CompTIA Linux+ XK0-005 - All Domains Linux Administration Security Scripting

SSH hardening: key-based auth, disable root login, change port optional, fail2ban for brute force.

/etc/ssh/sshd_config vs ssh_config — server vs client. systemctl restart sshd after edits.

Bash script: shebang #!/bin/bash, chmod +x, set -e exit on error. Variables $1 $2 positional params.

[[ ]] vs [ ] — modern bash prefers [[ for tests. $(command) for subshell capture.

systemctl start/stop/enable/disable — enable = boot persistence. journalctl -u nginx for service logs.

SysV init scripts legacy — exam still mentions runlevels vs systemd targets (multi-user.target).

cron vs systemd timers — crontab -e for user jobs, /etc/cron.d for packages. @reboot special string.

First five fields: min hour dom month dow. Check system time TZ for scheduled jobs.

grep -r 'pattern' /etc — ripgrep faster in real life but exam says grep. sed/awk for stream edit.

Pipe stderr: 2>&1 | grep error. Know stdout 1 vs stderr 2 file descriptors.

SELinux enforcing vs permissive — getenforce, restorecon after moving files breaks contexts. AppArmor on Ubuntu.

chcon temporary vs semanage fcontext permanent — troubleshoot denials in /var/log/audit/audit.log.

chmod 750 = rwxr-x--- (owner rwx, group rx, other none). Octal: 4 read, 2 write, 1 execute.

Setuid bit (4xxx) runs as file owner — /usr/bin/passwd example. Sticky bit on /tmp prevents others deleting your files.

find /home -user jdoe -perm -0020 finds group-writable files owned by jdoe — know find predicates (-name, -mtime, -size).

locate uses DB updated by updatedb — faster but not real-time. find is live filesystem walk.

chown -R user:group /data/files — recursive ownership change. chmod changes mode bits, chown changes owner.

Only root or current owner can chown (unless CAP_FOWNER nuances). Use sudo.