linux.md

Linux

Shared Directories

Linux distributions typically follow the Filesystem Hierarchy Standard (FHS), which defines where different types of files are placed. Some common system-wide (shared) directories are:

• /etc: System-wide configuration files.
• /usr: Contains the majority of userland programs and data.
  • /usr/bin: Executables for most user programs.
  • /usr/sbin: Executables for system administration.
  • /usr/lib: Shared libraries and internal binaries used by programs in /usr/bin and /usr/sbin.
  • /usr/local: For programs installed locally (outside of the distribution’s package manager), typically in subdirectories like /usr/local/bin and /usr/local/lib.
  • /usr/share: Architecture-independent data such as icons, documentation, and locale files.
• /var: Variable data that changes during system operation.
  • /var/lib: State information and variable data for system programs.
  • /var/log: Log files.
• /opt: Optional or third-party software, often self-contained packages.
• /lib, /lib64: Core system libraries (for /bin and /sbin).
• $HOME/.config/autostart: Applications that run at startup for a specific user. Similar to systemctl, but autostart is for user-specific applications, whereas systemctl manages system-wide services.

When you install packages using a package manager like apt, the files typically go into these shared directories depending on their type:

• Executables usually end up in /usr/bin or /usr/sbin.
• Libraries go into /usr/lib.
• Configuration files go into /etc.
• Documentation and data files often end up in /usr/share.

These directories are accessible system-wide, so all users on the system can use the installed programs without additional per-user setup.

Cheat Sheet

File and Directory Management

Command	Description
ls	List files and directories in the current directory.
ls -l	List files with detailed information.
ls -a	List all files, including hidden ones.
cd [directory]	Change the current directory to the specified one.
pwd	Display the current working directory path.
mkdir [directory]	Create a new directory.
rmdir [directory]	Remove an empty directory.
rm [file]	Delete a file.
rm -r [directory]	Recursively delete a directory and its contents.
cp [source] [destination]	Copy files or directories.
mv [source] [destination]	Move or rename files or directories.
touch [file]	Create an empty file or update the timestamp of an existing file.
cat [file]	Display the contents of a file.
less [file]	View the contents of a file one screen at a time.
head [file]	Display the first 10 lines of a file.
tail [file]	Display the last 10 lines of a file.
diff [file1] [file2]	Compare two files line by line and display the differences.
readlink [link]	Display the target of a symbolic link.
readlink -f [path]	Display the absolute path, resolving all symbolic links.
ln [target] [link_name]	Create a hard link to a file.
ln -s [target] [link_name]	Create a symbolic (soft) link to a file or directory.

File Permissions and Ownership

Command	Description
chmod [permissions] [file]	Change the permissions of a file or directory.
chown [owner]:[group] [file]	Change the owner and group of a file or directory.
chgrp [group] [file]	Change the group of a file or directory.

Process Management

Command	Description
ps	Display information about active processes.
pgrep: ps + grep	Search for processes by name or other attributes and display their PIDs.
top	Display real-time system information, including active processes.
htop	Interactive process viewer (requires installation).
nvtop	Interactive monitor for NVIDIA GPUs (requires NVIDIA GPUs).
kill [PID]	Terminate a process by its Process ID (PID).
killall [process_name]	Terminate all processes with the specified name.
bg	Resume a suspended job in the background.
fg	Bring a background job to the foreground.

Disk Usage and Storage

Command	Description
df -h	Display disk space usage in human-readable format.
du -sh [directory]	Display the size of a directory and its contents.
mount [device] [mount_point]	Mount a device to the filesystem.
umount [device]	Unmount a device from the filesystem.

Networking

Command	Description
ifconfig	Display or configure network interfaces.
ip a	Display all network interfaces and their IP addresses.
ping [host]	Send ICMP ECHO_REQUEST packets to network hosts.
wget [url]	Download files from the internet.
curl [url]	Transfer data from or to a server.
ssh [user]@[host]	Connect to a remote host via SSH.

File Transfer

Command	Description
scp [source] [user@host:destination]	Securely copy files between hosts over a network.
rsync [options] [source] [destination]	Synchronize files and directories between two locations efficiently.

User Management

Command	Description
adduser [username]	Create a new user.
passwd [username]	Change the password for a user.
deluser [username]	Delete a user.
usermod -aG [group] [username]	Add a user to a group.

System Information

Command	Description
uname -a	Display all system information.
uname -r	Display the kernel version.
uptime	Show how long the system has been running.
date	Display or set the system date and time.
who	Show who is logged into the system.
whoami	Display the current logged-in user's username.

Package Management (Debian-based systems)

Command	Description
apt update	Update the package index.
apt upgrade	Upgrade all installed packages to their latest versions.
apt install [package]	Install a new package.
apt remove [package]	Remove an installed package.
apt search [package]	Search for a package in the repositories.

Text Processing

Command	Description
grep [pattern] [file]	Search for a pattern in a file.
sed 's/[old]/[new]/' [file]	Replace text in a file using stream editor.
awk '{print $1}' [file]	Pattern scanning and processing language.

Compression and Archiving

Command	Description
tar -cvf [archive.tar] [files]	Create a tarball archive of files.
tar -xvf [archive.tar]	Extract files from a tarball archive.
gzip [file]	Compress a file using gzip.
gunzip [file.gz]	Decompress a gzip compressed file.

This cheat sheet provides a quick reference to common Linux commands. For more detailed information, refer to the manual pages by typing command --help or man command in the terminal.

Examples

# Copy a local file to a remote host:
scp /path/to/local/file.txt user@remote_host:/path/to/remote/directory/

# Copy a file from a remote host to the local machine:
scp user@remote_host:/path/to/remote/file.txt /path/to/local/directory/

# Synchronize a local directory to a remote host:
# `-a`: Archive mode (preserves permissions, times, symbolic links, etc.).
# `-v`: Verbose output.
# `-z`: Compress data during transfer.
rsync -avz /path/to/local/directory/ user@remote_host:/path/to/remote/directory/

# Synchronize a remote directory to the local machine:
rsync -avz user@remote_host:/path/to/remote/directory/ /path/to/local/directory/

# Find the process ID(s) of a running program:
# pgrep -ifa porcesss_name: -i: ingore case, -f only process id, -a: full command, similar to ps aux | grep process name
pgrep process_name

# Find processes by user:
pgrep -u username

# Find differences
diff --color -U 0 file1 file2

# Display the absolute path
readlink -f $HOME

# To create a symbolic link named `my_link` that points to a file `myfile.txt`
ln -s myfile.txt my_link

# To unzip a file to a specific location
unzip filename.zip -d /path/to/destination