Intermediate Linux filesystem: 50 paths admins use daily

All 50 cards

1. 1
   What is /sys for?
   A virtual filesystem (sysfs) that exposes devices, drivers and kernel settings as files.

   udev uses it to create the device files in /dev.
2. 2
   What is /run for?
   Runtime data since the last boot: PID files, sockets, lock files. Emptied at every boot.

   Added in FHS 3.0; /var/run is now a symlink to it.
3. 3
   What is /srv for?
   Data this system serves to others, such as web or FTP sites.

   Many distributions still put web content in /var/www by default.
4. 4
   What is /usr/bin for?
   Most user commands installed by the distribution’s packages.

   Not administrative commands: those are in /usr/sbin.
5. 5
   What is /usr/sbin for?
   Most system administration commands and daemons installed by packages (useradd, sshd).
6. 6
   What is /usr/lib for?
   The libraries (and internal program files) for the programs in /usr/bin and /usr/sbin.
7. 7
   What is /usr/local for?
   Software the administrator compiles or installs by hand, outside the package manager (the third hierarchy).

   The package manager never touches it. /usr/local/bin is the place for your own scripts for all users.
8. 8
   What is /usr/share for?
   Architecture-independent data: documentation, man pages, icons, locales, time zones.
9. 9
   What is /var/cache for?
   Cached data that can be deleted and regenerated, e.g. downloaded packages (/var/cache/apt).
10. 10
    What is /var/lib for?
    Programs’ persistent state: package databases, database files.

    e.g. /var/lib/dpkg, /var/lib/rpm, /var/lib/mysql.
11. 11
    What is /var/tmp for?
    Temporary files that must survive a reboot.

    Contrast /tmp, which may be emptied at boot.
12. 12
    What is /etc/skel for?
    Template files copied into every new user’s home directory when the account is created.

    Typically .bashrc, .bash_profile (or .profile) and .bash_logout.
13. 13
    What is /etc/sudoers?
    The rules for who may run which commands with sudo.

    Edit it only with visudo, which checks the syntax before saving.
14. 14
    What is /etc/profile?
    The system-wide startup script that every login shell runs.

    It also runs the scripts in /etc/profile.d/.
15. 15
    What is ~/.bash_profile?
    Your personal login-shell startup file, for environment variables set once at login.

    Only the first that exists of ~/.bash_profile, ~/.bash_login and ~/.profile is read.
16. 16
    What is ~/.bash_history?
    Your saved command history, written when the shell exits.

    HISTFILESIZE sets how many lines it keeps.
17. 17
    What is /etc/resolv.conf?
    The DNS servers to query (nameserver lines) and the search domains.

    Often generated by systemd-resolved or NetworkManager rather than edited by hand.
18. 18
    What is /etc/ssh/sshd_config?
    The configuration of the SSH server, sshd.

    Mind the d: ssh_config is the client. Restart sshd after editing.
19. 19
    What is ~/.ssh/authorized_keys?
    The public keys allowed to log in to this account.

    ssh-copy-id appends your key to it on the server.
20. 20
    What is ~/.ssh/known_hosts?
    The host keys of servers you’ve connected to, used to detect an impostor.
21. 21
    What is /etc/crontab?
    The system-wide crontab, with a user field on each line.

    Users’ own crontabs (crontab -e) have no user field.
22. 22
    What is /etc/apt/sources.list?
    The package repositories APT uses on Debian/Ubuntu.

    More repositories go in /etc/apt/sources.list.d/.
23. 23
    What is /etc/os-release?
    The distribution’s name and version (NAME, VERSION_ID, PRETTY_NAME).
24. 24
    What is /proc/cpuinfo?
    Details of each processor: model, cores, flags.
25. 25
    What is /proc/meminfo?
    Detailed memory statistics: total, free, available, swap.

    free reads its numbers from here.
26. 26
    Which virtual filesystem (sysfs) exposes devices, drivers and kernel objects as files?
    /sys

    udev uses it to create the device files in /dev.
27. 27
    Which directory holds runtime data such as PID files and sockets, and is emptied at every boot?
    /run

    Added in FHS 3.0; /var/run is now a symlink to it.
28. 28
    Which directory is meant for the data a system serves, such as web or FTP sites?
    /srv

    Many distributions still put web content in /var/www by default.
29. 29
    Which directory holds most user commands installed by packages (not just the essential ones)?
    /usr/bin

    Not administrative commands: those are in /usr/sbin.
30. 30
    Which directory holds most system administration commands and daemons installed by packages?
    /usr/sbin
31. 31
    Which directory holds the shared libraries for most installed programs?
    /usr/lib
32. 32
    Where does software go that the administrator compiles or installs by hand, outside the package manager?
    /usr/local

    The package manager never touches it. /usr/local/bin is the place for your own scripts for all users.
33. 33
    Which directory holds architecture-independent shared data such as man pages, docs and icons?
    /usr/share
34. 34
    Which directory holds application caches that can be deleted and rebuilt, such as downloaded packages?
    /var/cache
35. 35
    Which directory holds programs’ persistent state, such as the package manager’s database?
    /var/lib

    e.g. /var/lib/dpkg, /var/lib/rpm, /var/lib/mysql.
36. 36
    Which temporary directory keeps its files across reboots?
    /var/tmp

    Contrast /tmp, which may be emptied at boot.
37. 37
    Which directory’s contents are copied into a new user’s home directory?
    /etc/skel

    Typically .bashrc, .bash_profile (or .profile) and .bash_logout.
38. 38
    Which file defines who may use sudo, and for which commands?
    /etc/sudoers

    Edit it only with visudo, which checks the syntax before saving.
39. 39
    Which system-wide file does every login shell run at startup?
    /etc/profile

    It also runs the scripts in /etc/profile.d/.
40. 40
    Which personal file does a Bash login shell read first, if it exists?
    ~/.bash_profile

    Only the first that exists of ~/.bash_profile, ~/.bash_login and ~/.profile is read.
41. 41
    Which file stores your Bash command history between sessions?
    ~/.bash_history

    HISTFILESIZE sets how many lines it keeps.
42. 42
    Which file lists the DNS servers the system queries?
    /etc/resolv.conf

    Often generated by systemd-resolved or NetworkManager rather than edited by hand.
43. 43
    Which file configures the SSH server?
    /etc/ssh/sshd_config

    Mind the d: ssh_config is the client. Restart sshd after editing.
44. 44
    Which file lists the public keys allowed to log in as a user?
    ~/.ssh/authorized_keys

    ssh-copy-id appends your key to it on the server.
45. 45
    Which file stores the host keys of the SSH servers you’ve already connected to?
    ~/.ssh/known_hosts
46. 46
    Which system crontab file has a user field on each line?
    /etc/crontab

    Users’ own crontabs (crontab -e) have no user field.
47. 47
    On Debian/Ubuntu, which file lists the package repositories APT uses?
    /etc/apt/sources.list

    More repositories go in /etc/apt/sources.list.d/.
48. 48
    Which file identifies the distribution and its version?
    /etc/os-release
49. 49
    Which virtual file shows details about the processors?
    /proc/cpuinfo
50. 50
    Which virtual file shows detailed memory statistics?
    /proc/meminfo

    free reads its numbers from here.