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LINUX CLI CHEATSHEET
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BASIC NAVIGATION
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pwd
  Print Working Directory - shows where you are.

ls
  List files in current directory.

ls -l
  Long format (permissions, owner, size, date).

ls -a
  Show ALL files, including dotfiles (.bashrc, .git, etc.).

ls -lah
  l = long, a = all, h = human file sizes (KB/MB/GB).

cd /path/to/dir
  Change directory to /path/to/dir.

cd ..
  Go up one directory.

cd ~
  Go to your home directory.

cd -
  Go back to the previous directory (like a back button).


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WORKING WITH FILES AND DIRECTORIES
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mkdir NAME
  Make a directory called NAME.

mkdir -p path/to/nested/dir
  Make nested directories; "-p" creates parents as needed.

touch file.txt
  Create an empty file (or update its timestamp).

cp source dest
  Copy file from source to dest.

cp -r dir1 dir2
  Copy directory dir1 to dir2 (recursive).

mv old new
  Move or rename a file or directory.

rm file.txt
  Remove file.txt.

rm -r dir
  Recursively delete directory "dir" and its contents.
  DANGEROUS: there is no recycle bin.

rm -rf dir
  r = recursive, f = force (no prompts).
  VERY DANGEROUS - double-check before running.

cat file.txt
  Print the file to the terminal.

less file.txt
  View file one screen at a time, scroll with arrows, q to quit.

head file.txt
  Show first 10 lines.

tail file.txt
  Show last 10 lines.

tail -f logfile.log
  Follow a log file as it grows. Ctrl+C to stop.


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GLOBBING / WILDCARDS (PATTERNS)
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*  Matches any number of characters (including none)
   Example:  *.php   -> all .php files

?  Matches exactly one character
   Example: file?.txt -> file1.txt, fileA.txt, but NOT file10.txt

[abc]  Matches one character in the set
       Example: file[12].txt -> file1.txt, file2.txt

[0-9]  Range: any digit
       Example: file[0-9].txt -> file0.txt ... file9.txt


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REDIRECTION AND PIPES (|, >, >>, 2>&1)
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command > file
  Redirect STDOUT (normal output) to a file (overwrite).

command >> file
  Append STDOUT to the file (do not overwrite).

command < file
  Use "file" as STDIN (input) to the command.

command 2> errors.log
  Redirect STDERR (error output) to errors.log.

command > output.log 2>&1
  Redirect BOTH normal output and errors to output.log.

Breakdown of "2>&1":
  2  = file descriptor 2 (STDERR)
  >  = redirect
  &1 = to the same place as 1 (STDOUT)

command1 | command2
  Take output of command1 and feed it as input to command2.
  Example:   ps aux | grep apache2


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PROCESSES AND SYSTEM INFO
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ps aux
  Show all processes.

ps aux | grep NAME
  Find processes with NAME in their line.

top
  Live view of CPU and memory usage and processes.
  (q to quit)

htop   (if installed)
  Nicer version of top with colors and better UI.

btop   (if installed)
  System monitor with CPU, RAM, disks, etc.

kill PID
  Ask process PID to terminate (polite).

kill -9 PID
  Force kill (last resort).

df -h
  Disk free space (human readable).

du -sh *
  Show size of directories/files in current folder.

free -h
  Memory usage (human readable).

uptime
  How long system has been running and load averages.


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HANDLING SYSTEM FREEZES
(TTY, GNOME SHELL, SAFE REBOOT)
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Ctrl+Alt+F3   (or F4, F5)
  Switch from frozen desktop (GUI) to a text console (TTY).
  Log in with your user (jcc) and password.

Ctrl+Alt+F1 or Ctrl+Alt+F2
  Switch back to the graphical desktop (GNOME) from a TTY.
  (Exact key can vary; try F1 then F2.)

# Find what is misbehaving
top
  Basic live process view (CPU%, MEM%, etc.).
htop
  Nicer process view (if installed). Look for processes pegging CPU
  or eating tons of RAM.

# Kill an offending GUI app from TTY
pkill soffice.bin
  Kill LibreOffice (Calc/Writer/etc.) if it has frozen the desktop.

killall -9 PROGRAM
  Force kill a specific program by name.
  Example: killall -9 chrome

# Restart GNOME Shell without reboot (Xorg sessions)
# (Run this from a TTY when the desktop is frozen.)
export DISPLAY=:0
gnome-shell --replace &
  Restart the GNOME Shell compositor. On Xorg this can revive
  a frozen desktop without reboot. If on Wayland, you usually
  need to log out / back in instead.

# Clean reboot from TTY
sudo reboot
  Reboot the system cleanly (preferred over holding power button).

sudo poweroff
  Power the system down cleanly.

# Emergency "magic" reboot when EVERYTHING is stuck (REISUB)
Alt+SysRq+R E I S U B
  Hold Alt + SysRq (Print Screen), then slowly press:
    R  - take keyboard control back
    E  - send TERM to processes
    I  - send KILL to remaining processes
    S  - sync disks
    U  - remount disks read-only
    B  - reboot
  Safer than just yanking power, but you may still lose unsaved work.


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USER AND PERMISSIONS (chmod, chown)
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whoami
  Show current user.

id
  Show your user id, group id, etc.

sudo command
  Run command as root (superuser).
  WARNING: you can break things; think before running.

chmod 755 file
  Set permissions (rwxr-xr-x).
  7 = read + write + execute
  5 = read + execute

chmod +x script.sh
  Make script executable.

chown user:group file
  Change owner and group of a file.


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APT PACKAGE MANAGEMENT
(Ubuntu / Pop!_OS / Raspberry Pi OS)
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sudo apt update
  Refresh package lists.

sudo apt upgrade
  Upgrade installed packages.

sudo apt install package
  Install a package.

sudo apt remove package
  Remove a package.

sudo apt autoremove
  Remove unused dependencies.


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SSH AND SERVER MANAGEMENT
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ssh user@host
  Connect to remote machine (host can be IP or domain).

ssh -p PORT user@host
  Connect using a non-default port.

scp file user@host:/path
  Copy a file to a remote host (over SSH).

scp user@host:/path/file .
  Copy file from remote host to current directory.

sudo systemctl status service
  Show status of a service (example: apache2, ssh).

sudo systemctl restart service
  Restart a service.

sudo systemctl stop service
  Stop a service.

sudo systemctl start service
  Start a service.

journalctl -u servicename
  View logs for a specific service.

journalctl -u servicename -f
  Follow logs in real time (like tail -f).


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GREP / RIPGREP (SEARCHING INSIDE FILES)
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grep "text" file
  Search for "text" inside file.

grep -R "text" /path
  Recursive search for "text" under /path.

grep -R --line-number "text" /path
  Show line numbers too.

rg "text"
  If ripgrep (rg) is installed:
  Fast recursive search for "text" from current directory.

rg "text" path/
  Search only under "path/".


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FIND / FD-FIND (SEARCH FOR FILES)
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find . -name "*.php"
  Find all .php files under current directory.

find /var/www -type f -name "index.*"
  Find all regular files named index.* under /var/www.

fd pattern
  If fd-find (fd) is installed:
  Simpler, faster file search.
  Examples:
    fd index .
      Find "index" in current tree.
    fd ".php$"
      Find PHP files by regex.


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GIT BASICS (FOR YOUR TIMEcard PROJECT)
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git status
  Show changed files, branch, etc.

git add file
  Stage a specific file.

git add .
  Stage all changes in current directory and below.

git commit -m "message"
  Create a commit with message.

git push
  Push commits to remote repository.

git pull
  Fetch and merge latest changes from remote.

git log --oneline --graph --decorate
  Pretty log view with branches.


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BASH HISTORY AND SHORTCUTS (!, !$, !!, etc.)
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Up Arrow / Down Arrow
  Scroll through command history.

Ctrl+R
  Reverse search through history.
  Start typing part of a previous command and it finds matches.

history
  Print your command history with numbers.


!!     (double bang)
  Repeat the ENTIRE previous command.
  Example:
    apt install somepackage    (fails with permission)
    sudo !!                    (runs: sudo apt install somepackage)

Breakdown:
  !  = pull from history
  !! = previous command again


!$     (bang-dollar)
  Use LAST ARGUMENT of previous command.
  Example:
    sudo mkdir MyNewFolder
    cd !$

What actually runs:
  cd MyNewFolder

Breakdown:
  ! = pull from history
  $ = the last argument


!*     (bang-star)
  Use ALL arguments of previous command.
  Example:
    cp file1.txt file2.txt backup/
    ls !*

What actually runs:
  ls file1.txt file2.txt backup/


!string
  Run the most recent command that starts with "string".
  Example:
    systemctl restart apache2
    ...
    !sys
  This re-runs "systemctl restart apache2".


!?string?
  Run the most recent command that CONTAINS "string".
  Example:
    sudo nano /etc/apache2/sites-available/000-default.conf
    ...
    !?apache2?
  Runs that nano command again.


Alt+.
  Insert the last argument from the previous command into the current prompt.
  Example:
    sudo mkdir some-long-folder-name
    cd <Alt+.>
  Becomes:
    cd some-long-folder-name

  Press Alt+. repeatedly to cycle through last args from earlier commands.


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COMMON CONTROL KEYS (TERMINAL)
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Ctrl+C
  Send interrupt signal to the running process (stop it).

Ctrl+Z
  Suspend the current process (background it).
  Then you can use "fg" to bring it back.

fg
  Bring the last suspended job to the foreground.

bg
  Continue the last suspended job in the background.

Ctrl+D
  End-of-input (log out of a shell, or end input to a program).

Tab
  Auto-complete file and command names.
  Double Tab to show possible completions.