Linux command line: 50 LPIC-1 essentials

All 50 cards

1. 1
   Find regular files over 100 MB under /var
   bash

   find /var -type f -size +100M

   -size units: k, M, G; + means more than, - less than.
2. 2
   Find files with the SUID bit set
   bash

   find / -perm -4000 -type f 2>/dev/null

   -perm -MODE matches files with all of those bits set; -perm /MODE matches any of them.
3. 3
   Run a command on each file find matches
   bash

   find . -name "*.bak" -exec rm {} \;

   End with {} + instead to pass many files to one command, like xargs.
4. 4
   What does grep -E enable?
   Extended regular expressions: +, ?, |, () and {} work without backslashes

   The old egrep command is equivalent. grep -F matches fixed strings, with no regex at all.
5. 5
   grep flags -v, -i, -c and -l
   -v prints non-matching lines, -i ignores case, -c counts matching lines, -l lists only the names of matching files
6. 6
   Show the lines of a config file that are neither comments nor blank
   bash

   grep -Ev '^(#|$)' /etc/ssh/sshd_config

7. 7
   Find files by name instantly from a database
   bash

   locate sshd_config

   The database is rebuilt by updatedb, usually daily from cron, so very new files may be missing.
8. 8
   The special permission bits in octal
   4 = SUID, 2 = SGID, 1 = sticky, written before the usual three digits, as in chmod 4755
9. 9
   What does SUID do on an executable?
   The program runs with the file owner’s privileges instead of the caller’s

   /usr/bin/passwd is SUID root so users can update /etc/shadow. It shows as s in the owner’s execute slot: -rwsr-xr-x.
10. 10
    What does SGID do on a directory?
    New files created inside inherit the directory’s group instead of the creator’s primary group

    Handy for shared team directories.
11. 11
    What does the sticky bit do on a directory?
    Only a file’s owner, the directory’s owner or root can delete or rename files in it

    /tmp is mode 1777, shown as drwxrwxrwt.
12. 12
    With umask 022, what permissions do new files and directories get?
    Files 644 (rw-r--r--), directories 755 (rwxr-xr-x)

    The umask removes bits from the defaults of 666 for files and 777 for directories.
13. 13
    Change a file’s owner and group at once
    bash

    chown alice:developers report.txt

    Add -R to recurse into directories. chgrp changes only the group.
14. 14
    Hard link vs symbolic link
    A hard link is another name for the same inode (same filesystem only, not for directories); a symlink is a small file holding a path, which can cross filesystems and breaks if the target is removed

    bash

    ln data.txt data-hard.txt
    ln -s /etc/nginx/nginx.conf nginx.conf

15. 15
    What is an inode?
    The structure that stores a file’s metadata (type, permissions, owner, size, timestamps, data block locations), but not its name

    ls -i shows inode numbers; df -i shows inode usage per filesystem.
16. 16
    The six fields of an /etc/fstab line
    Device (or UUID=/LABEL=), mount point, filesystem type, mount options, dump flag, fsck pass order

    Example: UUID=… /home ext4 defaults 0 2. For the fsck pass, 1 is the root filesystem, 2 others, 0 skips the check.
17. 17
    Mount everything in /etc/fstab that isn’t mounted yet
    bash

    mount -a

    A good test after editing fstab, before you reboot. Entries with the noauto option are skipped.
18. 18
    MBR vs GPT partition tables
    MBR: up to 4 primary partitions (or 3 plus an extended one holding logical partitions) and disks up to 2 TiB. GPT: 128 partitions by default and far larger disks; used with UEFI

    fdisk and parted handle both; gdisk is GPT-only.
19. 19
    Create an ext4 filesystem, then check it
    bash

    mkfs.ext4 /dev/sdb1
    fsck /dev/sdb1

    Only run fsck on unmounted filesystems. For XFS, use xfs_repair.
20. 20
    Set up and enable a swap partition
    bash

    mkswap /dev/sdb2
    swapon /dev/sdb2

    Check with swapon --show or free -h; add a swap line to /etc/fstab to keep it after reboot.
21. 21
    SysV runlevels 0, 1, 3, 5 and 6
    0 halt · 1 single-user (maintenance) · 3 multi-user, text mode with networking · 5 multi-user, graphical · 6 reboot

    Runlevels 2 and 4 vary by distribution.
22. 22
    systemd targets that replace the runlevels
    poweroff.target (0), rescue.target (1), multi-user.target (3), graphical.target (5), reboot.target (6)
23. 23
    Show and change the default boot target
    bash

    systemctl get-default
    systemctl set-default multi-user.target

24. 24
    Switch to rescue mode now, without rebooting
    bash

    systemctl isolate rescue.target

    The SysV equivalent is telinit 1.
25. 25
    Start a service now and enable it at boot
    bash

    systemctl enable --now nginx

    systemctl status nginx shows whether it is running and its latest log lines.
26. 26
    Show a service’s log messages since the last boot
    bash

    journalctl -u nginx -b

    -f follows new messages; -p err shows only errors and worse.
27. 27
    Regenerate the GRUB 2 configuration after editing /etc/default/grub
    bash

    grub-mkconfig -o /boot/grub/grub.cfg

    Debian and Ubuntu wrap this as update-grub; Red Hat systems run grub2-mkconfig -o /boot/grub2/grub.cfg.
28. 28
    Load a kernel module together with its dependencies
    bash

    modprobe usb_storage

    insmod loads a single module file without resolving dependencies. lsmod lists loaded modules; modprobe -r removes one.
29. 29
    List the shared libraries a program needs
    bash

    ldd /usr/bin/ssh

    After adding a library directory under /etc/ld.so.conf.d/, run ldconfig to rebuild the cache.
30. 30
    Debian: install a local .deb, list installed packages, list a package’s files
    bash

    dpkg -i package.deb
    dpkg -l
    dpkg -L openssh-server

31. 31
    Which package owns a file? (Debian and RPM)
    bash

    dpkg -S /usr/bin/ssh
    rpm -qf /usr/bin/ssh

32. 32
    apt remove vs apt purge
    remove uninstalls the package but keeps its configuration files; purge deletes them too

    apt update refreshes the package lists; apt upgrade installs newer versions.
33. 33
    RPM: install, upgrade and erase a package
    bash

    rpm -ivh package.rpm
    rpm -Uvh package.rpm
    rpm -e package

    rpm doesn’t resolve dependencies; dnf (formerly yum) and zypper (openSUSE) do.
34. 34
    The seven fields of /etc/passwd
    username : password placeholder (x) : UID : GID : comment (GECOS) : home directory : login shell
35. 35
    Where are password hashes stored?
    /etc/shadow, readable only by root

    It also holds password ageing data, managed with chage.
36. 36
    Create a user with a home directory and Bash as the shell
    bash

    useradd -m -s /bin/bash alice
    passwd alice

37. 37
    Add a user to a supplementary group, keeping their other groups
    bash

    usermod -aG wheel alice

    Without -a, -G replaces the user’s supplementary groups.
38. 38
    The five time fields of a crontab line
    Minute, hour, day of month, month, day of week

    bash

    # 02:30 every Monday
    30 2 * * 1 /usr/local/bin/backup.sh

39. 39
    How does /etc/crontab differ from a user’s crontab?
    System crontabs (/etc/crontab, /etc/cron.d/) have a sixth field naming the user to run as

    Edit your own with crontab -e; list it with crontab -l.
40. 40
    Run a job once at a later time
    bash

    echo "/usr/local/bin/report.sh" | at 23:00

    atq lists pending jobs; atrm removes one.
41. 41
    Nice values: the range, the default, and which is the highest priority
    -20 (highest priority) to 19 (lowest); the default is 0

    Start with nice -n 10 cmd; change with renice -n 5 -p PID. Only root can lower a nice value.
42. 42
    Suspend a foreground job, resume it in the background, then bring it back
    Ctrl+Z, then bg, then fg

    jobs lists them; %1 refers to job 1, as in fg %1.
43. 43
    Signal numbers of SIGHUP, SIGINT, SIGKILL and SIGTERM
    1, 2, 9 and 15

    SIGTERM is what kill sends by default; SIGHUP makes many daemons reload their configuration.
44. 44
    Create and extract a gzip-compressed tar archive
    bash

    tar -czvf etc.tar.gz /etc
    tar -xzvf etc.tar.gz -C /tmp

    -j for bzip2, -J for xz, -t to list the contents.
45. 45
    Replace text in a file in place with sed
    bash

    sed -i 's/old/new/g' file.txt

    Use -i.bak to keep a backup of the original.
46. 46
    Which files does Bash read for a login shell?
    /etc/profile, then the first of ~/.bash_profile, ~/.bash_login and ~/.profile that exists

    Interactive non-login shells read ~/.bashrc instead.
47. 47
    vi: save and quit, quit without saving, delete a line, search
    :wq (or ZZ), :q!, dd, /pattern

    Press i to insert text and Esc to return to normal mode.
48. 48
    Show IP addresses and the routing table
    bash

    ip addr show
    ip route show

    The ip command replaces the older ifconfig and route.
49. 49
    List listening TCP and UDP ports with their processes
    bash

    ss -tulpn

    ss replaces netstat. Run it as root to see every process.
50. 50
    Which file sets the order of name lookups (local files, DNS)?
    /etc/nsswitch.conf, e.g. hosts: files dns

    Static names live in /etc/hosts; DNS servers are listed in /etc/resolv.conf.