Getting help

Vijay Adusumalli will be in the CS majors lab in the basement of the Love Building from 11:00am until 1:00pm everyday from Wednesday, January 23, until Monday, January 28, (except for Sunday, January 27) to help you if you cannot get set up.

Getting connected: "logging in"

Fundamental to the idea of Unix is the idea of "logging in". This means

1. Presenting some sort of credentials (maybe in an automated fashion)
2. Having a "shell" process created for you
3. Using the "shell" process

Unconnecting: logging out

• "exit" should work for any shell, and is usually your best choice
• CTRL-D is very likely to work also
• Most login shells also will accept "logout"
• "bye" is sometimes accepted, but don't count on it

Who else is on the system?

• who command
• w command
• users command

Communicating with
other users:

• traditional Mail
• write
• wall
• talk
• irc

Finding out more with man

• man CMD
• man FILE
• man -k KEYWORD

changing file permissions

To change permissions, we use the chmod command:

  chmod u+x   # add execute permission for the user
  chmod g+x   # add execute permission for group
  chmod o+x   # add execute permission for other
  chmod a+x   # add execute permission for all users

  chmod a-x   # remove execute permission for all users
  chmod g-r   # remove read permission for group

File permission defaults

Shells generally have an "built-in" command called umask which allows the user to specify the default permissions for newly created files.

It actually specifies the complement of the default read-write permissions for newly created files.

umask 077    # no one except the user can read or write new files
umask 000    # anyone can read or write all new files (dangerous!)
umask 022    # anyone can read but not write new files 
umask 777    # no one (not even the owner) can read or write new files

Working with files

% cal 1776 > 1776.calendar.txt
% cal 1752 > 1752.calendar.txt

We can use many programs to examine this file: cat, less, and od are among the most popular, but also head, tail, and even grep are also quite useful.

Joining files with cat

You can use cat to join multiple files together:

cat 1776.calendar.txt 1752.calendar.txt > both.txt
cat 17{76,52}.calendar.txt > both.txt

# or with append
cat 1776.calendar.txt > both.txt
cat 1752.calendar.txt >> both.txt

Copying with cp

cp both.txt copy.txt   # copies, but uses new date and 
                       # umask default permissions for new file
cp -a both.txt         # creates an exact copy
cp -r .mozilla .mozilla-backup   # creates a new copy of a dir
                                 # but times are new and permissions
                                 # are umask-controlled
cp -a .mozilla .mozilla-exact    # creates an exact copy of a dir

Moving files

Moving files with mv is usually far faster than copying the same files with cp. If a file is in the same filesystem (and if it's in your home directory it very likely is), then mv just changes some directory information rather than doing anything with the contents. cp of course must work with the contents (unless you are using a very sophisticated filesystem that understands on-the-fly deduplication.)

mv both.txt old.txt              
mv .mozilla .mozilla-2013-01-22   # notice that no option is necessary
                                  # to specify directory operations

Removing files

Removing files is easy — maybe too easy!

rm old.txt
rm -rf .mozilla        # remove a directory without complaining
rm -rf /               # remove everything in the system (bad idea!)
rmdir .somedir/        # doesn't work unless .somedir/ is empty