Operating Systems and Command-Line Concepts
Kernel, processes, files
The kernel runs in privileged mode and mediates hardware access; user programs reach it through system calls. A process is a running program with a PID and its own address space; threads share that space. On Linux, know the filesystem layout: /etc for configuration, /var for variable data and logs, /home for users, /tmp for scratch, /bin and /usr/bin for executables, /proc for kernel state. Boot order is firmware (BIOS/UEFI) → bootloader → kernel → init/systemd.
Permissions
Linux permissions are three triads - owner, group, other - with r=4, w=2, x=1. chmod 750 yields rwxr-x---. chown changes ownership. Processes are listed by ps aux (snapshot) or top (live); kill sends SIGTERM by default, kill -9 sends SIGKILL (no cleanup, last resort). systemctl manages services; crontab schedules jobs.
Windows and text tools
Windows: dir, ipconfig /all, ipconfig /flushdns, ping, tracert, nslookup, netstat -ano, tasklist, taskkill /PID, net user, whoami, sfc /scannow; PowerShell offers Get-Process and Get-Service. grep searches file contents, find searches the filesystem by attributes, which/locate find binaries. A pipe (|) chains commands; > overwrites, >> appends, 2> redirects stderr.
Traps
chmod 777 vs 755; expecting rm to delete a directory without -r; confusing grep with find; assuming top and ps give the same view.
Worked example
chmod 640 file → owner rw-, group r--, others ---. Rule of thumb: sum the triad values (4+2+1), and remember order is always owner, group, other.