IT 244: Introduction to Linux/Unix
Class 11
Tips and Examples
Review
New Material
Microphone
Graded Quiz
You can connect to Gradescope to take weekly graded quiz
today during the last 15 minutes of the class.
Once you start the quiz you have 15 minutes to finish it.
You can only take this quiz today.
There is not makeup for the weekly quiz because Gradescope does not permit it.
Readings
If you have the textbook you should read chapter 5,
The Shell.
Homework 6
I have posted homework 6 here.
It is due this coming Sunday at 11:59 PM.
Midterm
The Midterm exam for this course will be held on Tuesday,
October 22nd.
It will be given in this room.
It will consist of 25 questions like those on the quizzes.
60% of the questions will come from the Weekly Graded Quizzes.
There is a link to the answers to the graded quizze on the class web page.
The other 40% of points will be questions that I create specially for this exam.
For these questions you will have to know
- Absolute and relative pathnames
- The PATH system variable
- Access permissions
- Redirection & and pipes
- grep
- Utilities
The last class before the exam, Thursday, October 17th, will be a review session.
You will only be responsible for the material in the Class Notes for that class
on the exam.
You will find the Midterm review Class Notes
here.
If for some reason you cannot take the exam on the date mentioned above
you must contact me to make alternate arrangements.
The Midterm is given on paper.
I scan each exam paper and upload the scans to Gradescope.
I score the exam on Gradescope.
You will get an email from Gradescope with your score when I am done.
The Midterm is a closed book exam.
You are not allowed to any resource, other than what is in your head,
while taking the exam.
Cheating on the exam will result in a score of 0 and will be reported to the Administration.
Remember your Oath of Honesty.
To prevent cheating, certain rules
will be enforced during the exam.
Remember, the Midterm and Final determine 50% of your grade.
Questions
Are there any questions before I begin?
Tips and Examples
Studying for the Midterm with Flashcards
- 60% of the point on the Midterm come from Class Quiz Questions
- You can use the flashcards you create for these questions
when studying for the Midterm
- But not all Class Quiz Questions will appear on the Midterm
- If the flashcard question covers a topic not in the Midterm Review ...
- you do not have to study it for the exam
- You should remove these flashcards from your collection
Getting Your First IT Job
- Students sometimes come to me asking for advice on how to get an IT
job
- I have not had a job in industry for many years
- So I usually refer them to
Career Services
- They provide a web site called
Handshake
where companies can find interns
- In IT, as in many fields, your first job is the stepping stone
to your career in the field
- But it can be hard to find an internship or or a job
when you have no IT experience
- If this is a situation you face, there are things
you can do
- If you have a job, check with your current employer
- They must have computers somewhere and maybe you can
help keeping them running
- If they have an IT Department ask if there is something you
can do for them in your free time
- You can do something similar with local organization
like a church, temple, mosque or youth group
- They probably use a computer for some of the work they do
or perhaps they need a web page
- Volunteer to do some IT work for them in return for a letter
talking about the work you did
- You can cite this work in your resume
- Another place to find volunteer opportunities is
Volunteer Match
- They have virtual opportunities that in many different areas
- Or perhaps you can find some open source project that needs
help
- The Free Software Foundations
is based in Boston and often needs volunteers
- Many people who are looking to hire people say they are
having a hard time ...
- finding people who take their work seriously
- Whenever you get a job, no matter how menial ...
- be sure to do your best ...
- and take the work seriously
- Employers don't want people who don't give a damn
- The economy goes through cycles and sometimes jobs are hard
to find
- Just get in the habit constantly looking for opportunities
- But above all don't give up
- When solving a problem, one of your best resources ...
- is the people you work with
- Sometimes you get you get too wrapped up in a problem ...
- and can't see something that is obvious to others
- Other times you need a function or technique ...
- that you have never used before
- Nobody in IT knows everything
- Of course you could always Google for an answer
- But someone you work with might be able to explain it ...
- saving you hours of searching
- All of us who work in IT are critically dependent on our peers
- This is why I urge all of you to keep in touch with those you
meet in your classes ...
- or on the job
- Most of the jobs I have had in my life ...
- have come from leads from people I know
- Make sure you have the email or text address of everyone
you study or work with ...
- and keep in regular contact
- Maybe go out for a beer with them one a month
- You won't regret it
Viewing Directory Permissions
- Running
ls -l
on a directory will show the permission of everything inside that directory
$ ls -l tmp
total 8
-rw-r--r-- 2 ghoffman faculty 22 Jun 19 14:15 lines.txt
-rw-r--r-- 2 ghoffman faculty 22 Jun 19 14:15 test.txt
- What if you wanted to see the permission on the directory itself?
- You have two options
- You can run
ls -l
on the parent directory
$ ls -l ~
total 80
drwxr-xr-x 4 ghoffman grad 4096 Oct 15 2016 bin
drwxr-xr-x 6 ghoffman faculty 4096 Jan 20 14:44 code
drwxr-xr-x 6 ghoffman faculty 4096 Sep 9 2016 course_files
...
drwxr--r-- 2 ghoffman faculty 4096 Jun 19 14:15 tmp
- Or you can run
ls -ld
on the directory itself
$ ls -ld tmp
drwxr--r-- 2 ghoffman faculty 4096 Jun 19 14:15 tmp
- The
-d
option tells ls
to show information on the directory
- Not the things inside the directory
Never Use . . in an Absolute Pathname
Write Permissions on a Directory
- Write permission on a directory only applies to what is inside the directory
- It does not apply to the directory itself
- If you have write permission on a directory you can
- Create a file or directory inside the directory
- Delete a file or directory inside the directory
- Rename a file or directory inside the directory
- To delete or change the name of the directory itself ...
- you must have write permission on the parent directory
Using the ls
Command
The chmod
Chant
Using . When Copying a File
Review
The root Account
- On every Unix or Linux system there is a special account named
root
- root can access any file or run any program
- root is an administrator account
- It is used for system configuration and maintenance
- Even a system administrator should not log in as root
- Instead he or she should use a regular Unix account ...
- and use
sudo
when running a command that needs root privileges
sudo
allows a user to run a command that normally only root can run
- When you run
sudo
it asks you for your password
- Not the password of the root account
- In order to run
sudo
you must be on the sudoers list
- Only root or someone running
sudo
can change this list
Directory Access Permissions
- The Unix access permissions
work a little differently for directories than they do for files
- Read and write permissions for a directory are similar to those for a file
- Read permission allows you to list the names of things inside that directory using
ls
- Write permission allows you to create, delete or rename the things inside the directory
- Write permission on a directory does not allow you to change the contents of a file in that directory
- If you have write permission on a directory you can change what's inside it
- But you cannot rename or delete the directory itself
- To do that you have write permission on its parent directory
- Execute permission on a directory allows you to
cd
into that directory
Links
- Links are like shortcuts on Windows
- Or aliases on a Mac
- Links are pointers to files or directories
- Each of you has an entry in your home directory called it244
- In my home directory, I have such a link
$ ls -l it244
lrwxrwxrwx 1 ghoffman faculty 39 May 30 14:08 it244 -> /courses/it244/f24/ghoffman/ghoffman
- This is a link to/courses/it244/f24/ghoffman/ghoffman
- If you
cd
into this location and use pwd
you will see
$ cd it244
$ pwd
/home/ghoffman/it244
- This path reflects the route you took to get here
- But it is not the real pathname of the directory
- You can only see your true location ...
- if you use
pwd
with the -P (note the capitalization) option
$ pwd -P
/courses/it244/f24/ghoffman/ghoffman
The Two Types of Links
- There are two types of links
- Hard links
- Symbolic, or soft, links
- Hard links are older
- A hard link is like a duplicate file name
- Hard links can only point to files not directories
- You can only have a hard link to a file ...
- if that file is on the same hard disk partition as the link
- Symbolic links are much more flexible
- A symbolic link can point to a file or directory anywhere in the filesystem
- Deleting a symbolic link does not delete the file or directory it points to
ln
Setuid and Setgid Permissions
- Sometimes a program needs to read or modify a file ...
- to do the work it was designed to do
- The
passwd
command is used to change the password for an account
- To do this it must change to the file /etc/shadow
- But /etc/shadow is owned by the root account
- No other account can change it
- To deal with situations like this, two special permissions were created
- Both permissions are set on programs
- But they affect the files that the program acts upon
- setuid permission
allows anyone who runs the program ...
- to do what the owner can do
- They can modify the any file the owner can modify
- Let's say the owner of a executable file is also the owner of another file
- If another user ran the executable file and it did not have setuid permission ...
- they could not change that other file
- But if the executable file has the setuid permission ...
- they could change it
- If you run a program with setuid permission ...
- you can do anything the owner can do
passwd
is owned by root
passwd
needs to change /etc/shadow ...
- which is also owned by root
- But
passwd
has setuid permission
- So when you run it you can change /etc/shadow
- setgid permission
applies to the group
- Anyone who runs the program can do whatever the group can do
- setuid and setgid permissions are only given to executable files and scripts
- But while the permission is set on the executable files it works on other files
- setuid permission appears as an s in the owner's executable column
- setgid permission appears as an s in the group's executable column
- Since setuid and setgid permission apply only to executable files ...
- there is no ambiguity in replacing x
with s
Attendance
New Material
Running a Unix Command
- You run a Unix command by typing the name of the command on the command line
- The name of the command is really the name of the executable file ...
- that contains the code for the command
- It turns out that there is another way you can run a Unix command
- You can use a
pathname
for the executable file ...
- to run that command
- We can use
which
to find the
absolute pathname
of ls
$ which ls
/usr/bin/ls
- I can run
ls
using this absolute pathname
$ ls /usr/bin/ls
bin data groups lib32 media proc snap swap.img users
boot dev home lib64 mnt root sources sys usr
cdrom etc home.ORIG libx32 nobackup run spool tmp var
courses etc.ORIG.tar lib lost+found opt sbin srv tools
- I can also use a
relative pathname
$ ls ../../usr/bin/ls
bin data groups lib32 media proc snap swap.img users
boot dev home lib64 mnt root sources sys usr
cdrom etc home.ORIG libx32 nobackup run spool tmp var
courses etc.ORIG.tar lib lost+found opt sbin srv tools
- So there are two way you can run a Unix command
- Using the name of the executable file for the command
- Using a pathname for the executable file
Syntax of the Command Line
Command Options
- Many commands have options
- Options modify the behavior of the command
- Options are usually preceded by one or two dashes, -
- GNU programs frequently have options that are preceded by two dashes, --
- The options in GNU programs are usually words
- The options in other Unix programs are usually a single letter
- When a command uses a single dash, -, before an option ...
- you can combine single letter options after the dash
- An example of this is
ls -ltr
- This means run
ls
- To get a long listing
- Sorted by modification date and time
- In reverse order
- Options using two dashes, --, cannot usually be combined
- In this case, each option must be written separately ...
- and preceded by two dashes
- Sometimes the option can have it's own argument
- When this happens, the argument is usually separated from the option by spaces
gcc -o prog prog.c
- Utilities that report the size of files usually do so in bytes
- This works well with small files
- But with large files, a size in bytes can be hard to read
- Such utilities often have a
-h
, or --human-readable
, option
- With this option, the file size will be displayed in kilobytes, megabytes or gigabytes ...
- as appropriate
df
(disk free) shows the amount of space on the various filesystems
$ df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda1 12253360 6027788 5580096 52% /
none 4 0 4 0% /sys/fs/cgroup
udev 2013316 4 2013312 1% /dev
tmpfs 404836 560 404276 1% /run
none 5120 0 5120 0% /run/lock
none 2024172 0 2024172 0% /run/shm
none 102400 0 102400 0% /run/user
blade66:/disk/sd0g/courses/it244 8260768 2615904 5562240 32% /courses/it244
blade82:/disk/sd0f/home/jdu 8260768 8171264 6912 100% /home/jdu
mx1:/disk/sd1e/spool/mail 4129312 3403648 684384 84% /spool/mail
blade82:/disk/sd1h/home/ghoffman 78805984 32988544 45029408 43% /home/ghoffman
- When used with the -h option
df
produces more readable output
$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 12G 5.8G 5.4G 52% /
none 4.0K 0 4.0K 0% /sys/fs/cgroup
udev 2.0G 4.0K 2.0G 1% /dev
tmpfs 396M 568K 395M 1% /run
none 5.0M 0 5.0M 0% /run/lock
none 2.0G 0 2.0G 0% /run/shm
none 100M 0 100M 0% /run/user
blade66:/disk/sd0g/courses/it244 7.9G 2.5G 5.4G 32% /courses/it244
blade82:/disk/sd0f/home/jdu 7.9G 7.8G 6.8M 100% /home/jdu
mx1:/disk/sd1e/spool/mail 4.0G 3.3G 669M 84% /spool/mail
blade82:/disk/sd1h/home/ghoffman 76G 32G 43G 43% /home/ghoffman
- The
-h
also works with ls
- If I run
ls -l
on a directory the file size will be given in bytes
$ ls -l
total 1236
-rw-rw-r-- 1 ghoffman faculty 26211 Jun 12 09:23 01_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty 26138 Jun 21 15:25 02_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty 29395 Jun 6 11:21 03_class_notes_it244.html
...
- But if I use
ls -lh
I get a much more readable result
$ ls -lh
total 1.3M
-rw-rw-r-- 1 ghoffman faculty 26K Jun 12 09:23 01_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty 26K Jun 21 15:25 02_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty 29K Jun 6 11:21 03_class_notes_it244.html
- Many commands display a help message when run with the --help option
$ mkdir --help
Usage: mkdir [OPTION]... DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.
Mandatory arguments to long options are mandatory for short options too.
-m, --mode=MODE set file mode (as in chmod), not a=rwx - umask
-p, --parents no error if existing, make parent directories as needed
-v, --verbose print a message for each created directory
-Z, --context=CTX set the SELinux security context of each created
directory to CTX
--help display this help and exit
--version output version information and exit
Report mkdir bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
For complete documentation, run: info coreutils 'mkdir invocation'
- All GNU utilities accept this option
Device Drivers
- All operations that need to access any hardware ...
- must do so through the
kernel
- The kernel controls access to
- RAM (short term memory)
- Disc (long term memory)
- CPU
- How does the kernel know how to talk to new pieces of hardware?
- Like a printer?
- The answer is software known as a
device driver
- Every manufacturer of a device that works with a computer ...
- creates a device driver for it
- The device driver lets the hardware and the operating system communicate
tty
- In the very early days of Unix people used a machine called a teletype ...
- in place of a screen and keyboard
- It consisted of a keyboard and a printer
- Here is a picture
- A user would type a command on the keyboard
- The output of the command would be printed on a continuous flow of paper
- The name of this device was soon shortened to tty
- The printer part of these devices was soon replaced with a video monitor
- Any device that allows you to send text to a computer and see the output ...
- is called a terminal
- But Unix still refers to it as tty
The Unix tty Device Driver
- A terminal is a device connected to the computer
- So it needs a device driver
- The device driver for the terminal is called tty
- It is built into the kernel
- Otherwise you would not be able to talk to the machine
- The tty device driver allows your keyboard to talk to the kernel ...
- and for the kernel to send output to your screen ...
- as you type on the command line
- The characters you type are examined by the tty device driver
- It does different things with different characters
- Most of the time, it places the character in a
buffer
- A buffer is a space in memory that holds data for later processing
- But the tty device driver responds differently to certain special characters
- When the character you type is the backspace
- It erases the previous character from the buffer
- When the character is Control U something different happens
- tty erases the buffer from the current insertion point ...
- to the beginning of the line
- tty is responsible for all
command line editing
- When the tty gets a newline character ...
- it passes the contents of the buffer to the shell ...
- so the shell can run the command
- Newline is the character you get from hitting Enter on Windows ...
- or Return on a Mac
Parsing the Command Line
- The shell now takes the contents of the buffer ...
- and breaks it up into tokens
- Tokens are the strings of text separated by spaces
- This action is called
parsing
- Parsing is the act of making a list of all the strings on the command line ...
- without the spaces
- Next, the shell looks for the name of the command
- Usually, the command name is the first string on the command line
- The command can be specified by a simple filename
ls
- Or by using a pathname to the executable file
/bin/ls
The PATH System Variable
- When you run a program using a pathname at the terminal
- The shell knows where to find the executable file
- So we can run any command using the pathname of the executable file
$ /usr/bin/ls /
bin data groups lib32 media proc snap swap.img users
boot dev home lib64 mnt root sources sys usr
cdrom etc home.ORIG libx32 nobackup run spool tmp var
courses etc.ORIG.tar lib lost+found opt sbin srv tools
- But when we run ls we use
ls
- Not
/usr/bin/ls
- How can the shell know where to find the executable file?
- Programs are executable files that can be stored anywhere in the filesystem
- So how does the shell find the correct file?
- The shell checks a system variable called PATH
- PATH contains a list of directories to search for an executable file
- It checks each directory for a file with the right name
- It checks each directory in order ...
- and stops when it finds the first match
- It stores the locations in memory ...
- so it won't have to look more than once
- The
hash
command will show you this list
$ hash
hits command
1 /usr/bin/which
1 /bin/ls
- PATH always has a default value ...
- which is created when the system is installed
- Here is the default value on our system
$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/lib/oracle/12.1/client64/bin
- The absolute pathname of each directory is separated from the next by a colon, :
- If the shell can't find the file it prints an error message
- You will also get an error message if you don't have permission to run the file
- You can modify the PATH variable in your own Unix environment
- We'll see how to do this in a few classes
Running a Program in the Current Directory
Running the Command Entered on the Command Line
Setgid Permission on Directories
- The setgid permission works differently on directories
- Every Unix account has a default group assigned to it
- Whenever you create a file or directory ...
- this default group will be assigned to it
- You can change this group afterwards
- But a new file or directory is always assigned the default group ...
- when it is created
- The default group for all your Unix accounts is ugrad
- But my default group is faculty
- If I go to a directory and create a file ...
- it will be assigned the faculty group
$ touch foo.txt
total 0
-rw-r--r-- 1 ghoffman faculty 0 Feb 27 17:30 foo.txt
- However if I go to my ex directory ...
- and create a directory for today's Class Exercise
mkdir ex10
it will be assigned to a different group
$ ls -ld ex10
drwxr-sr-x 2 ghoffman it244-1G 4096 Jun 18 08:26 ex10
- Any file or directory I create inside ex10 ...
- will also be assigned to this group
- Look closely at the permissions assigned to this new directory
drwxr-sr-x 2 ghoffman it244-1G 4096 Jun 18 08:26 ex10
- Notice that the directory has setgid permission
- setgid permission works differently on a directory
- Any file or directory I create in this directory will have the same group
- And every directory created there will have setgid permission
- Since ex10 is assigned the group it244-1G
- Every file or directory inside it will have the same group
- The group it244-1G is a grading group
- It only contains my username and that of the Class Assistant
- Look at the permission of the it244 class directory
drwxrwsr-x 12 ghoffman it244-1G 4096 May 30 14:07 /courses/it244/f24/ghoffman
- This directory contain all your IT 244 directories
- Notice the setgid permission and group is it244-1G
- Since by default every file and directory has read permission set
- I can read all your files ...
- and copy them
Class Exercise
Class Quiz