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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. Today. Linux Folders Permissions Directories.

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Homework review
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


Today
Today

  • Linux Folders

  • Permissions

  • Directories


Hi again tom
Hi Again Tom

  • I like Tom – it’s a great visualization of paths


Important directories
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


Quick aside
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”


Folders bin sbin and dev
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


Folders etc home opt
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)


Folders root tmp var
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)


Permissions
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


Octal
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


Oct wait what
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?


Octal math
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 =


Octal permissions partial
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)


Symbolic permissions
Symbolic Permissions

  • Read, write, and execute are now letters

  • Read: r

  • Write: w

  • Execute: x

  • No permissions: -

  • Now there are three spaces: ---


Symbolic permissions cont d
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


Groups of groups
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!


Groups of groups again
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


Groups of groups 3
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


Real example
Real Example

  • [[email protected] ~]$ 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


Manipulation
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


Chmod
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 ~


Chmod symbolic
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


Your turn interpret the following
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


Necessary file permissions
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)


Necessary directory permissions
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


Questions on permissions
Questions on Permissions?

  • Symbolic or octal

  • Three groups of three

  • Read, Write, Execute, None

  • User/owner, Group, Others

  • chmod


Switch user
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


Corporate environments
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


Quiz monday
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…


Own study
Own Study

  • Folders review

    • SobellCh 4 – The Filesystem (81-89)

  • Permissions

    • SobellCh 4 – Access Permissions (98-103)


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