🐧 Linux

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Command-line utilities and Linux system administration. Covers file operations, text processing, systemd service management, networking, and useful tools for daily operations. For scripting and shell syntax, see the Bash cheatsheet.

Resources

Files & Archives

Common operations for working with files, directories, and compressed archives.

• Tar — extract / create:

  tar -xzvf archive.tar.gz          # extract
  tar -czvf archive.tar.gz dir/     # create
  tar -tzvf archive.tar.gz          # list contents

  (-x extract, -c create, -z gzip, -v verbose, -f file)
• Zip / Unzip:

  zip -r archive.zip directory/
  zip archive.zip *
  unzip archive.zip -d output_dir/

• Find files:
  find /home/user -name "*.zip"
find . -type f -mtime -7
  (-type f = files only; -mtime -7 = modified in last 7 days)

• Symlink:
  ln -s /path/to/target /path/to/link
  (e.g. to expose a Flatpak app to dmenu/rofi)

Text Processing

Tools for searching, transforming, and inspecting text files and command output. These are most useful when composed with pipes.

• Grep:
  grep "pattern" file.txt
grep -r "pattern" ./dir
  (-i case-insensitive, -n show line numbers, -l filenames only, -v invert match)

• Sed — find & replace:
  sed 's/old/new/g' file.txt
sed -i 's/old/new/g' file.txt
  (-i edits in place; omit to preview)

• Awk — print column:
  ls -l | awk '{print $9}'
awk -F: '{print $1}' /etc/passwd
  ($NF = last column; -F sets field separator)

• Cut — extract columns:
  cut -d',' -f1,3 file.csv
cut -c1-10 file.txt
  (-d delimiter, -f fields, -c character positions)

• Sort & Uniq:

  sort file.txt                  # sort alphabetically
  sort -n file.txt               # sort numerically
  sort -rn file.txt              # reverse numeric sort
  sort file.txt | uniq           # remove duplicate lines
  sort file.txt | uniq -c        # count occurrences
  sort file.txt | uniq -d        # show only duplicates

• Wc — count lines/words/chars:
  wc -l file.txt
ls | wc -l
  (-l lines, -w words, -c bytes)

• Tr — translate/delete characters:
  echo "hello" | tr 'a-z' 'A-Z'
echo "a::b::c" | tr -s ':' ','
  (-s squeezes repeated characters, -d deletes)

• Head / Tail:
  head -n 10 file.txt
tail -n 10 file.txt
tail -f app.log
  (-f follows the file live; useful for logs)

• Less:
  less large_file.log
  (/ to search, n/N to navigate matches, q to quit)

• Xargs — build commands from stdin:

  find . -name "*.log" | xargs rm
  find . -name "*.txt" | xargs grep "error"
  cat urls.txt | xargs -n1 curl -O    # -n1 = one arg per call

  (use -I{} for inline substitution: echo "world" | xargs -I{} echo "hello {}")
• Pipe & redirect:

  ls -l | grep ".zip"           # pipe stdout to grep
  ls > file_list.txt            # redirect stdout (overwrite)
  echo "new line" >> file.txt   # redirect stdout (append)
  command 2>&1 | tee out.log    # stderr to stdout, tee to file + screen

• Diff:
  diff file1.txt file2.txt
diff -u file1.txt file2.txt
  (-u unified format, easier to read; used by git diff)

systemd

systemd is the init system and service manager on most modern Linux distributions. systemctl manages units (services, timers, sockets); journalctl reads the unified log.

• Service lifecycle:

  sudo systemctl start nginx
  sudo systemctl stop nginx
  sudo systemctl restart nginx
  sudo systemctl reload nginx      # reload config without full restart

• Enable / disable on boot:

  sudo systemctl enable nginx      # start on boot
  sudo systemctl disable nginx     # don't start on boot
  sudo systemctl enable --now nginx  # enable + start immediately

• Status and inspection:

  systemctl status nginx
  systemctl is-active nginx        # exits 0 if active
  systemctl is-enabled nginx
  systemctl list-units --type=service --state=running

• Reload unit files after editing:
  sudo systemctl daemon-reload
  (required after creating or modifying a .service file)

• journalctl — view logs:

  journalctl -u nginx              # logs for a specific unit
  journalctl -u nginx -f           # follow live
  journalctl -u nginx --since "1 hour ago"
  journalctl -u nginx -n 50        # last 50 lines
  journalctl -p err                # errors only (emerg alert crit err warning notice info debug)
  journalctl --disk-usage          # how much space logs use

• Minimal unit file example:

  [Unit]
  Description=My App
  After=network.target
  
  [Service]
  ExecStart=/usr/bin/myapp --config /etc/myapp.conf
  Restart=on-failure
  User=myuser
  
  [Install]
  WantedBy=multi-user.target

  (place in /etc/systemd/system/myapp.service, then daemon-reload + enable)

Networking

Tools for managing network connections and monitoring traffic.

• Connect to WiFi (nmcli):

  nmcli device wifi list
  nmcli device wifi connect "SSID" password "PASSWORD" ifname wlp2s0

• Set display brightness (xrandr):
  xrandr --output HDMI-1 --brightness 0.8
xrandr --output eDP-1 --brightness 0.8

System Administration

Common administration tasks for managing services, permissions, and audio on Linux.

• PipeWire — set default audio output (Fedora):

  pactl list sinks short              # list available sinks
  pactl set-default-sink <sink_id>   # set default

• Expose Flatpak app to PATH (e.g. for dmenu/rofi):

  sudo ln -s /var/lib/flatpak/exports/bin/com.stremio.Stremio /usr/bin/stremio

• NGINX — fix file permissions:

  sudo chown -R www-data:www-data /var/www/example/
  sudo chmod -R 755 /var/www/example/

yt-dlp

Download audio and video from YouTube and other sites. Add --cookies-from-browser BROWSER if the site requires authentication.

• Best audio quality (Opus):

  yt-dlp --restrict-filenames \
      -P "~/Downloads/YouTube" \
      -o "%(title)s-%(id)s.%(ext)s" \
      -f "bestaudio" -x \
      --no-playlist --audio-format opus \
      <URL>

• Specific quality video + audio (1080p WebM):

  yt-dlp --restrict-filenames \
      -P "~/Downloads/YouTube" \
      -o "%(title)s-%(height)sp-%(id)s.%(ext)s" \
      -f "bestvideo[height=1080]+bestaudio" \
      --no-playlist --merge-output-format webm \
      <URL>

• Best video + audio quality (WebM):

  yt-dlp --restrict-filenames \
      -P "~/Downloads/YouTube" \
      -o "%(title)s-%(height)sp-%(id)s.%(ext)s" \
      -f "bestvideo+bestaudio" \
      --no-playlist --merge-output-format webm \
      <URL>