Linux command line survival guide for hosting customers

pwd                 # print working directory — where am I?
ls -lah             # list files, human sizes, hidden files
cd /var/www         # change directory
cd ..               # up one level
cd ~                # home directory

less /etc/nginx/nginx.conf     # page through a file (q to quit, / to search)
head -50 file.log              # first 50 lines
tail -50 file.log              # last 50 lines
tail -f file.log               # follow a log live (Ctrl-C to stop)
grep "error" file.log          # find lines containing "error"

find /var/www -name "*.php"            # by name
grep -r "password" /etc/nginx          # search inside files, recursively
which nginx                            # where is this command?

cp file1 file2        # copy
mv file1 file2        # move / rename
rm file               # delete (no undo — be careful)
mkdir -p a/b/c        # make directories, including parents
chmod 644 file        # permissions: owner rw, group r, others r
chown www-data:www-data file   # change owner and group

top                   # live process list (q to quit); htop is nicer if installed
ps aux | grep nginx   # is nginx running?
kill <PID>            # politely ask a process to stop
kill -9 <PID>         # force it (last resort)
free -h               # memory
df -h                 # disk space
du -sh /var/log       # size of a directory

systemctl status nginx        # is it running? recent logs?
systemctl restart nginx       # restart it
systemctl reload nginx        # reload config without dropping connections
systemctl enable nginx        # start automatically at boot
journalctl -u nginx -n 100    # last 100 log lines for a service
journalctl -u nginx -f        # follow the service log live

apt update                     # refresh the package index (Debian/Ubuntu)
apt upgrade                    # install available updates
apt install nginx              # install
apt remove nginx               # uninstall, keep config
apt purge nginx                # uninstall and delete config
apt search "web server"        # search

# RHEL-family equivalents
dnf check-update && dnf upgrade
dnf install nginx

ip -br addr            # my IP addresses
ip route               # routing table
ss -tulpn              # what is listening on which port
curl -I https://example.com    # fetch headers only
ping -c 4 1.1.1.1      # is a host reachable?
dig example.com        # DNS lookup