Advanced Linux filesystem: 50 config files and logs

All 50 cards

1. 1
   What is /lost+found for?
   Where fsck puts recovered files that lost their directory entry after a crash.

   One at the top of each ext2/3/4 filesystem; normally empty.
2. 2
   What is /usr/share/man for?
   The manual pages, one subdirectory per section (man1, man5, man8…).
3. 3
   What is /var/spool for?
   Queues of work waiting to be processed: print jobs, outgoing mail, users’ crontabs.

   e.g. /var/spool/cron (edit crontabs with crontab -e, never directly) and /var/spool/cups.
4. 4
   What is /etc/profile.d for?
   Extra *.sh scripts that /etc/profile runs at every login.

   The tidy place for your own system-wide environment settings.
5. 5
   What is /etc/sudoers.d for?
   Extra sudo rule files, included by /etc/sudoers.

   Edit them with visudo -f /etc/sudoers.d/NAME.
6. 6
   What is /etc/systemd/system for?
   The administrator’s own and overriding systemd unit files.

   They take precedence over the packaged units in /usr/lib/systemd/system (/lib/systemd/system on older Debian).
7. 7
   What is /etc/cron.daily for?
   Scripts that run once a day (siblings: cron.hourly, cron.weekly, cron.monthly).

   Started by cron through run-parts, or by anacron on machines that aren’t always on.
8. 8
   What is /etc/yum.repos.d for?
   The .repo files that define the package repositories for yum/dnf.

   The Debian counterpart is /etc/apt/sources.list plus /etc/apt/sources.list.d/.
9. 9
   What is /lib/modules for?
   The kernel modules, in one subdirectory per kernel version.

   The running kernel’s modules are in /lib/modules/$(uname -r)/.
10. 10
    What is /etc/login.defs?
    Defaults for new accounts: UID and GID ranges, password aging, home directory creation.
11. 11
    What is /etc/shells?
    The list of valid login shells.

    chsh accepts only a shell listed here.
12. 12
    What is /proc/mounts?
    The filesystems mounted right now, as the kernel sees them.

    /etc/mtab is usually a symlink to the same list.
13. 13
    What is /boot/grub/grub.cfg?
    GRUB 2’s generated boot menu configuration; don’t edit it by hand.

    RHEL: /boot/grub2/grub.cfg. Regenerate it with grub-mkconfig -o (grub2-mkconfig on RHEL, update-grub on Debian).
14. 14
    What is /etc/default/grub?
    The GRUB 2 settings you edit (timeout, default entry, kernel parameters) before regenerating grub.cfg.
15. 15
    What is /etc/nsswitch.conf?
    The order of sources for lookups, e.g. hosts: files dns.
16. 16
    What is /etc/services?
    Maps service names to port numbers and protocols, e.g. ssh 22/tcp.
17. 17
    What is /etc/ssh/ssh_config?
    The system-wide configuration of the SSH client, ssh.

    Per-user client settings go in ~/.ssh/config.
18. 18
    What is /etc/ld.so.conf?
    Extra directories the dynamic linker searches for shared libraries.

    Plus /etc/ld.so.conf.d/. Run ldconfig after a change to rebuild /etc/ld.so.cache.
19. 19
    What is /etc/localtime?
    The system time zone: a symlink to a file in /usr/share/zoneinfo.

    timedatectl set-timezone changes it.
20. 20
    What is /proc/cmdline?
    The parameters the running kernel was booted with.
21. 21
    What is /var/log/syslog?
    The general system log on Debian/Ubuntu.

    Red Hat-family systems use /var/log/messages.
22. 22
    What is /var/log/messages?
    The general system log on Red Hat-family systems.

    Debian/Ubuntu use /var/log/syslog.
23. 23
    What is /var/log/auth.log?
    Authentication events on Debian/Ubuntu: logins, sudo, SSH.

    Red Hat-family systems use /var/log/secure.
24. 24
    What is /var/log/secure?
    Authentication events on Red Hat-family systems: logins, sudo, SSH.

    Debian/Ubuntu use /var/log/auth.log.
25. 25
    What is /dev/zero?
    A device that supplies an endless stream of zero bytes when read.

    e.g. dd if=/dev/zero of=swapfile bs=1M count=1024.
26. 26
    Where does fsck put recovered files that lost their directory entry?
    /lost+found

    One at the top of each ext2/3/4 filesystem; normally empty.
27. 27
    Where are the man pages stored?
    /usr/share/man
28. 28
    Which directory holds queues of pending work, such as print jobs, outgoing mail and users’ crontabs?
    /var/spool

    e.g. /var/spool/cron (edit crontabs with crontab -e, never directly) and /var/spool/cups.
29. 29
    Which directory holds *.sh snippets that /etc/profile runs at every login?
    /etc/profile.d

    The tidy place for your own system-wide environment settings.
30. 30
    Which directory holds extra sudo rule files that /etc/sudoers includes?
    /etc/sudoers.d

    Edit them with visudo -f /etc/sudoers.d/NAME.
31. 31
    Where does an administrator put custom or overriding systemd unit files?
    /etc/systemd/system

    They take precedence over the packaged units in /usr/lib/systemd/system (/lib/systemd/system on older Debian).
32. 32
    Which directory holds the scripts that run once a day?
    /etc/cron.daily

    Started by cron through run-parts, or by anacron on machines that aren’t always on.
33. 33
    On a Red Hat-family system, which directory holds the .repo files that define the package repositories?
    /etc/yum.repos.d

    The Debian counterpart is /etc/apt/sources.list plus /etc/apt/sources.list.d/.
34. 34
    Where are the kernel modules stored?
    /lib/modules

    The running kernel’s modules are in /lib/modules/$(uname -r)/.
35. 35
    Which file sets defaults such as UID ranges and password aging for newly created users?
    /etc/login.defs
36. 36
    Which file lists the valid login shells?
    /etc/shells

    chsh accepts only a shell listed here.
37. 37
    Which file shows the kernel’s list of currently mounted filesystems?
    /proc/mounts

    /etc/mtab is usually a symlink to the same list.
38. 38
    Which file is GRUB 2’s generated boot menu configuration?
    /boot/grub/grub.cfg

    RHEL: /boot/grub2/grub.cfg. Regenerate it with grub-mkconfig -o (grub2-mkconfig on RHEL, update-grub on Debian).
39. 39
    Which file do you edit to change GRUB 2’s timeout or the kernel parameters?
    /etc/default/grub
40. 40
    Which file decides whether /etc/hosts or DNS is consulted first?
    /etc/nsswitch.conf
41. 41
    Which file maps well-known service names to port numbers?
    /etc/services
42. 42
    Which file configures the SSH client for all users?
    /etc/ssh/ssh_config

    Per-user client settings go in ~/.ssh/config.
43. 43
    Which file lists extra directories to search for shared libraries?
    /etc/ld.so.conf

    Plus /etc/ld.so.conf.d/. Run ldconfig after a change to rebuild /etc/ld.so.cache.
44. 44
    Which file sets the system’s time zone?
    /etc/localtime

    timedatectl set-timezone changes it.
45. 45
    Which file shows the parameters the running kernel was booted with?
    /proc/cmdline
46. 46
    On Debian/Ubuntu, which file is the main general-purpose system log?
    /var/log/syslog

    Red Hat-family systems use /var/log/messages.
47. 47
    On a Red Hat-family system, which file is the main general-purpose system log?
    /var/log/messages

    Debian/Ubuntu use /var/log/syslog.
48. 48
    On Debian/Ubuntu, which log records logins, sudo use and SSH authentication?
    /var/log/auth.log

    Red Hat-family systems use /var/log/secure.
49. 49
    On a Red Hat-family system, which log records logins, sudo use and SSH authentication?
    /var/log/secure

    Debian/Ubuntu use /var/log/auth.log.
50. 50
    Which device supplies an endless stream of zero bytes, e.g. for dd?
    /dev/zero

    e.g. dd if=/dev/zero of=swapfile bs=1M count=1024.