Linux Basics
File Structure
Linux uses the filesystem hierarchy standard which defines the way directories/folders are laid out.
It’s important to understand this structure to understand how the operating system works.
The top most directory is called the root directory(/).
Since the file system is organised in a hierarchy, every file and folder is located under the root directory. Here’s the most common layout of the root directory:
/bin- contains programs in the form of binary files that user can run/boot- contains the files needed to start the system/dev- used to represent devices(mostly virtual) that correspond to particular functions/etc- contain configuration files for services on a computer and some system files/usr- contain executable files(programs) for most system programs/home- contains home directories for personal users/lib- contain libraries(which contain extra functions) that are used by executable files in the /bin directories/var- mostly contain files that store information about how services run(also known as log files)/proc- contains information about processes running on the system/mnt- used to mount file systems. Mounts are usually used when users want to access other file systems on their system/opt- contains optional software/media- contains removable hardware e.g. USB/tmp- contains temporary files. This folder is usually cleared on reboot so doesn’t store persistent files/root- contains files created by the super user(more on this later)
Password File
The format of the file would be:
[1] username
[2] usually an x character and is used to represent their passwords
[3] User ID
[4] Group ID
[5] Extra comments about a user
[6] Home Directory
[7] Shell Location - Most actual users will use the aforementioned shells, but accounts can also belong to particular services.
These services won’t have paths to shells but files like /sbin/nologin which means that the user can’t access this account through a shell(sometimes not at all)
Another note to mention is that the /etc/passwd file is world readable. This is a useful file to display as a proof of concept to show that you have access to a system. A similar such file is the /etc/shadow file that actually contains password hashes of the user.
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