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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
  • [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
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)