Ubuntu-L4-Lieutenant

Debugging & Inspection

strace — Trace system calls of a process

Traces system calls made by a process. Essential for low-level debugging.

strace ls

lsof — List open files and network connections

Lists open files and network sockets. Used to find port and file locks.

lsof -i :8080

watch -n — Re-run a command at intervals

Executes a command repeatedly at intervals. Useful for live system monitoring.

watch -n 2 free -h

time — Measure command execution time

Measures real, user, and system execution time. Used for performance profiling.

time python script.py

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Networking & Remote Access

ssh — Secure remote login

Securely logs into a remote system. Primary tool for server access.

$ ssh user@server_ip

scp — Copy files over SSH

Copies files securely over SSH. Simple but less efficient than rsync.

$ scp file.txt user@server:/path

rsync — Fast file synchronization

Synchronizes files efficiently. Transfers only changed data.

$ rsync -av src/ dest/

nc (netcat) — Network testing and debugging

Reads and writes raw network data. Used for port testing and debugging.

$ nc -zv localhost 80

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Permissions & Security

getfacl — View Access Control Lists

Displays Access Control Lists. Shows fine-grained permissions.

$ getfacl file.txt

setfacl — Set Access Control Lists

Modifies Access Control Lists. Adds user-specific permissions.

$ setfacl -m u:user:r file.txt

id — Show user/group IDs

Displays user and group IDs. Used to verify privilege context.

$ id

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Filesystem & Storage

mountpoint — Check if directory is mounted

Checks if a directory is a mount point. Useful in scripts and safety checks.

$ mountpoint /mnt

sync — Flush write buffers

Flushes write buffers to disk. Prevents data loss.

$ sync

fallocate — Preallocate file space

Preallocates space for a file. Common in storage testing.

$ fallocate -l 1G test.img

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Logs & Monitoring

journalctl — Query system logs

journalctl Queries systemd logs. Primary log inspection tool.

$ journalctl -xe

dmesg — Kernel ring buffer messages

Displays kernel messages. Used for hardware debugging.

$ dmesg | less

vmstat — Memory statistics

Reports memory and CPU stats. Useful for performance monitoring.

$ vmstat 2

iostat — Disk I/O stats (install first)

Shows disk I/O statistics. Helps detect bottlenecks.

$ sudo apt install sysstat

$ iostat

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Shell Power Tools

xclip — Clipboard access from terminal

Copies terminal output to clipboard. Bridges CLI and GUI workflows.

$ echo "text" | xclip -selection clipboard

column — Align output into columns

Aligns text into columns. Improves readability.

$ column -t data.txt

John   Smith  25
Alice  Brown  30
Bob    Lee    22

select — Bash menu creation

Creates interactive shell menus. Useful for scripts.

$ select opt in start stop exit; do echo $opt; break; done

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User & System Control

loginctl — Manage user sessions

Manages user sessions. Used in systemd environments.

$ loginctl list-sessions

ulimit — Control resource limits

Sets resource usage limits. Prevents resource exhaustion.

$ ulimit -n

hostnamectl — Manage system hostname

Views or sets system hostname. Updates system identity.

$ hostnamectl set-hostname mymachine

poweroff — Shut down system

Shuts down the system safely. Requires root access.

$ sudo poweroff

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