Linux command reference

hostnamectl                       # system identity
uptime                            # uptime and load
free -h                           # memory and swap
df -hT                            # filesystem capacity and type
du -sh * 2>/dev/null | sort -h    # size of entries here
ps aux --sort=-%cpu | head        # top CPU processes
systemctl --failed                # failed units
timedatectl                       # clock, timezone, and time sync
last reboot                      # recent reboot history
dmesg --level=err,warn            # current boot's kernel warnings

ls -lah                           # detailed listing
find /path -type f -name '*.log'  # find by name and type
less file                         # inspect safely
tail -F file                      # follow a rotating log
grep -Rin 'pattern' /path         # recursive content search
cp -a source destination          # preserve metadata when copying
rsync -a --dry-run src/ dest/     # preview synchronization
namei -l /full/path/to/file       # permissions on every path component
file archive.tar.gz               # detect file type from content
sha256sum file                    # calculate an integrity checksum

tar -czf backup.tar.gz directory/ # create a gzip-compressed archive
tar -tzf backup.tar.gz            # list archive contents before extraction
tar -xzf backup.tar.gz            # extract the archive here
rsync -aHAX --dry-run src/ dest/  # preview a metadata-preserving local copy
scp file user@host:/tmp/          # copy one file over SSH
sftp user@host                    # interactive transfer over SSH

whoami
id
sudo -l
stat file
chmod 640 file
chown user:group file
getent passwd deploy
getent group www-data
getent hosts example.com
sudo -u deploy command             # run one command as the service user
umask                              # default permissions for new files

pgrep -af nginx                    # find matching PIDs and full commands
ps -o pid,ppid,user,%cpu,%mem,etime,cmd -p PID
sudo lsof -p PID                   # files and sockets opened by a process
nice -n 10 command                 # start with lower CPU scheduling priority
renice 10 -p PID                   # adjust a running process priority
systemctl list-timers --all        # systemd timers and their next runs
crontab -l                         # current user's cron jobs
sudo crontab -u deploy -l          # another user's cron jobs

systemctl status SERVICE --no-pager
systemctl restart SERVICE
systemctl reload SERVICE
systemctl enable --now SERVICE
journalctl -u SERVICE -n 100 --no-pager
journalctl -u SERVICE -f
journalctl --since '-30 min' -p warning
systemctl cat SERVICE              # unit file plus drop-in configuration
systemctl show SERVICE -p User -p MainPID -p ExecStart
journalctl -b -u SERVICE           # service logs from the current boot

ip -brief address
ip route
sudo ss -tulpn
dig +short example.com A
nc -vz example.com 443
curl -fsS http://127.0.0.1:3000/health
curl -vI https://example.com
sudo ufw status numbered
resolvectl status                  # configured DNS resolvers
ip route get 1.1.1.1               # route chosen for a destination
openssl s_client -connect example.com:443 -servername example.com

lsblk -f                           # block devices, filesystems, and mount points
findmnt                            # mounted filesystems and options
df -hT                             # capacity by filesystem
df -ih                             # inode usage
du -xhd1 /var | sort -h            # directory sizes on one filesystem
sudo lsof +L1                      # deleted files still held open
journalctl --disk-usage            # journal storage use

docker compose ps
docker compose logs -f --tail=100 app
docker compose pull
docker compose up -d --remove-orphans
docker compose exec app sh
docker inspect CONTAINER
docker system df
docker stats --no-stream
docker inspect --format '{{json .State.Health}}' CONTAINER

sudo ss -lntup                    # listening ports and owning processes
sudo last -a | head               # recent login history
sudo journalctl -u ssh --since today   # Ubuntu / Debian SSH events
sudo journalctl -u sshd --since today  # RHEL-family SSH events
sudo find / -xdev -perm -4000 -type f  # setuid files on the root filesystem
sudo getenforce                   # SELinux mode, when installed
sudo aa-status                    # AppArmor status, when installed

sudo cp -a config config.bak.$(date +%Y%m%d%H%M%S)
sudoedit config
VALIDATOR -t                     # nginx -t, sshd -t, compose config --quiet
sudo systemctl reload SERVICE
systemctl status SERVICE --no-pager