Linux Essentials and LPIC-1 Flashcards

The kernel. Shells, desktop apps, and package managers run in user space and ask the kernel to do privileged work.

A copyleft license. BSD-style licenses are permissive. Creative Commons is used for content, not only for programs.

A colon-separated list of directories the shell searches for commands.

Memory note: The current directory is usually not on PATH, so run a local script as ./script.sh.

man ls. ls --help prints a shorter usage summary.

> replaces the file. >> appends. A pipe sends output to the next command instead of to a file.

tar -czf archive.tar.gz dir/. Extract with tar -xzf archive.tar.gz.

It tells the kernel to run the script with bash. The line must be the first line of the file.

/etc/shadow. Only root can read it. /etc/passwd is world-readable and stores the account name, UID, home, and shell.

Read, write, and execute. 755 is owner read-write-execute, and group and others read-execute.

It is a sticky directory, usually mode 1777. Anyone can create files, and only the owner can delete their own.

lspci. lsusb lists USB devices. lsmod lists loaded kernel modules.

Firmware, boot loader, kernel and initramfs, then PID 1. On current systems PID 1 is systemd.

multi-user.target. graphical.target starts a display manager. Set the default with systemctl set-default.

grub-mkconfig. grub-install writes the boot loader itself. Older GRUB used menu.lst or grub.conf.

apt update refreshes the package index. apt install downloads a package and its dependencies. dpkg -i installs one file and does not fetch dependencies.

ldd. ldconfig rebuilds the library cache from /etc/ld.so.conf.

The default is 0. -20 is the highest priority and 19 is the lowest. Only root can raise priority.

A device name such as /dev/sda1 can change. The UUID stays with the filesystem. Look it up with blkid.

When the filesystem is unmounted. Checking a mounted filesystem can corrupt it. The same rule applies to xfs_repair.

A hard link is another name for the same inode and cannot cross filesystems. ln -s stores a path and can point at a missing target.

crontab -e. crontab -l lists it. The five fields are minute, hour, day of month, month, and day of week.

at runs a command once. Cron repeats on a schedule. atq lists pending at jobs.

LC_ALL. LANG is the default when a specific LC_ variable is unset.

timedatectl. hwclock reads the hardware clock. Zone data lives under /usr/share/zoneinfo.

journalctl -u name. rsyslog text logs are files such as /var/log/syslog or /var/log/messages.

It sends a message to the system log so you can test a facility and priority without waiting for a real event.

newaliases. mailq lists the outgoing mail queue. ~/.forward redirects one user’s mail.

lp or lpr submits a job. lpstat or lpq lists the queue. CUPS configuration is under /etc/cups.

It locks the account password. userdel removes the account. chage changes password aging.

The starter files for a new home directory, such as shell startup files, when useradd creates an account with a home.

ip addr. ip route shows the routing table. ip link shows whether the interface is up.

/etc/resolv.conf. /etc/hosts is a local name list. The order is set in /etc/nsswitch.conf.

ping tests whether an address answers ICMP. host looks up a name in DNS. A failed ping does not prove that DNS is broken.

visudo checks the syntax before installing the file, so a typo is less likely to lock out every administrator.

/etc/hosts.allow and /etc/hosts.deny. They filter services built with libwrap. They are not a firewall.

In ~/.ssh/authorized_keys on the server. The private key stays on the client. ssh-keygen creates the pair.

SSH host keys and the D-Bus machine id. Otherwise two machines present the same identity.

Encrypt, decrypt, sign, and verify, plus the idea of revoking a key. Key material lives under ~/.gnupg.

The X server and screen an application should draw on, such as :0. Wayland is awareness beside X11, not a replacement you must configure in detail.

It blocks logins for ordinary users. Root can still sign in on the console for maintenance.