and -o pipefail makes a pipeline fail if any stage fails, hardening scripts.

sudoers editing

Edit /etc/sudoers only with visudo, which locks the file and validates syntax. Entries follow user host=(runas) command, and NOPASSWD skips the password prompt.

3-2-1 backup rule

Keep three copies of data on two different media with one copy offsite. Restores must be tested regularly for the strategy to be reliable.