Linux (General)
General topics relating to Linux and Unix operating systems
Basic Commands Overview
| Command | Description | Package |
|---|---|---|
base64 |
base64 encode/decode data and print to standard output | coreutils |
basename |
strip directory and suffix from filenames | coreutils |
cat |
concatenate — combine the output of files and print to standard output | coreutils |
cd |
Change Directory — navigate directories on the filesystem | built-in |
chmod |
change file mode bits (permissions) for files and directories | coreutils |
chown |
change file owner and group | coreutils |
clear |
clear the terminal content | clear |
cp |
copy files | coreutils |
csplit |
split a file into sections determined by context lines | coreutils |
dd |
convert and copy a file | coreutils |
df |
Disk Free — report overall file system space usage | coreutils |
diff |
show differences between two files line by line | diffutils |
dirname |
print the directory path if the passed file | coreutils |
du |
Disk Usage — show file system usage for a given directory | coreutils |
echo |
display a line of text | built-in |
find |
search for files in a given directory | findutils |
grep |
search for text | grep |
head |
display the first X lines of a file (default 10) | coreutils |
history |
print the history of issued commands | built-in |
jobs |
display currently running jobs | built-in |
kill |
terminate a process | util-linux |
ls |
list directory contents | coreutils |
man |
Manual — display the documentation of a given command | man |
mkdir |
create a directory | coreutils |
mv |
move (and rename) files | coreutils |
ping |
check if a server is reachable | iputils |
printenv |
print information about currently set environment variables | coreutils |
printf |
format and print data | coreutils |
pwd |
Print Working Directory — returns the current working directory | built-in |
rm |
remove files and directories (caution: no undo!) | coreutils |
scp |
OpenSSH secure file copy | openssh |
sha256sum |
compute and check SHA256 message digest (variants for SHA1, SHA224, SHA384 and SHA512 exist) | coreutils |
sort |
sort lines of text files | coreutils |
sudo |
Super User Do — execute commands as root | sudo |
tail |
display the last X lines of a file (default 10) | coreutils |
tar |
archiving utility, compression optional | tar |
top |
display Linux processes | procps-ng |
touch |
create an empty file | coreutils |
uname |
Unix Name — print system information | coreutils |
unzip |
list, test and extract compressed files in a ZIP archive | unzip |
useradd |
create a new user or update default new user information | shadow |
userdel |
delete a user account and related files | shadow |
wget |
download files from the internet | wget |
zip |
package and compress (archive) files | zip |
Filesystem Hierarchy
The Filesystem Hierarchy Standard (FHS) is a set of guidelines that define the directory structure of a Linux operating system. This standard specifies the names and locations of directories, files, and other file system objects that are essential for the proper functioning of the Linux operating system.
| Directory | Description |
|---|---|
/bin |
binary files of basic commands |
/boot |
static files of the bootloader |
/dev |
device files |
/etc |
host-specific system configuration |
/lib |
basic dynamic libraries and kernel modules |
/media |
mount point for removable media |
/mnt |
for temporarily mounted file system |
/opt |
additional application programs |
/run |
relevant data for running processes |
/sbin |
essential binary files of the system |
/srv |
data for services |
/tmp |
temporary files |
/usr |
secondary hierarchy |
/var |
variable data |
Root Directory (/)
The root directory is the highest level of the file system hierarchy. It contains all other directories and files in the system. All directories and files in the system are located either directly or indirectly under the root directory.
/bin Directory
NOTE: On most modern Linux distributions, /bin is actually a symbolic link pointing to /usr/bin.
The /bin directory contains essential binary files that are required for the system to function properly. These files are used by all users of the system, and they include basic system commands such as ls, cp, and mv. The /bin directory must not contain any subdirectories. Each installed package places its main executables here.
/boot Directory
The /boot directory contains the files needed by the bootloader for the Linux kernel, including the kernel itself, initrd (initial RAM disk), and boot loader configuration files. The bootloader is the program that loads the operating system when the computer starts up. The files in /boot are essential for the bootloader to function properly.
/dev Directory
The /dev directory contains device files that represent hardware devices connected to the system. These files allow applications and users to interact with the hardware devices such as keyboards, mice, and disk drives using standard input/output operations.
There's different types of device files:
| Type | Description |
|---|---|
| Character device | Communication with a character device is based on the exchange of characters, such as individual bytes |
| Block device | Communication with a block device is based on the exchange of entire blocks of data |
| Pseudo device | Devices that are not corresponding to any real hardware actually present on the system |
Each device file in the /dev directory represents a specific hardware device. These files are created dynamically and disappear when the device is disconnected.
Some examples of device files:
| File | Description |
|---|---|
/dev/dm-X |
Logical volumes created using LVM (device manager).
|
/dev/hdX |
IDE hard drives or other storage devices. Common in older systems.
|
/dev/hdXY |
Partitions on IDE storage device.
|
/dev/jsX |
Device file for a joystick/gamepad.
|
/dev/mdX |
Software RAID devices (meta disk).
|
/dev/mmcblkX |
MMC/SD cards and eMMC storage devices.
|
/dev/nvmeXnY |
NVMe solid-state drives.
|
/dev/nvmeXnYpZ |
Partitions on NVMe storage device.
|
/dev/null |
Pseudo-device that always outputs EOF (end of file). Output redirected to it is immediately discarded. |
/dev/random |
Pseudo-device that generates random data (blocking). |
/dev/sdX |
SCSI or SATA hard drives and other storage devices. Common in modern systems.
|
/dev/sdXY |
Partitions on SCSI or SATA storage device.
|
/dev/srX |
CD-ROM or DVD-ROM drives.
|
/dev/ttyX |
Device file for the terminal.
|
/dev/urandom |
Pseudo-device that generates random data (non-blocking) |
/dev/vdX |
Virtual storage device.
|
/dev/vdXY |
Partitions on a virtual storage device.
|
/dev/videoX |
Device file for a webcam.
|
/etc Directory
The /etc (editable text configuration) directory contains system configuration files. These files are used to configure the system and its applications. Examples of files in this directory include passwd and group files, which contain user and group information, and fstab file, which contains information about file systems that are mounted at boot time. The /etc directory is one of the most important directories in the system, as it contains configuration files that enable system administrators to customize the system to their needs.
Some examples of what might be of particular interest:
| Directory/File | Description |
|---|---|
/etc/opt/ |
Configuration files for software installed to /opt. |
/etc/security/ |
Configuration files for PAM (Pluggable Authentication Modules). |
/etc/skel/ |
Configuration files that are copied to a newly created user's /home directory. |
/etc/X11/ |
Configuration files for the X Window System which provides a graphical user interface for Linux. |
/etc/crontab |
Configuration file uniquely formatted to automate system tasks on a set schedule. |
/etc/fstab |
Contains information about file systems and partitions automatically mounted at system startup. |
/etc/group |
Contains information about user groups, including group names and group IDs. |
/etc/hosts |
Configuration file that maps hostnames to IP addresses. |
/etc/logrotate.conf |
Configuration file for the logrotate utility, managing system log files. |
/etc/passwd |
Stores user account information such as usernames, user IDs, home directories, and login shells. |
/etc/profile |
Contains system-wide environment variables and other startup scripts. |
/etc/resolv.conf |
DNS resolver file which specifies how the system leverages DNS to resolve hostnames. |
/etc/shadow |
Contains encrypted user passwords and other password-related information. |
/etc/ssh/sshd_config |
Configuration file for the SSH server, which allows secure remote access to the system. |
/etc/sudoers |
Contains rules for allowing or denying sudo privileges to users and groups. |
/home Directory
The /home directory contains the personal files for all users in the system. Each user has a subdirectory of their name in the /home directory that contains their documents, pictures, music, and individual application settings. Each user's home directory is private, and other users cannot access the files in that directory without the user's permission.
NOTE: A user's personal home directory can be shortened to ~.
NOTE: Directories and files starting with a period . are considered hidden.
Some examples for directories/files in a user's home directory could include:
| Directory/File | Description |
|---|---|
~/.config/ |
Contains configuration files for applications that are specific to a user. |
~/.gnupg/ |
Contains user-specific configuration files for GnuPG (GNU Privacy Guard). |
~/.local/ |
Contains user-specific data files for applications that are installed system-wide. |
~/.mozilla/ |
Contains user-specific configurations and settings for Firefox. |
~/.ssh/ |
Contains user-specific configuration files and private/public keys for SSH (Secure Shell) connections. |
~/.bashrc, ~/.zshrc |
Contains user-specific configurations for the user's terminal emulator. |
/lib Directory
NOTE: On most modern Linux distributions, /lib is actually a symbolic link pointing to /usr/lib.
The /lib directory (or /lib64 for 64-bit systems) is a crucial component of the operating system. It contains shared libraries that are required by programs in /bin and /sbin directories, as well as other applications. These libraries provide functionality for many basic system operations, such as networking, file handling, and encryption. Without these libraries, the system would not be able to perform many of its fundamental functions. It is important to ensure that the files in /lib are always up-to-date and compatible with the system's other components to guarantee smooth operation.
In addition to the shared libraries, the /lib directory contains other important files, such as kernel modules (/lib/modules/) and firmware files (/lib/firmware/). Kernel modules are small programs that can be loaded and unloaded dynamically into the kernel without having to reboot the system. These modules provide additional functionality to the system, such as support for new hardware or file systems. Firmware files, on the other hand, contain low-level software that is used to control hardware devices, such as network adapters (especially WiFi) or graphics cards.
| Directories/Files | Description |
|---|---|
/lib/firmware/ |
Contains firmware files needed by devices to function properly, e.g. WiFi adapters and graphics cards. |
/lib/security/ |
Contains authentication modules for PAM (Pluggable Authentication Modules). |
/lib/systemd/ |
Contains files for the systemd system and service manager. Also contains systemd unit files. |
/media Directory
The /media directory contains mount points for removable media devices such as USB drives and CD/DVDs. When a removable device is mounted, its contents appear in a subdirectory under the /media directory. The /media directory is used to manage removable media devices, and it allows users to access the files on those devices as if they were part of the file system.
/mnt Directory
The /mnt directory is used to temporarily mount file systems. This directory is typically used by system administrators when they need to mount a file system for a short period of time. The /mnt directory is used to access file systems that are not part of the main file system, such as network file systems or file systems on removable media devices.
/opt Directory
The /opt directory is used for optional software packages that are not part of the main operating system. Examples of software that might be installed in the /opt directory include proprietary software packages or third-party software that is not included with the distribution's main software repositories.
Examples of software you might see in this directory:
| Directory/File | Description |
|---|---|
/opt/1Password/ |
Password manager from Agile Bits |
/opt/discord/ |
Discord VoiP client |
/opt/google/ |
Google Chrome web browser |
/opt/quake3/ |
Quare 3 Arena from id Software |
/opt/spotify/ |
Spotify music streaming client |
/opt/visual-studio-code/ |
Visual Studio Code editor from Microsoft |
/proc Directory
The /proc directory is a virtual directory that contains information about running processes and system resources. This directory provides a way for processes to communicate with the kernel and allows system administrators to monitor system performance. The files in the /proc directory are not actual files, but rather a virtual representation of system resources. The /proc directory is used by system administrators to monitor system performance and diagnose system problems.
| Directory/File | Description |
|---|---|
/proc/cpuinfo |
Contains information about the CPU, e.g. model name, number of cores, and clock speed. |
/proc/meminfo |
Contains memory usage information including swap storage use. |
/proc/modules |
Contains a list of all the modules being used by the kernel. |
/proc/swaps |
Contains information only about swap storage. |
/proc/sys |
Contains files that are used to manage kernel parameters. |
/proc/version |
Contains Linux version information. |
/root Directory
The /root directory is the home directory for the root user. This directory contains the root user's personal files and settings. The root user is the most powerful user on the system, and has access to all files and directories on the system.
/run Directory
The /run directory is a unique and important directory in the Linux operating system that contains runtime data that is used by the system and applications. This data is volatile as it is mounted as a tmpfs filesystem that is only stored in system memory and its contents are lost when the system is rebooted.
The data stored in the /run directory is updated frequently and automatically by the system and applications. The directory is also used as a mount point for systemd, a system and service manager that is used in most modern Linux distributions. Systemd is responsible for managing system services, and it also uses the /run directory to store runtime data for these services.
The /run directory is also used for user mounted storage mediums like CDs, DVDs, USB drives and network shares.
/sbin Directory
NOTE: On most modern Linux distributions, /sbin is actually a symbolic link pointing to /usr/bin.
The /sbin directory contains essential system binaries that are used for system administration tasks, such as configuring the network or managing user accounts. These files are typically used by system administrators and are not intended for everyday use.
/srv Directory
The /srv directory is used to store data that is served by the system, and it is typically used by web servers, FTP servers, and other server applications.
Other directories that are used for different purposes include:
| Directory/File | Description |
|---|---|
/srv/ftp/ |
Contains files that an FTP server uses to store all of the files that can be accessed by the FTP server. |
/srv/www/, /srv/http/ |
Contains files that are served by a web server (sub-directory is distribution specific). |
/sys Directory
The /sys directory is a fundamental component of the Linux filesystem hierarchy and an essential tool for hardware management and configuration. This directory is structured in such a way that each hardware device is represented by a directory tree and a set of files, providing users with detailed information about the device's properties and allowing them to adjust, among other things, power management settings, device parameters, and monitor the device's status. The files in the /sys directory are not actual files, but rather a virtual representation of hardware devices. They are re-created upon every boot of the system and mounted as a virtual sysfs filesystem.
/tmp Directory
The /tmp directory is used for temporary files. This directory is typically used by applications to store non-critical files and it is typically cleaned out on a regular basis to prevent the accumulation of unnecessary files. It is writable to any user and is ideal for using it as a scratchpad type of directory. The /tmp directory is mounted as a tmpfs type of filesystem, which means its contents are stored in the system's memory for fast access. However, this also means that it is generally not recommended putting large files there as to not exhaust the system's memory with large amoounts of data.
/usr Directory
The /usr (Unix System Resources) directory is used to store programs, libraries, documentation and data that are not part of the core operating system, but that are still important for the system to function. Examples of programs that might be installed in the /usr directory include text editors, email clients, and games.
Some other examples of contents of the /usr directory:
| Directory/File | Description |
|---|---|
/usr/bin/ |
Contains executable binaries of applications |
/usr/bin/gcc/ |
Binaries for compiling all sorts of source code of different programming languages |
/usr/include/ |
Source code header files for including functionality of one application into another during development |
/usr/lib/ |
Shared system libraries, e.g. libc.so (C library), libssl.so (SSL library) |
/usr/lib/systemd/ |
Systemd service unit files |
/usr/local/ |
Typically used to install applications compiled from source, otherwise goes largely unused |
/usr/share/ |
Architecture-independent data files, e.g. game assets, icons, cursors, desktop themes, wallpapers, etc. |
/usr/share/doc/ |
System documentation |
/usr/share/locale/ |
Translations for applications |
/usr/share/man/ |
Application manual pages |
/usr/src/ |
Linux kernel source code files |
/var Directory
The /var directory contains variable data files. These files are typically files that change frequently, such as log files, locally delivered mail and spooler files for printer job queues.
Some other examples of contents of the /var directory:
| Directory/File | Description |
|---|---|
/var/cache/ |
Frequently accessed data, e.g. browser cache and downloaded software packages |
/var/lib/ |
Varying application data used by system libraries (e.g. /var/lib/libvirt/images/ for libvirt virtual machine disk images) |
/var/log/ |
Log files of all sorts of different applications |
/var/mail/ |
Locally delivered mail messages |
/var/mysql/ |
MySQL database files |
/var/spool/ |
Printer job queue |