Homework & Review

1. Homework & Review • Questions? • Everything after this is for accuracy • “All your permanent teeth are in, okay? You’re playing for keeps now!” -Psych • Review: • Absolute Path • Relative Path • Linux Directory Structure • Shell look and feel

2. Today • Linux Folders • Permissions • Directories

3. Hi Again Tom • I like Tom – it’s a great visualization of paths

4. Important Directories • /bin - commands • /dev - devices • /etc - system configuration • /home - user files • /opt - application directory • /root - root user’s home directory • /sbin- commands • /tmp - temporary files • /var - changing files • There are more, but we’re starting here

5. Quick Aside • Windows has .exe files for ‘executable’ • How you differentiate programs from other files • Linux doesn’t have this • You can only tell executables by the permissions • What flag on the ls command would show permissions? • Commands are “programs”

6. Folders - /bin, /sbin and /dev • /bin and /sbin – commands • /bin/ls • /sbin/ip • /dev – devices • /dev/hdd – IDE hard drives • /dev/sda – SATA/SAS hard drives • /dev/tty – userspace • /dev/cciss – HP’s RAID • /dev/dvd – DVD • /dev/vg_it136centos65vm – Virtual HDD space • Run the df command to see

7. Folders - /etc, /home, /opt • /etc – configuration files • Configures applications on the system • Both integrated and not • /etc/sudoers, /etc/httpd/conf/httpd.conf • /etc/named.conf, /etc/dhcpd.conf • /home – default user space • /home/<username> • Users will then create/manage their own files/directories • /opt – third party application space • Anything you buy from someone that “runs on Linux” should be here • If it’s not that is a BIG red flag (usually)

8. Folders - /root, /tmp, /var • /root – home directory for the root user, usually contains backups of configuration files or is a staging area for administrative tasks • /tmp – temporary space, some OS “lock” files, staging area for updates/installations • /var – files that will change through the course of normal system operation (mail, logs, some databases/web servers)

9. Permissions • Permissions in Linux are shown by the ls –l command • -l stands for ‘long list’ which includes the permissions, owner, and group • Linux permissions are shown in two ways • Octal and symbolic • Each break permissions down into three groups • 1) user/owner of the file • 2) group set to use the file • 3) everyone else

10. Octal • Three groups of numbers, each number runs from 0 through 7 • 777, 733, 755 (three very common permissions), 456, 523, 123, are other possibilities • So each numerical value has a defined level of access: • 0 – no access for anybody • 1 – execute permissions • 2 – write permissions • 4 – read permissions

11. Oct-wait, what?! • 1, 2, 4?! • Yes, how’s your math? • Every unique combination now stands for a unique type of access • If read was 3 (and not 4), then what would happen if we wanted to make a file that was executable, and writeable?

12. Octal Math • Unique options are 3, 5, 6, and 7 from • 1) execute • 2) write • 4) read • So, what do these unique options mean • 3 = • 5 = • 6 = • 7 =

13. Octal Permissions (partial) • So what does ‘read,’ ‘write,’ and ‘execute’ mean? • File: • 4) Read: You can read the contents (cat, vi but no saving) • 2) Write: You can edit the file (vi, plus saving!) • 3) Execute: If it’s a script, you can run it (./) • Directory: • 4) Read: You can see what’s inside (ls) • 2) Write: You can create new files (touch, vi + saving!) • 1) Execute: You can move inside the directory (cd)

14. Symbolic Permissions • Read, write, and execute are now letters • Read: r • Write: w • Execute: x • No permissions: - • Now there are three spaces: ---

15. Symbolic Permissions, cont’d • Meaning stays the same: • Files: • r - can cat the file, open in vi without saving • w - can save in editor (vi) • x - can run the script • No permissions: - • Directories: • r - can do an ls to see files inside • w - can create new files (vi/touch) • x - can move inside

16. Groups of Groups • So we have three options (read, write, execute) • These options combine into three groups • User/owner – the primary user (owner) of the file • Group – the primary group the file is associated with • Others – everyone else not the primary user (owner) or group • Don’t get ‘owner’ and ‘others’ confused!

17. Groups of Groups, again • Each group gets a full set of permissions • Octal • 000 (no permissions to anyone) • 777 (full permissions to everyone) • Symbolic • --------- (no permissions to anyone) • rwxrwxrwx (full permissions to everyone) • Or some combination

18. Groups of Groups, 3 • Each set of permissions stands for a different group • Octal • 7 7 7 • Owner Group Others • Symbolic • rwxrwxrwx • Owner Group Others

19. Real Example • [student@it136centos58vm ~]$ ls -l • total 20 • -rw-rw-r– 1 student student 29 Apr 17 16:53 err.out • -rwxrwxr-x1 student student 29 Apr 17 16:53 script.sh • -rw-rw-r– 1 student student 29 Apr 17 16:53 teams2.txt • -rw-rw-r– 1 student student 29 Apr 17 16:53 teams.txt • permissions owner group filename

20. Manipulation • Commands • mkdir <arg> – make new directory at (and named) <arg> - ~/backups or /tmp/test • cd <arg> – change directory into ‘arg’ (path) • ls -l <arg> – list file permissions at ‘arg’ (path) • chmod <arg1> <arg2> - change the permissions to ‘arg1’ (new permissions) on ‘arg2’ (file/path), explained next slide • chmod is awesome, works on directories or files

21. chmod • chmod <permissions> /path/to/anything • chmod <permissions> /etc/passwd • chmod <permissions> ~/myscript.sh • <permissions> • Can be octal or symbolic • Otcal: • chmod 755 ~/myscript.sh • chmod 700 ~

22. chmod, symbolic • Symbolically, we can update one specific group (user_owner, group, others) • chmodg+rwx file - give rwx to the group • chmodo+rwx file - give rwx to ‘others’ • chmodu+rwx file - give rwx to the user/owner • chmoda+rwx file - give rwx to everyone • chmod o-r file - remove read permissions from ‘others’ • chmod u-x file - remove execute permissions from user/owner

23. Your Turn – Interpret the Following • Octal: User full, group read, nothing for others • 755 • Symbolic: User read and write, group write, others read • r-xrw---x

24. Necessary File Permissions • To read a file (cat, less, grep, etc…) • As owner -r--------; group ----r-----; other_users -------r-- • To write to a file (vi, nano, emacs, etc…) • As owner -rw-------; group ----rw----; users -------rw- • You need to be able to read (see the contents of) the file you want to write to • You can’t write in a book unless you can open the book • To run a script (./script.sh) • As owner -r-x------; group ----r-x---; users -------r-x • Again, you need to be able to see the contents of the script to know what actions the script is telling you to do • These are all file-based (not directory)

25. Necessary Directory Permissions • To list the contents of a directory (ls backups) • Owner: -r--------; group: ----r-----; users: -------r-- • To move into a directory (cd backups) • Owner: ---x------; group: ------x---; users: ---------x • Note that execute-only will not allow the user to do an ls • Create or modify files inside a directory (touch/echo/vi) • Owner: --wx------; group: ------wx---; users: --------wx • Note - you will still not be able to do an ls as you do not have ‘read’ permissions • Run scripts inside a directory (./script.sh) • Owner: --wx------; group: ------wx---; users: --------wx • Note - again, if something inside the script requires read permissions, it will not work, but the script will run

26. Questions on Permissions? • Symbolic or octal • Three groups of three • Read, Write, Execute, None • User/owner, Group, Others • chmod

27. Switch User • su <username> • Will switch to the account named <username> • su • Will switch to the ‘root’ or admin account • Root has all privileges • Used for adding users, patching/updating/installing, reading log files, troubleshooting and other administrative tasks

28. Corporate Environments • As an admin you’ll get several passwords • 1) Your own (as a “power user”) • 2) An account that can access all devices (if needed) • 3) Administrator/root password • #3 is what is known as the ‘keys to the kingdom’ – Windows group doesn’t share with Linux group, which doesn’t share with network group • Keep the root pw extremely confidential • Is usually something clever like we have so they can reference it out loud without others knowing exactly what it is

29. Quiz Monday • 30 minutes • 10 questions • From Day 1 to now • So expect a question on each topic listed in the “review” or “today” slides from each lecture • And at least one “what is a kernel/os/etc…” question • Open book, open note • No collaboration • Email, chat, text, social network, etc…

30. Own Study • Folders review • SobellCh 4 – The Filesystem (81-89) • Permissions • SobellCh 4 – Access Permissions (98-103)