CompTIA Linux+ cheat sheet

uname -r prints the kernel release; hostnamectl displays or sets the static hostname, and timedatectl shows time zone and NTP status.

lsblk -f shows block devices with filesystem type, label, UUID, and mount point; blkid prints UUID and filesystem type for a device.

/etc/fstab fields are device/UUID, mount point, filesystem type, options, dump pass, and fsck pass; mount -a mounts entries not marked noauto.

df -h reports filesystem free and used space; du -sh /path totals apparent disk usage for a directory tree.

LVM creation order is pvcreate, vgcreate, then lvcreate; after lvextend, run resize2fs for ext4 or xfs_growfs for XFS.

tar -czf archive.tar.gz dir creates a gzip-compressed tar archive; tar -xzf extracts it; rsync -a preserves permissions, times, and symlinks.

systemctl daemon-reload reloads unit definitions; journalctl -b shows current-boot journal entries; dmesg shows the kernel ring buffer.

A systemd mount unit is named from its mount path, for example /data becomes data.mount; systemctl status data.mount checks it.

systemctl enable unit creates boot-time symlinks; systemctl start unit starts it now; systemctl --failed lists failed units.

systemctl mask unit links the unit to /dev/null and prevents any start; systemctl unmask reverses it.

systemctl get-default shows the default target; multi-user.target is comparable to runlevel 3 and graphical.target to runlevel 5.

useradd -m -s /bin/bash user creates a home directory and login shell; usermod -aG group user adds a supplementary group without removing existing ones.

/etc/passwd has seven colon-separated fields: name, password placeholder, UID, GID, GECOS, home directory, and login shell.

/etc/shadow stores password hashes and aging data; chage -l user shows last change, minimum, maximum, warning, inactive, and expiry fields.

Cron format is minute, hour, day-of-month, month, day-of-week, command; /etc/crontab adds a user field before the command.

ss -tulnp lists listening TCP and UDP sockets with numeric ports and processes; loginctl list-sessions shows login sessions.

Numeric permissions use read=4, write=2, execute=1; chmod 750 file gives owner rwx, group rx, and others no access.

umask 027 produces file mode 640 and directory mode 750 because the mask is subtracted from default modes 666 and 777.

Special permission bits are setuid=4, setgid=2, and sticky=1; find / -perm -4000 locates setuid files.

SELinux modes are enforcing, permissive, and disabled; getenforce shows the current mode and sestatus shows policy and mode.

SELinux file context changes use semanage fcontext -a -t type '/path(/.*)?' followed by restorecon -Rv /path.

ssh-keygen -t ed25519 creates an SSH key pair; the public key goes in ~/.ssh/authorized_keys; sshd_config controls PermitRootLogin and PasswordAuthentication.

Edit sudo rules with visudo; %wheel ALL=(ALL) ALL grants sudo to group wheel, and sudo -l lists allowed commands.

firewall-cmd --add-service=https changes runtime firewalld; add --permanent and run firewall-cmd --reload to persist it.

A script's first line #!/bin/bash is the shebang; the kernel uses it to select the interpreter for execution.

In shell, $? is the last exit status, $# is the argument count, and $@ expands to all positional arguments.

set -e exits on command failure; set -u errors on unset variables; set -x prints commands as they are executed.

Use [ -f file ] for file tests, -eq for numeric equality, and = for string equality; [[ ]] adds pattern matching in Bash.

$(command) captures standard output; command > file 2>&1 redirects both stdout and stderr to the same file.

Ansible playbooks are YAML task lists; ansible-playbook -i inventory site.yml runs them, and modules are designed to be idempotent.

sed -i 's/old/new/g' file edits in place; awk '{print $1}' prints the first whitespace-separated field.

systemctl status unit shows state and recent logs; journalctl -u unit -xe shows unit errors with explanations.

ip addr shows interface addresses, ip route shows routes, and ip route add default via GATEWAY sets the default gateway.

DNS checks include dig name, getent hosts name, and the nameserver entries in /etc/resolv.conf.

For disk-full issues, df -h checks filesystems, du -sh * checks directory usage, and lsof +L1 finds deleted files still held open.

fsck /dev/device must run on an unmounted filesystem; checking a mounted filesystem can cause corruption.

strace -p PID traces system calls, and lsof -p PID lists open files and sockets for a process.

namei -l /path/file prints permissions for each path component, locating where access is denied.