Technical - Linux/OS Flashcards
Hard vs Soft/Sym Link (+how to create)
Hard Link:
Definition: Filesystem entry pointing directly to the inode of a file.
Deletion Behavior: No problem
Creation: ln <source></source> <target></target>
Soft/Symbolic Link:
Definition: File system entry that creates a new inode that references the path to the target file.
Deletion Behavior: Broken link
Creation: ln -s <source></source> <target></target>
File Descriptor (Explain stdout, stderr, and stdin)
Definition: An integer that uniquely identifies an open file in a process.
Standard Descriptors:
stdin (0): Standard input.
stdout (1): Standard output.
stdout (2): Standard error.
Process vs Thread
Process:
Definition: An independent program in execution, with its own memory space.
Resource Sharing: None, separate memory space and resources.
Thread:
Definition: A smaller unit of execution within a process.
Resource Sharing: Separate registers and stack, but shares memory space.
Linux Boot Process (BIOS > MBR > GRUB/LILO > Kernel > Init System)
BIOS: Initializes hardware, loads and executes bootloader from MBR (Master boot record).
GRUB/LILO: Bootloader(GRUB) loads and executes the Linux kernel and mounts initial RAM disk
Kernel: Initializes system hardware and mounts the root filesystem.
Init System: Starts user-space processes (e.g., systemd, SysVinit).
Full Disk Encryption LUKS (TPM Integration)**
LUKS: Linux Unified Key Setup; provides full disk encryption.
TPM: Trusted Platform Module; stores encryption keys securely.
Integration: LUKS can be used with TPM to protect encryption keys against tampering.
Secure Boot
Purpose: Ensures that only signed and authorized software is loaded during boot.
Mechanism: Checks digital signatures of bootloaders and kernels to prevent unauthorized modifications.
Inode (How to check inode number and inode usage)
Index Node:
Definition: Data structure containing metadata about a file (e.g., size, permissions), but not file name or contents.
Check: ls -i <file> shows the inode number. df -i shows inode usage.</file>
Zombie Process
Definition: A process that has completed execution but still has an entry in the process table.
Cause: Parent process has not yet read the exit status.
Check: ps aux | grep Z
Iptables vs UFW**
Definition: User-space utility for configuring Linux kernel firewall.
Usage: iptables -A INPUT -p tcp –dport 22 -j ACCEPT (Allow SSH)
DF vs DU
DF: Shows filesystem disk space usage.
Command: df -h
DU: Shows disk usage of files and directories.
Command: du -sh <directory></directory>
SELinux**
Definition: Security-Enhanced Linux; provides mandatory access controls.
Commands:
Check Status: sestatus
Manage Policies: semanage, setenforce
How to check disk space, memory, cpu, and io
Disk Space: df -h
Memory: free -h
CPU: top or htop
IO: iostat (requires sysstat package)
How to check open files (+why significant)
Command: lsof
Significance: Identifies files and sockets opened by processes, useful for troubleshooting and security.
How to check open ports on local system
netstat -tuln
How to scan external system for open ports (+flags)
nmap -p <port-range> <target></target></port-range>
Systemd (How to create, status, stop, start)
Create Service: Write a .service file in /etc/systemd/system/
Status: systemctl status <service>
Start: systemctl start <service>
Stop: systemctl stop <service></service></service></service>
How to trace a network path from source to target
traceroute <destination></destination>
How to lookup IP for DNS record and reverse
DNS Record: dig <domain> or nslookup <domain>
Reverse Lookup: dig -x <IP> or nslookup <IP></IP></IP></domain></domain>
Linux Filesystem Architecture
Standard Dirs:
/root - home dir for root user
/bin - essential user command binaries
/boot - static files of boot loader
/dev - rep hardware devices
/etc - system-wide config
/home - user home dirs
/lib - shared libraries and kernel modules
/media - mount point for removable media
/mnt - mount point for temp mounted fs
/sbin - system bins
/srv - data for system services
/opt - add-on optional application pkgs
/root - Home dir for root user
/tmp - temporary fs
/usr - user-related programs
/var - variable data files w/frequent change
Special FSs:
/proc - virtual fs providing process and kernel info
/sys - virtual fs providing info and control over kernels devices and modules
/dev/shm - Temp file storage for inter-process comms
Linux Kernel Architecture**
Monolithic Kernel: A single large kernel that includes the core functionalities and all device drivers.
System Call Interface: Provides the interface between user-space applications and kernel services.
How to configure basic LAN:
Use netplan or ip commands to configure interface, addresses, gateway, and nameservers.
How to check running processes (+how to kill)
Running Processes: ps aux or top
Kill Process: kill <PID> or kill -9 <PID> (force kill)</PID></PID>
/proc**
/sys**
