Add more Linux-related commands

src/linux.md

@@ -184,7 +184,12 @@ uptime
 
 ## Package management
 ```bash
+# Show manually installed packages.
+apt-mark showmanual
+apt list --manual-installed
+
 # List installed packages.
+apt list --installed
 dpkg --list
 
 sudo dpkg --audit
@@ -220,6 +225,8 @@ apt install sudo
 
 # Grant a user sudo.
 usermod -aG sudo <username>
+# Add the current user to a group (<group-name>).
+sudo usermod -aG <group-name> ${USER}
 ```
 
 ## Permissions
@@ -232,6 +239,8 @@ getent passwd
 compgen -g
 getent group
 
+# List all groups current user is in.
+groups
 # List all groups a user (username) is in.
 groups username