ECE/CS 250 – Summer 2020 – Prof. Bletsch Recitation #1 – Unix
Introduction to Unix – CS 21
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Transcript of Introduction to Unix – CS 21
Introduction to Unix – CS 21
Lecture 4
Lecture Overview * cp, mv, and rm Looking into files
The file command head and tail cat and more
What we’ve seen so far
Homework and Quiz Homework #1 now available at
http://www.cs.ucr.edu/~villarre/cs21/homework1.html
Due next Thursday (January 20th) at the beginning of class
Quiz #1 next Thursday (January 20th) Everything covered up through today will
be fair game Homework assignments, lab
assignments, reading assignments, and lecture material
The First Wildcard * (asterisk) Wildcards are just like wild cards in
poker They can be anything (within reason) In Unix, this means that * can be
replaced by anything in the current directory
* actually gets replaced by everything in the current directory
Example Of * Usage
Why Is This Useful? At first glance, this doesn’t seem
to mean much After all, ls without the * will list all
the files anyway You can specify a little more than
every file *.txt will match all files that end in .txt
Example Of Advanced * Usage
Moving Files Around There are a couple of essential
commands to move files around cp mv
Remember, you can use touch to create an empty file to play around with
cp Usage The cp command makes a copy of
a file Usage: cp OLDFILE NEWFILE
Examples Of cp
Problems With cp?
Commonly Used Flags With cp -i flag
Asks if you want to write over a file that already exists
-r flag Recursively will copy all files and
subdirectories
What Does Recursion Mean? The same program gets called
over and over again In this context, cp gets called on all
files in all subdirectories Parent directories and other files
“above” the current directory are not affected
A Graphical Representation Of Recursion
/
/home
/usr/zzz
/usr
/usr/bin /usr/misc
cp –r /usr ~/temp
~/temp
~/temp/usr
Keep In Mind All Of The Permissions You will only be able to copy a file
that you have read permission on You will only be able to create a
file in a directory that you have write permission in
mv Usage The mv command will move a file
from one location to a new location Usage: mv OldFile NewFile You can also think of this as a
rename command
Examples Of mv
How Does This Compare To Windows? Windows will let you drag and drop
files from one location to another Right click if you would like to copy A little progress bar shows up
animating the file being moved over to the new folder
Other than that, it is exactly the same
rm Usage The rm command will remove a file
This doesn’t normally include directories
Usage: rm filename
Examples Of rm
Commonly Used Flags -i
Verify the delete -f
Force the removal of a file If you have permission, the file is gone,
regardless of any warnings that might pop up “Yeah, yeah, just do it” Overrides the –i flag
-r Recursively delete files
Dangers Of rm
Unix Is missing something you are probably used to rm is probably one of the most
dangerous commands in Unix
Once It’s Gone, It’s Gone… There is no way to get a file that
has been removed back Only run rm if you are absolutely
sure you want to remove a file The –i flag provides a little
protection Prompts the user if they are really
sure they want to delete the file
Extreme Dangers Of rm rm –rf ~
Say “Bye, bye” to your home directory rm –rf /
You won’t have permission to delete much, but…
If you are root, say goodbye to the entire system!
rm –rf . rm –rf *
Don’t look as dangerous, but you have to be absolutely certain you know where you’re at
So Why Use rm –rf Then? The most destructive command in
Unix, why would you ever want to use it?
Just so happens that you WILL want to remove large portions of your files at some time (most likely many times) Much easier to run “rm – rf” than delete
each file individually
Avoiding The Dangers Of rm The best way is to make sure you
are always using the –i flag and only use –f when you are certain
Always check where your current directory is so you don’t delete the wrong file
Make copies of important files just in case
Unix, The Multichoice OS What’s the difference?
rm –rf subdirectory/ rmdir subdirectory/
Multichoice Again What’s the difference?
mv fileA fileB cp fileA fileB rm fileA
Examining Files Closer As previously stated, everything in
Unix is a file But different files have different uses How do you tell what type a file is? Example: In Windows, a *.doc file is a
Word Document No such restriction is enforced in Unix
A *.doc file might even be an executable!
The file Command A helpful command to get you
started in your quest for knowledge: file
Checks the first few bytes of the file in question and takes its best guess as to what type of file it is Sometimes file gets it wrong, but
most of the time it is pretty good
Example Different Types Of Files
What If I Want To Actually Look Inside The File Myself? If the file is a text file, several
options exist head tail cat more
If the file is a binary (executable), you don’t want to read it! (trust me)
The head Command Print out the first few lines of a text
file 10 by default
Provides a quick way to see if this is the file you’re looking for
Doesn’t bombard you with a million line file scrolling off the screen
Usage: head FILE
Common Flags For The head Command -6
Only print out the first 6 lines Actually, any number works here the
same way What counts as a line?
Everything up until the new line terminator
Examples Of head Usage
The tail Command Pretty much the opposite of head Prints out the last few lines of a file
10 by default Usage: tail FILE Just like head, -NUM
Prints out the last NUM lines
Examples Of tail Usage
The cat Command The cat command will print out an
entire file to the screen Usage: cat FILE Cat?
Short for concatenate This command can be used to print
out multiple files one right after the other
cat FILE1 FILE2
Problem With cat If the file is very large, it will scroll
off the screen too fast to read No way to read a scrolling file
without stopping the program Cntrl-C will kill a running program
The more Command Works exactly like cat, but doesn’t
automatically scroll the screen Usage: more FILE This is the first truly interactive
program we’ve seen You can control how the program runs
while it is running
More Example
How do you scroll to the next screen? Hit the space bar
Advanced more usage Return will move you one line NUM followed by return will move
you NUM lines Example: 5 [Return] displays the next
5 lines q will quit more without finishing
less Is more A better version of more exists: less Allows all of the same options as more Allows easier moving through the file
Arrow keys and page up, page down will move you both forward and backward
Which you choose to use is, of course, up to you
What About Binary Files?
You Now Should Have Enough Info To Cause Some Damage You now have the ability to:
Wander about the system Create simple files and change all the
properties of these files Copy and move files around Check out what type of files you are
looking at and read the interesting ones Delete files you no longer want
The Class So Far… History of Unix and the hacker connection Logging on and getting help
man Environment variables The Unix Directory Structure
ls cd pushd and popd Relative and absolute pathnames . And ..
Class Summary Continued Disk usage
du Compressing files Symbolic Links Ownership and permissions
chmod umask
The Class So Far Continued Moving files
cp mv
Checking the contents and type of files file head and tail cat and more
In Lab Today You will practice setting
permissions and the effect they have chmod and umask
Start creating simple files and moving them around the system
Read files using head, tail, cat, and more
Next Week Things really start to get
interesting as we start getting programs to work together and tie everything together
We’ll look at more uses of wildcards and start making Unix “sentences” with pipes and filters
We’ll look at the oddly named but strangely powerful command: grep