LIS508
using Debian GNU/Linux
Thomas Krichel
2010-01-10
ssh
• The main protocol we use to communicate
with the server is the secure shell “ssh”.
• The server has to have ssh server software
installed.
• Any rented server will have this.
• Otherwise run “aptitude install opensshserver”
installing putty
• Go to your favorite search engine to search
for putty.
• If you have administrator rights install the
installer version.
• Since you have already installed winscp,
you should have no further problems.
putty options
• In the window/translation choose UTF-8,
always.
• Find out what the size of your screen for the
font that you are using, and save that in
your session.
• For wotan, the port is 22, ssh.
• You can choose to disable the annoying
bell.
how is ssh secure
• ssh uses public key cryptography.
• It can cope with serveral crytographic
algorithm. The most common one is rsa.
• In public key cryptography, you have two
keys.
• One is the private key.
• The second is the public key.
private key
• Using the private key, I can decrypt a
message that has been encrypted using my
public key.
• Somebody can check that I am who I claim
to be because by keeping the private key
private, I am the only one who can decrypt
it.
authorizing keys
• If you want to give your friend access to an
account, you can ask her for her public key.
• You will then authorize the public key by
storing it in a file with authorized keys.
• When an somebody comes along and
pretends to be your friend you can
challenge her to decode a message
encoded with your friends public key.
public key
• Using the public key, I can encrypt a
message and send it to you.
• Once the message is encrypted, I can not
decode it any more. The public key can not
be used to decode a message.
• This is the reason why the encryption key
can be made public.
communication with the server
• Assume we are using a Microsoft Windows
client.
• For file editing and manipulation, we use
putty.
• For file transfer, we use winscp.
• Both are available on the web.
password authentication
• When we login to a machine, we give a
password for the user that we login as.
• This a common form of authenticiation.
• The idea is that we keep the password
secret.
key authenticatiton
• ssh uses public key
• This a common form of authenticiation.
• The idea is that we keep the password
secret.
key creation
• “ssh-keygen” is the command to use to
create a key. Answer all questions with the
<ENTER> key.
• “cd .ssh” and “ls -l” shows you the contents
of the directory .ssh created in the first step.
• The file “id_rsa.pub” has your public key.
authorized_keys
• In .ssh, you can maintain a file
“authorized_keys” that contains the public
keys of all users you authorize to access
the account, one line per user.
• “cp id_rsa.pub authorized_keys” will
authorize yourself. Then “ssh user@wotan”
will allow you to login again as you on
wotan.
• You can also create a public key with putty.
issuing commands
• While you are logged in, you talk to the
computer by issuing commands.
• Your commands are read by command line
interpreter.
• The command line interpreter is called a
shell.
• You are using the Bourne Again Shell,
bash.
bash features
• bash allows to browse the command history
with the up/down arrow keys.
• bash allows to edit commands with the
left/right arrow keys.
• You can complete command and file names
with <TAB>.
• bash comes with a language of commands
that allows to write batch files.
• “exit” is the command to leave the shell.
environment variables
• These are variables used by the shell.
• Two important ones are
– $HOME your home directory
– $PATH the location where bash will search for
executable files.
• echo $HOME will show you your home
directory.
• “env” is a command that can be used to see
all environment variables.
bash initialization
• Files that start with a dot are hidden. They
are only seen with “ls -a”.
• .bashrc is a file written in bash language
that is run every time bash is started.
• .bash_profile or .profile is run when the
shell is started at login.
• You can customize these files.
files, directories and links
• Files are continuous chunks data on disks
that are required for software applications.
• Directories are files that contain other files.
Microsoft calls them folders.
• In UNIX, the directory separator is “/”
• The top directory is “/” on its own.
home directory
• When you first log in to wotan you are placed
in your home directory /home/username
• “cd” is the command that gets you back to the
home directory.
• The home directory is also abbreviated as “~“
• cd ~user gets you to the home of user user.
• “cd ~” does what?
~/public_html
• The web server on wotan will map requests
to http://wotan.liu.edu/~user to show the file
~user/public_html/index.html
• The web server will map requests to
http://wotan.liu.edu/~user/file to show the
file ~user/public_html/file
• The server will do this by virtue of a
configuration option.
changing directory, listing files
• “cd directory” changes into the directory
directory
• the current directory is “.”
• its parent directory is “..”
• “ls” lists files
users and groups
• “root” is the user name of the superuser.
• The superuser has all privileges.
• There are other physical users, i.e. persons
using the machine
• There are users that are virtual, usually
created to run a daemon. For example, the
web sever in run by a user www-data.
• Arbitrary users can be put together in
groups.
permission model
• Permission of files are given
– to the owner of the file
– to the group of the file
– and to the rest of the world
• A group is a grouping of users. Unix allows
to define any number of groups and make
users a member of it.
• The rest of the world are all other users
who have access to the system. That
includes www-data!
listing files
• “ls” lists files
• “ls -l” make a long listing. It contains
– elementary type and permissions (see next
slide)
– owner
– group
– size
– date
– name
first element in ls -l
• Type indicator
– d means directory
– l means link
– - means ordinary file
•
•
•
•
3 letters for permission of owner
3 letters for permission of group
3 letters for permission of rest of the world
r means read, w means write, x means
execute
• Directories need to be executable to get in
them.
change permission: chmod
• usage: chmod permission file
• file is a file
• permission is three numbers, first for owner,
2nd for group and 3rd rest of the world.
• Each number is sum of
– 4 for read
– 1 for execute
- 2 for write
- 0 for no permission
• Example: chmod 764 file
general structure of commands
• commandname –flag --option
• Where commandname is a name of a
command
• flag can be a letter
• Several letters set several flags at the same
time
• An option can also be expressed with - and a word, this is more user-friendly than
flags.
example command: ls
• ls lists files
• ls -l makes a long listing
• ls -a lists all files, not only regular files but
some hidden files as well
– all files that start with a dot are hidden
• ls -la lists all files is long listing
• ls --all is the same as ls -a. --all is known as
a long listing.
copying and removing files
• cp file copyfile copies file file to file copyfile.
If copyfile is a directory, it copies into the
directory.
• mv file movedfile moves file file to file
movedfile. If movedfile is a directory, it
moves into the directory.
• rm file removes file, there is no recycling
bin!!
directories and files
• mkdir directory makes a directory
• rmdir directory removes an empty directory
• rm -r directory removes a directory and all
its files
• more file
– Pages contents of file, no way back
• less file
– Pages contents of file, “u” to go back, “q”
to quit
soft links
• A link is a file that contain the address of
another file. Microsoft call it a shortcut.
• A soft link can be created with the
command
• ln -s file link_to_file where file is a file that is
already there and link_to_file is the link.
file transfer
• You can use winscp to upload and
download files to wotan.
• If uploaded files in the web directory remain
invisible, that is most likely a problem with
permission. Refer back to permissions.
• chmod 644 * will put it right for the files
• chmod 755 . (yes with a dot) will put it right
for the current directory
• * is a wildcard for all files.
• rm -r * is a command to avoid.
editing
•
•
•
•
•
There are a plethora of editors available.
For the neophyte, nano works best.
nano file edits the file file.
nano -w switches off line wrapping.
nano shows the commands available at the
bottom of the screen. Note that ^letter,
where letter is a letter, means pressing
CONTROL and the letter letter at the same
time.
emacs
• This is another editor that is incredibly rich
and complex.
• Written by Richard M. Stallman, of GNU
and GPL fame.
• Get an emacs cheat sheet of the web
before you start it. Or look at next slide.
emacs commands
(here ^ stands for the control character)
• ^x^s saves buffer
• ^x^c exits emacs
• ^g escapes out of a troublesome situation
• control+space sets the mark
• ^w removes until the mark (cut)
• ^y pastes
common emacs/bash commands
• ^k kills until the end of the line or removes
empty line
• ^y yank what has been killed (paste)
• ^a get to the beginning of the line
• ^e get to the end of the line
• These commands also work in the shell.
emacs modes
• Just like people get into different moods,
emacs gets into different modes.
• One mode that will split your pants is the
PHP mode.
• Then look how emacs checks for
completion of parenthesis, braces,
brackets, and the ; and use the tab
character to indent.
copy and paste
• Putty allows to copy and paste text between
windows and wotan.
• On the windows machine, it uses the
windows approach to copy and paste
• On wotan machine,
– you copy by highlighting with the mouse’ left
button
– you paste using the middle button
– if you don't have a middle button, use left and
right together
man
• man is the manual lookup command
• You usually say “man command” if you
want to have an overview over the
command.
• man -k keyword looks up the man pages on
the computer for pages with the keyword
keyword. I don't find this very effictive ;-(
echo
• echo is a command to desplay a line of text.
• Example: echo foo
cat
• This is a program that displays the contents
of a file.
• Use like “cat file” to display the contents of
the file file.
•
more
• This is a paging utility.
• It is mostly used as part of a pipe.
• You take the output of a command and pipe
it to the input of the next command
• Example “cat longfile | more”
• | is the piping operator.
less
• This is a paging utility. It does more than
more.
• You can use the command “u” to go up.
• You must use the command “q” to leave the
page.
du
• du is a command to look at disk usage.
• du -s makes a summary, rather than listing
the usage of every.
• The size is give in kilobytes. A kilobyte is
1024 bytes, not 1000 bytes.
ln -s
• ln -s origin target makes a link from a target
file target to an original file origin.
• Then when you access target you get the
same contents as in origin.
• This concept is knows as a shortcut in
Microsoft windows.
ssh
• ssh user@host logs you in as user user on
the host host. host can be a DNS name or
an IP address.
• If you don't have you public keys in the
remote account's .ssh/autorized_keys file,
you will be prompted for a password.
slogin
• slogin is a bit of a synonym for ssh.
• I use it often “slogin host -l user”.
scp
• This copies files from one host to another.
You can use it has complicated as “scp
user1@host1:file1 user2@host2:file2.
• But often one of the users is the current
user on thee current host. In that case the
user@host: bit can be left out.
• If you don't have permissions via keys you
will be prompted for passwords.
date
• date says what time it is.
• This depends on the locale, as set of
conventions to deal with language issues.
find
• find finds file in a directory.
• This is a very important and powerful
command.
• Example: “find . -type f -name '*.deb'” finds
all regular files ending with “deb”
• The most powerful feature is “-exec
command \;” that executes a command on
the files found. Each file is represented by
{}
find examples
• find ~/public_html -name '*.html' -exec cp {}
{}.org
• find ~/public_html -name '*.css' -exec cat {}
>> /tmp/master.css
output and error
• In programs that ran on the shell, there are
two concepts, the output and the error.
• The output is what is written by the
command in normal operation, e.g. a list of
files for ls. The output is often empty, for
example for the cp command.
• The error of a command is what is reported
when an error occurs. Example “cp foo bar”
will generate an error when there is no file
foo.
output redirection
• The redirect the output of a command to a
file, use > or >>
• Example “echo foo > /tmp/foo.file”
• When you use “>” the file will be created
anew.
• When you use “>>” the output will be
appended to an existing file, if any.
error redirection
• The redirect the output of a command to a
file, use “2>” or “2>>”
• Example “cp foo 2> /tmp/error.file”
• When you use “2>” the file will be created
anew.
• When you use “2>>” the error will be
appended to an existing file, if any.
pipes
• The piping operator | connects the output of
one command to the input of another.
• Example: echo “hi thomas” | mutt
krichel@openlib.org
grep
• grep is a very important utility to look up a
pattern in a file, as in “grep pattern file”.
• Pattern is often just a string of what we
want to find.
• But in the pattern, the following will have
special meaning: ( ) \ + . ? * [].
• grep -r finds the pattern recursively.
sort and uniq
• sort sorts entries in it's inputExample: “last |
cut -f 1 | sort” gives you a sorted list of last
users.
• uniq gives uniq values in an sorted list. The
list has to be sorted first. Example “last | cut
-f 1 | sort | uniq”
crontab files
• /etc/crontab is the system wide crontab.
• /etc/cron.d, /etc/cron.monthly,
/etc/cron.daily, /etc/cron.weekly have more
cron examples for your viewing pleasure.
• Output and error from a crontab entry is
sent by local mail to the user.
• This is one reason of having mail at least
set up locally, i.e. from the machine to users
on the same machine.
crontab entries
• You can create a file that contains
commands you want to schedule regularly,
and the schedule
• The file cantains lines of the form minutes
hour day_of_month month day_of_week.
There day_of_week ranges form 0 to 6 with
0 being Sunday.
• Save this in a file say etc/crontab.
http://openlib.org/home/krichel
Thank you for your attention!
Please switch off machines before leaving!