Essential Commands

These are the commands that form the backbone of daily work. Learn them until they’re muscle memory.

Navigation & filesystem

# Where am I?
pwd

# List files — human-readable sizes, show hidden
ls -lah

# Move around
cd /etc/nginx
cd -          # back to previous directory
cd ~          # home directory

# Find files
find /var/log -name "*.log" -mtime -1    # modified in last 24h
find . -type f -size +100M               # files over 100 MB

# Search inside files
grep -rn "ERROR" /var/log/app/
grep -v "DEBUG" app.log                  # exclude DEBUG lines

Inspecting files

# Quick look
cat /etc/os-release
head -20 /var/log/syslog
tail -f /var/log/nginx/access.log        # follow in real time

# Paged view
less /etc/nginx/nginx.conf               # q to quit, / to search

# Word/line count
wc -l access.log

Process management

Inspect running processes

$ ps aux | grep nginx root 1234 0.0 0.1 10824 3456 ? Ss 09:00 0:00 nginx: master www-data 1235 0.0 0.0 11280 2048 ? S 09:00 0:00 nginx: worker

$ # Show process tree $ pstree -p 1234

$ # Interactive monitor $ htop

# Kill a process
kill -15 1234      # SIGTERM — graceful shutdown
kill -9 1234       # SIGKILL — force kill (last resort)
killall nginx

# Background jobs
long-running-command &    # run in background
jobs                      # list background jobs
fg %1                     # bring job 1 to foreground

Networking

# Socket / port inspection
ss -tlnp                  # listening TCP sockets + process
ss -s                     # summary statistics

# Interface configuration
ip addr show
ip route show

# DNS lookup
dig almamy.net
dig +short almamy.net A
nslookup almamy.net

# Connectivity test
curl -v https://almamy.net
curl -o /dev/null -s -w "%{http_code}" https://almamy.net

# Transfer files
scp user@host:/path/file .
rsync -avz --progress ./local/ user@host:/remote/

File permissions

# Read the permission string: drwxr-xr-x
# d = directory | rwx = owner | r-x = group | r-x = others

chmod 755 script.sh          # rwxr-xr-x
chmod 600 ~/.ssh/id_rsa      # rw------- (private key)
chmod +x deploy.sh           # add execute for all

chown www-data:www-data /var/www/html
chown -R app:app /opt/app/

# Check effective permissions
stat -c "%a %n" /etc/shadow   # octal + name

Disk & memory

# Disk usage
df -h                         # filesystem usage
du -sh /var/log/*             # directory sizes

# Memory
free -h
vmstat 1 5                    # 5 samples, 1 second apart

# I/O
iostat -x 1
iotop                         # per-process I/O

Systemd services

# Status
systemctl status nginx
systemctl list-units --state=failed

# Control
systemctl start nginx
systemctl stop nginx
systemctl restart nginx
systemctl reload nginx        # reload config without restart

# Enable at boot
systemctl enable nginx
systemctl disable nginx

# Logs
journalctl -u nginx           # all nginx logs
journalctl -u nginx -f        # follow
journalctl -u nginx --since "1 hour ago"
journalctl -p err             # only errors

Text processing

# Column extraction
cat /etc/passwd | cut -d: -f1         # usernames only
awk '{print $1, $NF}' access.log      # first and last columns

# Sort and unique
sort -rn counts.txt | head -10        # top 10 by count
sort | uniq -c | sort -rn             # frequency count pattern

# Stream editing
sed 's/old/new/g' file.txt
sed -i 's/localhost/0.0.0.0/g' config.ini   # in-place edit

# JSON
cat response.json | jq '.items[].name'
curl -s api.example.com | jq '.[] | select(.status == "active")'

Shortcuts worth knowing

Shortcut	Action
Ctrl+C	Interrupt current process
Ctrl+Z	Suspend to background
Ctrl+R	Reverse search history
Ctrl+L	Clear screen
!!	Repeat last command
!$	Last argument of previous command
Alt+.	Insert last argument (same as !$)