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Linux ​

Commands ​

Users & Groups ​

  • Users having UID 0 are superusers (root)
  • Users having UID >= 1000 are normal users & < 1000 are system users
  • System users are helpful when we want to run something/cron in background
  • View users in /etc/passwd file & groups in /etc/group file
  • If someone has permission like rw- for directory, it means user can view the content of the directory and add files to it but can't navigate to it.
shell
sudo useradd myuser # create a user

sudo userdel myuser # delete a user

sudo useradd -m myuser # create a user with home directory

sudo userdel -r myuser # delete a user along with home directory

sudo useradd -r mybot # create a system user using `-r` flag

passwrd # change password of currently logged in user

sudo passwd myuser # change password of a user "myuser"

cat /etc/passwd # list all users
# username:password:UID:GID:comment:home:shell

# --- Group ---

groups # list groups of current user

groups myuser # list groups of user "myuser"

sudo groupadd mygroup # create a group

sudo groupdel mygroup # delete a group

sudo usermod -aG mygroup myuser # add user "myuser" to group "mygroup" (logout & login required)
# sudo gpasswd -a myuser mygroup # Same thing as above but using `gpasswd`

sudo gpasswd -d myuser mygroup # Remove user "myuser" from the group "mygroup"

cat /etc/group # list all groups
# groupname:password:GID:users

Permissions ​

type#
read4
write2
execute1
  • rwx => 7
  • rw- => 6
  • r-x => 5
shell
chmod +w file.txt # add execute permission to file.txt for all

chmod -w file.txt # remove execute permission from file.txt for all

chmod u+w file.txt # add execute permission to file.txt for user

chmod g+w file.txt # add execute permission to file.txt for group

chmod o+w file.txt # add execute permission to file.txt for others

chmod g+rw file.txt # add read & write permission to file.txt for group

chmod 770 file.txt # add read, write & execute permission to file.txt. User & group have full permission & others have no permission

chmod -R 755 mydir # Recursively update permission

chown -R myuser:mygroup mydir # Change owner of directory "mydir" to "myuser" & group to "mygroup"

chown -R $USER:$USER mydir # Change owner of directory "mydir" to current user & group to current user

Utility ​

bash
mktemp # create a temporary directory

MYTEMPDIR=$(mktemp) # create a temporary directory and store the path in a variable
echo $MYTEMPDIR # print the path to the temporary directory

# ---

echo $RANDOM # print a random number

# Read latest data of constantly updating file
tail -f /var/log/syslog

Security ​

ufw - Uncomplicated Firewall ​

bash
# Enable firewall
sudo ufw enable

# Disable firewall
sudo ufw disable

# Check status
sudo ufw status
sudo ufw status verbose # for verbose output

# Allow port
sudo ufw allow 3000

# Close/Block port
sudo ufw deny 3000

# Delete rule
sudo ufw delete allow 3000

Hosting ​

Nginx ​

Useful Commands ​

bash
# Start Nginx
sudo systemctl start nginx

# Stop Nginx
sudo systemctl stop nginx

# Restart Nginx
sudo systemctl restart nginx

# Reload Nginx (without dropping connections):
sudo systemctl reload nginx

# Test Nginx Configuration for Syntax Errors
sudo nginx -t

# Create Symbolic Link to Enable a Site
sudo ln -s /etc/nginx/sites-available/your_site_name /etc/nginx/sites-enabled/

# View Nginx Error Logs
sudo tail -f /var/log/nginx/error.log

# View Nginx Access Logs
sudo tail -f /var/log/nginx/access.log

Systemd ​

Useful Commands ​

bash
# --- Working with systemd service ---

# Start service/Unit
sudo systemctl start nginx

# Stop service/Unit
sudo systemctl stop nginx

# Restart service/Unit
sudo systemctl restart nginx

# Reload service/Unit
sudo systemctl reload nginx

# Autostart service/Unit on boot
sudo systemctl enable nginx

# Disable autostart service/Unit on boot
sudo systemctl disable nginx

# --- Debugging ---

# Check status of a service/Unit
journalctl -u nginx

# Get recent logs
journalctl -e -u nginx

# Check status of a service/Unit and follow the logs (continuous)
journalctl -u nginx -f

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