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Most of my Linux knowledge was picked up by doing, not by studying. For years I glued things together with searches, copy-pasted commands, and more recently AI assistance, and that was usually enough to ship whatever sat in front of me. My day job never forced a deeper return to the fundamentals either: servers mostly worked, containers hid the rest, and anything unfamiliar could be worked around without truly understanding it.

Revisiting a flashcard set recently showed how much of the basics had slipped. I recognised every topic but could not always explain it cleanly without guessing, especially around permissions, signals, and the filesystem layout. Nothing here is a war story or a production outage tale; it is simply the gap between using Linux every day and being able to explain it. So these are my revision notes: the same twenty questions, answered compactly in my own words as a personal checkpoint, not as a tutorial and not as a study app.

Filesystem layout

Where do the main directories live? /bin holds essential user commands, /etc holds system configuration, /home holds user home directories, /var holds variable data such as logs and mail, /tmp holds temporary files, /usr holds secondary user programs and libraries, and /opt holds optional add-on software. On most current distributions /bin is a symlink to /usr/bin after the usrmerge, so the split matters more for the standard than for daily navigation.

Permissions and ownership

What does rwx mean, and what is rwxr-xr-- in numbers? r, w, and x stand for read (4), write (2), and execute (1), repeated in three sets for owner, group, and others. rwxr-xr-- therefore means the owner can do everything, the group can read and execute, and others can only read, which is 754.

How do I read a chmod command? Numeric mode sets all three sets at once, as in chmod 754 file, while symbolic mode adjusts one class at a time with u, g, o, or a combined with +, -, or =. chmod g+w,o-r file adds group write and removes other read while leaving everything else untouched.

What does chown change? chown user file changes the owner, chown :group file changes only the group, and chown user:group file changes both at once. Changing ownership normally requires root or sudo, which is why day-to-day fixes tend to be chown plus a chmod correction.

Processes and services

What is a process? A process is a running program with its own memory and process ID (PID), started as a child of another process except for the first one. PID 1 is the init system, which is systemd on most current mainstream distributions.

How do I list processes with ps? ps aux uses BSD-style options to show every process with user, CPU, and memory columns, while ps -ef shows the same idea in POSIX style with full command lines. ps -u user filters by owner, and piping any of these into grep narrows the list to one program.

What is top for? top shows a live view of running processes with CPU and memory usage plus the load average, refreshed every few seconds. Inside it, q quits, k prompts for a PID to kill, and M sorts by memory usage.

Which signals should I actually remember? SIGTERM (15) asks a process to terminate cleanly, SIGKILL (9) forces it to stop without cleanup and cannot be caught, SIGHUP (1) historically meant hang-up and now commonly asks daemons to reload, and SIGINT (2) is what Ctrl+C sends. The safe habit is kill PID or kill -15 first and kill -9 only when the process refuses to exit.

How do I manage a service with systemctl? start, stop, restart, and status control or inspect a service right now, while enable and disable control whether it starts at boot. journalctl -u name.service then shows that service’s logs, which is usually the next step after a failed restart.

Shell, pipes, and text search

How do I search text with grep? grep -r searches recursively through directories, -i ignores case, -n prints line numbers, and -v inverts the match to show lines that do not contain the pattern. The pattern itself is a regular expression, so quoting it keeps the shell from expanding characters first.

How do pipes and redirects fit together? | sends one command’s output into the next command’s input, > writes output to a file by truncating it, >> appends instead, and < feeds a file as input. 2> redirects error output, &> redirects output and errors together in Bash, and 2>&1 merges errors into the standard output stream so both travel through the same pipe or file.

How do I find files with find? -name matches by filename pattern, -type f limits results to regular files, -mtime -7 limits to files modified in the last seven days, and -size +100M limits to files larger than 100 MB. -delete removes matches directly, so I always run the same command without it first to see what would be deleted.

Storage and archives

How do I pack and unpack with tar? tar -czf archive.tar.gz dir/ creates a gzip-compressed archive, tar -xzf archive.tar.gz extracts one, and tar -tzf archive.tar.gz lists the contents without extracting. Adding v, as in -czvf, only makes the operation verbose.

What is the difference between df and du? df reports free and used space per mounted filesystem, which answers “which disk is full”, while du estimates space used by files and directories, which answers “which folder is big”. Both accept -h for human-readable sizes, and du -sh dir gives a single total for one path.

Remote access and environment

How do I connect and set up SSH keys? ssh user@host opens a remote shell, defaulting to port 22 unless the server or client config changes it. ssh-keygen creates a key pair, ssh-copy-id user@host installs the public key on the server, and entries in ~/.ssh/config save per-host names, users, keys, and ports.

How do environment variables work? env lists the current variables, echo $VAR prints one value, and export VAR=value sets one for the current shell and its children. ~/.bashrc is where interactive Bash shells pick up repeated settings, and the ones I touch most are PATH, HOME, USER, and SHELL.

How do I read a cron entry? crontab -e edits the current user’s schedule and crontab -l lists it. Each line has five time fields — minute, hour, day of month, month, and day of week — followed by the command, with shorthands such as @reboot, @daily, and @weekly available for common cases.

Which package manager do I use where? Debian and Ubuntu use apt, Fedora and current Red Hat systems use dnf with yum kept as a compatibility alias, and all of them resolve dependencies automatically. snap and flatpak sit alongside these as distribution-independent formats with stronger application sandboxing.

Where do I look when something breaks? Plain text logs live under /var/log, with exact filenames varying by distribution — for example syslog and auth.log on Debian-family systems versus messages and secure on Red Hat-family ones. On systemd systems journalctl queries the journal instead, and tail -f follows a growing file in real time.

What is the difference between a symlink and a hard link? ln -s target link creates a symbolic link, a small pointer to another path that can cross filesystems but breaks when the target moves. ln target link creates a hard link, a second name for the same inode on the same filesystem that keeps working until every name is removed.

What this changed

This round showed me which basics I can explain and which ones I only recognise. Going forward I am keeping three small habits:

  • Re-reading permission and redirect forms before pasting them from memory.
  • Checking systemctl status plus journalctl -u before assuming a config edit failed.
  • Using find with a dry run first whenever -delete is involved.

These notes revise the Linux Fundamentals flashcards from devops-daily; the answers above are rewritten in my own words.