/ (the root directory). Everything — files, devices, network connections — is represented as a file somewhere in this tree. Key directories to know for the A+: /etc (config files), /var (variable data/logs), /home (user files), /bin and /usr/bin (executables), /tmp (temporary files), /boot (kernel and bootloader). Linux uses ext4 as its primary file system. Permissions are controlled with chmod using octal notation (755, 644) or symbolic notation (rwxr-xr-x).
Directory definitions on this page are quoted from the Filesystem Hierarchy Standard 3.0 (dated 19 March 2015, still the current release); exam weightings are checked against CompTIA's published A+ Core 1 and Core 2 (V15) exam details. Both verified 31 August 2026.
The Linux Directory Tree
Unlike Windows (which uses drive letters like C:\ and D:\), Linux has a single hierarchical file system rooted at /. All storage devices — internal drives, USB drives, network shares — are mounted somewhere within this tree. This design is called the Filesystem Hierarchy Standard (FHS).
Key Directories — What's Actually in Each
/home/sean, /home/alice. Contains the user's personal files, desktop, downloads, and user-specific config files (dotfiles like .bashrc). Users own and control their own home directory./dev/sda, /dev/sdb; partitions are /dev/sda1. NVMe drives are /dev/nvme0n1. Special files like /dev/null (discard output) and /dev/zero (generate zeros) live here.These layouts are specified, not conventional. The Filesystem Hierarchy Standard 3.0 defines /etc as “host-specific system configuration”, /var as “variable data files”, and /opt as “reserved for the installation of add-on application software packages”. The per-directory reference shipped on most systems is the hier(7) man page; on systemd distributions, the additional layout rules that sit on top of the FHS are documented in file-hierarchy(7).
Linux File System Types
| File System | Used On | Key Features | A+ Relevance |
|---|---|---|---|
| ext4 | Linux (primary) | Journaling, max file size 16 TB, max volume 1 EB. Default on most Linux distros. Backward compatible with ext3/ext2. | Know this is the standard Linux file system |
| ext3 | Linux (older) | Journaling added over ext2. Slower than ext4. Legacy — still found on older systems. | Know it preceded ext4 |
| XFS | Linux (enterprise) | High-performance journaling file system. Default on RHEL/CentOS. Excellent for large files and high I/O workloads. | Know it's a Linux enterprise option |
| FAT32 | USB drives, cross-platform | Compatible with Windows, macOS, Linux. Max file size 4 GB — cannot store files larger than 4 GB. No permissions or journaling. | Common on USB drives — 4 GB limit is exam-tested |
| exFAT | USB, flash drives | FAT32 successor — no 4 GB file size limit. Cross-platform compatible. No journaling. Common on modern USB drives and SD cards. | Replacement for FAT32 for large files on removable media |
| NTFS | Windows (primary) | Journaling, file-level permissions, encryption (EFS), compression, large file support. Linux can read NTFS natively with ntfs-3g driver. | Know Linux can read NTFS drives |
| swap | Linux (virtual memory) | Not a traditional file system — a dedicated partition used as virtual memory overflow when RAM is full. Similar to Windows pagefile. | Know this is Linux's virtual memory partition |
FAT32 cannot store any single file larger than 4 GB. This is a hard limitation of the file system, not the drive size. A common A+ scenario: a user tries to copy a 6 GB video file to a USB drive and gets an error — the cause is FAT32 formatting. The fix is to reformat the drive as exFAT (cross-platform, no 4 GB limit) or NTFS (Windows/Linux only). The drive itself is not broken.
Defaults differ by distribution and change between releases. Red Hat's own documentation states that XFS “is default in RHEL 10”, while the Debian and Ubuntu installers still default to ext4. Check the installer for the release in front of you rather than assuming.
Linux File Permissions
Every file and directory in Linux has permissions assigned to three groups: the owner, the group, and others (everyone else). Each group gets three permission bits: read (r), write (w), and execute (x).
| Octal | Binary | Symbolic | Meaning |
|---|---|---|---|
| 7 | 111 | rwx | Read + Write + Execute |
| 6 | 110 | rw- | Read + Write (no execute) |
| 5 | 101 | r-x | Read + Execute (no write) |
| 4 | 100 | r-- | Read only |
| 0 | 000 | --- | No permissions |
| Common chmod Value | Who Can Do What | Typical Use |
|---|---|---|
| 755 | Owner: rwx · Group: r-x · Others: r-x | Executable scripts, directories — owner can modify, everyone can read/execute |
| 644 | Owner: rw- · Group: r-- · Others: r-- | Regular files — owner can modify, everyone else read-only |
| 700 | Owner: rwx · Group: --- · Others: --- | Private scripts — only the owner can access at all |
| 600 | Owner: rw- · Group: --- · Others: --- | Private files (SSH keys) — only the owner can read or modify |
| 777 | Owner: rwx · Group: rwx · Others: rwx | Everyone can do everything — avoid except /tmp-style directories |
chmod changes the permission bits on a file or directory. chmod 755 script.sh sets the permissions using octal. chmod +x script.sh adds execute permission for all.
chown changes the owner of a file. chown sean file.txt makes "sean" the owner. chown sean:developers file.txt sets both owner and group. Requires sudo/root to change another user's files.
chgrp changes the group associated with a file. chgrp developers file.txt assigns the file to the "developers" group. Users in that group then have group-level permissions on the file.
Inodes — What They Are
Every file in a Linux file system has an inode — a data structure that stores metadata about the file: owner, permissions, timestamps, file size, and pointers to the actual data blocks on disk. The filename is not stored in the inode — it's stored in the directory that points to the inode.
| Stored in Inode | NOT Stored in Inode |
|---|---|
| File size | Filename |
| Owner (UID) and group (GID) | File path |
| Permissions (rwx bits) | File content |
| Timestamps (created, modified, accessed) | Hard link names |
| Pointers to data blocks on disk | Extended attributes (stored separately) |
| Number of hard links to this inode |
Mount Points
In Linux, adding a new storage device doesn't give it a drive letter — it's mounted at a directory in the file system tree. The directory where a device's file system becomes accessible is the mount point.
| Command | What It Does |
|---|---|
| mount /dev/sdb1 /mnt/usb | Mounts partition sdb1 at the /mnt/usb directory — files on the USB now appear at /mnt/usb/ |
| umount /mnt/usb | Unmounts the device from /mnt/usb (must not be in use). Note: "umount" not "unmount" |
| df -h | Shows all mounted file systems and their disk usage in human-readable format |
| lsblk | Lists block devices (drives and partitions) with their mount points |
| /etc/fstab | The file that defines which partitions/devices should be automatically mounted at boot and at which mount points |
Linux uses forward slashes (/home/user/file) — Windows uses backslashes (C:\Users\user\file). Linux is case-sensitive — File.txt and file.txt are different files. Windows is case-insensitive. Linux has no drive letters — everything is under /. The Linux equivalent of C:\ is /, the equivalent of the Windows Desktop is ~/Desktop or /home/username/Desktop.
What This Page Doesn't Cover
This covers the directory tree, permissions, inodes and mount points at the depth CompTIA tests. It does not cover POSIX ACLs beyond the basic rwx triads, SELinux or AppArmor contexts, LVM, filesystem tuning, or recovery from a damaged superblock — those are Linux+ and administration topics rather than A+ ones. Where a distribution diverges from the standard (Ubuntu's Snap mount points, for instance), this page follows the FHS rather than the distribution.
For scope: CompTIA's published A+ Core 1 and Core 2 (V15) exam details put Operating Systems at 28% of Core 2, with a passing score of 700 out of 900. The current versions, 220-1201 and 220-1202, launched on 25 March 2025. Linux file-system questions sit inside that 28%, alongside Windows and macOS.
Exam Scenarios
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