Linux filesystem basics: 50 directories and files

All 50 cards

1. 1
   What is / for?
   The root directory: the top of the single directory tree, where every absolute path starts.

   Not to be confused with /root, the root user’s home.
2. 2
   What is /bin for?
   Essential commands for all users, needed even to boot and repair the system (ls, cp, cat).

   On current distributions it’s a symlink to /usr/bin (the usr merge).
3. 3
   What is /sbin for?
   Essential system administration commands (fsck, mkfs, reboot).

   Mostly for root. A symlink to /usr/sbin on merged systems.
4. 4
   What is /boot for?
   What’s needed to boot: the kernel (vmlinuz), the initramfs and the GRUB bootloader files.

   Often a small separate partition.
5. 5
   What is /dev for?
   Device files: the interface to hardware and pseudo-devices (/dev/sda, /dev/null).

   Created dynamically by the kernel and udev as devices appear.
6. 6
   What is /etc for?
   System-wide configuration files, in plain text.

   Host-specific and static: configuration only, no programs. /etc/passwd, /etc/fstab, /etc/hosts…
7. 7
   What is /home for?
   The home directories of regular users, e.g. /home/alice.

   Often a separate partition or network share, so users’ data survives a reinstall.
8. 8
   What is /root for?
   The home directory of the root user.

   Kept on the root filesystem, so it’s there even when /home isn’t mounted.
9. 9
   What is /lib for?
   The essential shared libraries for the commands in /bin and /sbin, and the kernel modules (/lib/modules).

   /lib64 holds the 64-bit libraries on some distributions. A symlink to /usr/lib on merged systems.
10. 10
    What is /media for?
    Mount points for removable media such as USB sticks and DVDs, usually created automatically.

    e.g. /media/alice/USBSTICK. Contrast /mnt, for manual mounts.
11. 11
    What is /mnt for?
    A place for temporary filesystems the administrator mounts by hand.

    e.g. mount /dev/sdb1 /mnt for a quick look. Removable media go in /media.
12. 12
    What is /opt for?
    Self-contained add-on software from third parties, each package in its own subdirectory.

    e.g. /opt/google/chrome. Software you build yourself goes in /usr/local instead.
13. 13
    What is /proc for?
    A virtual filesystem with kernel and process information: one numbered directory per running process.

    Nothing in it is on disk: /proc/cpuinfo is generated when you read it.
14. 14
    What is /tmp for?
    Temporary files for any user; may be emptied at reboot.

    World-writable with the sticky bit, so users can delete only their own files. Contrast /var/tmp.
15. 15
    What is /usr for?
    The second hierarchy: the bulk of installed, read-only software (programs, libraries, documentation).

    It has its own bin, sbin, lib, share and local.
16. 16
    What is /var for?
    Variable data that changes while the system runs: logs, spools, caches, databases.

    var = variable data, not variables.
17. 17
    What is /var/log for?
    The log files of the system and its services.

    e.g. syslog or messages, auth.log or secure. The systemd journal is read with journalctl.
18. 18
    What is /etc/passwd?
    One line per user account: name, password placeholder, UID, GID, comment, home directory, login shell.

    World-readable. An x in the password field means the hash is in /etc/shadow.
19. 19
    What is /etc/shadow?
    The users’ password hashes and password-aging data, readable only by root.

    Change aging with chage, passwords with passwd.
20. 20
    What is /etc/group?
    The groups: group name, GID and the list of members.
21. 21
    What is ~/.bashrc?
    Your personal startup file for every interactive shell: aliases, functions, prompt.

    Apply changes to the current shell with source ~/.bashrc.
22. 22
    What is /etc/fstab?
    The filesystems to mount at boot: device, mount point, type, options, dump and fsck order.

    A manual mount is lost at reboot; an /etc/fstab entry is persistent.
23. 23
    What is /etc/hosts?
    Static hostname-to-IP mappings, checked before DNS by default.
24. 24
    What is /etc/hostname?
    The system’s hostname.

    hostnamectl set-hostname writes it.
25. 25
    What is /dev/null?
    The bit bucket: discards everything written to it; reading it gives end-of-file.

    e.g. find / -name x 2> /dev/null hides the error messages.
26. 26
    Which directory is the top of the whole tree, where every absolute path starts?
    /

    Not to be confused with /root, the root user’s home.
27. 27
    Which top-level directory holds the essential commands every user needs, such as ls and cp?
    /bin

    On current distributions it’s a symlink to /usr/bin (the usr merge).
28. 28
    Which top-level directory holds the essential system administration commands, such as fsck and mkfs?
    /sbin

    Mostly for root. A symlink to /usr/sbin on merged systems.
29. 29
    Which directory holds the kernel image, the initramfs and the bootloader files?
    /boot

    Often a small separate partition.
30. 30
    Which directory holds the device files, such as /dev/sda and /dev/null?
    /dev

    Created dynamically by the kernel and udev as devices appear.
31. 31
    Which directory holds the system-wide configuration files?
    /etc

    Host-specific and static: configuration only, no programs. /etc/passwd, /etc/fstab, /etc/hosts…
32. 32
    Which directory holds the regular users’ home directories?
    /home

    Often a separate partition or network share, so users’ data survives a reinstall.
33. 33
    Which directory is the root user’s home?
    /root

    Kept on the root filesystem, so it’s there even when /home isn’t mounted.
34. 34
    Which top-level directory holds the essential shared libraries and the kernel modules?
    /lib

    /lib64 holds the 64-bit libraries on some distributions. A symlink to /usr/lib on merged systems.
35. 35
    Under which directory are removable media such as USB sticks mounted automatically?
    /media

    e.g. /media/alice/USBSTICK. Contrast /mnt, for manual mounts.
36. 36
    Which directory is meant for filesystems an administrator mounts temporarily by hand?
    /mnt

    e.g. mount /dev/sdb1 /mnt for a quick look. Removable media go in /media.
37. 37
    Which directory holds self-contained third-party add-on software, one subdirectory per package?
    /opt

    e.g. /opt/google/chrome. Software you build yourself goes in /usr/local instead.
38. 38
    Which virtual filesystem shows kernel and process information, with a numbered directory per PID?
    /proc

    Nothing in it is on disk: /proc/cpuinfo is generated when you read it.
39. 39
    Which directory is for temporary files that may be deleted at reboot?
    /tmp

    World-writable with the sticky bit, so users can delete only their own files. Contrast /var/tmp.
40. 40
    Which top-level directory holds the bulk of installed, read-only software: programs, libraries and documentation?
    /usr

    It has its own bin, sbin, lib, share and local.
41. 41
    Which top-level directory holds data that changes as the system runs, such as logs, spools and caches?
    /var

    var = variable data, not variables.
42. 42
    Which directory holds the system’s log files?
    /var/log

    e.g. syslog or messages, auth.log or secure. The systemd journal is read with journalctl.
43. 43
    Which file lists every user account with its UID, home directory and login shell?
    /etc/passwd

    World-readable. An x in the password field means the hash is in /etc/shadow.
44. 44
    Which file holds the users’ password hashes?
    /etc/shadow

    Change aging with chage, passwords with passwd.
45. 45
    Which file lists the groups with their GIDs and members?
    /etc/group
46. 46
    Which personal file holds your aliases and functions, read by every new interactive shell?
    ~/.bashrc

    Apply changes to the current shell with source ~/.bashrc.
47. 47
    Which file lists the filesystems to mount at boot?
    /etc/fstab

    A manual mount is lost at reboot; an /etc/fstab entry is persistent.
48. 48
    Which file maps hostnames to IP addresses locally, without DNS?
    /etc/hosts
49. 49
    Which file holds the system’s hostname?
    /etc/hostname

    hostnamectl set-hostname writes it.
50. 50
    Which device discards everything written to it?
    /dev/null

    e.g. find / -name x 2> /dev/null hides the error messages.