Unix Accounts

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Unix Accounts
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Unix Accounts

• To access a Unix system you need to have an
  account.
• Unix account includes
• username and password
• userid and groupid
• home directory
• shell

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username

• A username is (typically) a sequence of
  alphanumeric characters of length no more than 8.
• username the primary identifying attribute of
  your account.
• username is (usually) used as an email address
• the name of your home directory is usually
  related to your username.

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password

• a password is a secret string that only the user
  knows (not even the system knows!)
• When you enter your password the system encrypts
  it and compares to a stored string.
• passwords are (usually) no more than 8 characters
  long.
• It's a good idea to include numbers and/or
  special characters (don't use an english word!)

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userid

• a userid is a number (an integer) that identifies
  a Unix account. Each userid is unique.
• It's easier (and more efficient) for the system
  to use a number than a string like the username.
• You don't need to know your userid!

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Unix Groups and groupid

• Unix includes the notion of a "group" of users.
• A Unix group can share files and active
  processes.
• Each account is assigned a "primary" group.
• The groupid is a number that corresponds to this
  primary group.
• A single account can belong to many groups (but
  has only one primary group).

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Home Directory

• A home directory is a place in the file system
  where the account files are stored.
• A directory is like a Windows folder.
• Many unix commands and applications make use of
  the account home directory (as a place to look
  for customization files).

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Shell

• A Shell is a unix program that provides an
  interactive session - a text-based user
  interface.
• When you log in to a Unix system the program you
  initially interact with is your shell.
• There are a number of popular shells that are
  available.

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Logging In

• To log in to a Unix machine you can either
• sit at the console (the computer itself)
• access via the net (using putty, telnet, rsh,
  ssh, kermit, or some other remote access client).
• The system prompts you for your username and
  password.
• Usernames and passwords are case sensitive!

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Session Startup

• Once you log in, your shell will be started and
  it will display a prompt.
• When the shell is started it looks in your home
  directory for some customization files.
• You can change the shell prompt and a bunch of
  other things by creating customization files
  (more on this later)

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Your Home Directory

• Every Unix process has a notion of the current
  working directory.
• You shell (which is a process) starts with the
  current working directory set to your home
  directory.
• A process is an instance of a program that is
  currently running.

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Interacting with the Shell

• The shell prints a prompt and waits for you to
  type in a command.
• The shell can deal with a couple of types of
  commands
• shell internals - commands that the shell handles
  directly.
• External programs - the shell runs a program for
  you.

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UNIX commands and the filesystem
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Some Simple Commands

• Here are some simple commands to get you started
• ls lists file names (like DOS dir command).
• who lists users currently logged in.
• date shows the current time and date.
• pwd print working directory
• To shutdown a server (sys admin only)
• Init 0

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Files and File Names

• A file is a basic unit of storage (usually
  storage on a disk).
• Every file has a name.
• Unix file names can contain any characters
  (although some make it difficult to access the
  file).
• Unix file names can be long!
• how long depends on your specific flavor of Unix

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File Contents

• Each file can hold some raw data.
• Unix does not impose any structure on files
• files can hold any sequence of bytes.
• Many programs interpret the contents of a file as
  having some special structure
• text file, sequence of integers, database
  records, etc.

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Directories

• A directory is a special kind of file - Unix uses
  a directory to hold information about other
  files.
• We often think of a directory as a container that
  holds other files (or directories).
• On Windows, a directory is the same idea as a
  folder.

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The Filesystem
/
ls
who
netprog
unix
X
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The Filesystem
/
ls
who
netprog
unix
X
Syllabus
/users/bob/unix/Syllabus
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Absolute Pathnames

• The pathnames described in the previous slides
  start at the root.
• These pathnames are called "absolute pathnames".
• We can also talk about the pathname of a file
  relative to a directory.

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Relative Pathnames

• If we are in the directory /users/bob, the
  relative pathname of the file Syllabus is
• unix/Syllabus
• Most unix commands deal with pathnames!
• We will usually use relative pathnames when
  specifying files.

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Example The ls command

• Exercise login to a unix account and type the
  command "ls".
• The names of the files are shown (displayed) as
  relative pathnames.
• Try this
• ls /usr
• ls should display the name of each file in the
  directory /usr.

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Disk vs. Filesystem

• The entire hierarchy can actually include many
  disk drives.
• some directories can be on other computers

/
bob
fred
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The current directory and parent directory

• There is a special relative pathname for the
  current directory
• .
• There is a special relative pathname for the
  parent directory
• ..

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The ls command

• The ls command displays the names of some files.
• If you give it the name of a directory as a
  command line parameter it will list all the files
  in the named directory.

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Some things to try

• ls list files in current directory
• ls / list files in the root directory
• ls . list files in the current directory
• ls .. list files in the parent directory
• ls /usr list files in the directory /usr

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Command Line Options

• We can modify the output format of the ls program
  with a command line option.
• The ls command support a bunch of options
• l long format (include file times, owner and
  permissions)
• a all (shows hidden files as well as regular
  files)
• F include special char to indicate file types.
• hidden files have names that start with "."

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ls command line options

• To use a command line option precede the option
  letter with a minus
• ls -a or ls -l
• You can use 2 or more options at the same time
  like this
• ls -al

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General ls command line

• The general form for the ls command is
• ls options names
• The options must come first!
• You can mix any options with any names.
• An example
• ls -al /usr/bin

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ls options names

• The brackets around options and names in the
  general form of the ls command means that
  something is optional.
• We will see the general form of many commands
  described in this manner.
• Some commands have required parameters.

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Many names

• You can give the ls command many names
• ls /usr /etc
• ls -l /usr/bin /tmp /etc

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Moving Around in the Filesystem

• There cd command can change the current working
  directory
• cd change directory
• The general form is
• cd directoryname

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cd

• With no parameter, the cd command changes the
  current directory to your home directory.
• You can also give cd a relative or absolute
  pathname
• cd /usr
• cd ..

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Some more commands and command line options

• ls -R will list everything in a directory and in
  all the subdirectories recursively (the entire
  hierarchy).
• you might want to know that Ctrl-C will cancel a
  command (stop the command)!
• pwd print working directory
• df shows what disk holds a directory.

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Copying Files

• The cp command copies files
• cp options source dest
• The source is the name of the file you want to
  copy.
• dest is the name of the new file.
• source and dest can be relative or absolute.

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Another form of cp

• If you specify a dest that is a directory, cp
  will put a copy of the source in the directory.
• The filename will be the same as the filename of
  the source file.
• cp options source destdir

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Yet another form of cp

• If you specify more than two names, cp assumes
  you are using this form.
• cp options source... destdir
• In this case cp will copy multiple files to
  destdir.
• source... means at least one name (could be more
  than one)

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Deleting (removing) Files

• The rm command deletes files
• rm options names...
• rm stands for "remove".
• You can remove many files at once
• rm foo /tmp/blah /users/john/course

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rm Exercises

• Try to delete /etc/passwd
• Try to delete a directory
• Look at the man page for rm
• man rm

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File attributes

• Every file has some attributes
• Access Times
• when the file was created
• when the file was last changed
• when the file was last read
• Size
• Owners (user and group)
• Permissions

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File Time Attributes

• Time Attributes
• when the file was last changed ls -l
• when the file was created ls -lc
• when the file was last read (accessed) ls -ul
• actually its the time the file status last
  changed.

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File Owners

• Each file is owned by a user.
• You can find out the username of the file's owner
  with the "-l" option to ls,
• Each file is also owned by a Unix group.
• ls -l also shows the group that owns the file.

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File Permissions

• Each file has a set of permissions that control
  who can mess with the file.
• There are three kinds of permissions
• read abbreviated r
• write abbreviated w
• execute abbreviated x
• There are separate permissions for
• the file owner, group owner and everyone else.

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ls -l

• gt ls -l foo
• -rw-rw---- 1 bob grads 13 Jan 10 2305 foo

size
permissions
name
owner
group
time
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ls -l and permissions

• -rwxrwxrwx
• Owner Group Others

Type of file - means plain file d means
directory
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rwx

• Files
• r - allowed to read.
• w - allowed to write.
• x - allowed to execute
• Directories
• r - allowed to see the names of the file.
• w - allowed to add and remove files.
• x - allowed to enter the directory

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Changing Permissions

• The chmod command changes the permissions
  associated with a file or directory.
• There are a number of forms of chmod, this is the
  simplest
• chmod mode file

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chmod mode file

• Mode has the following form
• ugoa-rwx
• uuser ggroup oother aall
• add permission - remove permission
  set permission
• The form is really more complicated, but this
  simple version will do enough for now.

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chmod examples

• gt ls -al foo
• rwxrwx--x 1 hollingd grads
• gt chmod g-wx foo
• gt ls -al foo
• -rwxrw---- 1 hollingd grads
• gtchmod u-r .
• gtls -al foo
• ls . Permission denied

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Other filesystem and file commands

• mkdir make directory
• rmdir remove directory
• touch change file timestamp (can also create a
  blank file)
• cat concatenate files and print out to terminal.

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Some commands worth knowing

• finger user name
• who and whoami
• users
• Lists who is logged in
• whereis command name
• Checks your path
• find
• Find path name file name
• Find / -name whereis
• Find / -name wh

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Some commands worth knowing

• history
• !number in history
• head display top few lines of a file
• more operates like man (and supports searching)

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Job control

• command
• Run the command in the background
• For a running program, ctrl-z stops it (but
  doesnt kill it)
• fg brings a background job into the foreground
• Jobs list the processes running (stopped or not)
  from the current shell.
• bg put a stopped job to run in the background
• job number bring a stopped job back to the
  foreground
• Kill job number kills that job

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vi

• Remember there are two modes
• insert and command
• ESC will always get you back to command

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Basic vi commands

• a - Append
• A Append at the end of the current line
• i Insert before the current character
• I insert at the start of the current line
• r Replace
• replace one character under the cursor. Specify
  count to replace a number of characters
• u - undo the last change to the file. Typing u
  again will re-do the change.
• x - delete character under the cursor. The
  characters will be deleted after the cursor. 5x
  will delete 5 characters

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Navigation

• h move the cursor to the left one character
  position.
• j move the cursor down one line.
• k move the cursor up one line.
• l move the cursor to the right one character
  position.

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More navigation

• Move to the beginning of the line.
• Move to the end of the line.
• w
• Move to the end of the word.
• (
• Move to the beginning of the sentence.
• )
• Move to the end of the sentence
• Move to end of paragraph (defined by a blank
  line)
• Move to start of paragraph
• Every command can be repeated by putting a number
  before/after the command
• 5, 4, 5w,

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Cutting, Pasting and searching

• Cut with either delete command or yank (y or Y)
  command
• Paste with
• P paste before current position
• p past after current position
• Search with
• /search pattern search from this point on
• Repeat search with / of n
• Search pattern is a regular expression
• /Brley, /Br0-9, /BrR

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Variations on delete commands

• d
• deletes from current cursor position to the
  beginning of the line.
• d
• deletes from current cursor position to the end
  of the line.
• dw
• deletes from current cursor position to the end
  of the word.
• dd
• deletes a line from current cursor position
  downwards.
• d
• deletes to end of paragraph.
• d
• deletes to start of paragraph.
• Every command can be repeated by putting a number
  before/after the command
• 3dd, 5dw, 4d, d5w,
• Works with yank (y/Y) as well

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Substituting

• s/pattern/to_pattern/options
• s/Ronan/Robert
• Find first occasion of Ronan and replace with
  Robert
• s/Ronan/Robert/g
• Find every occasion of Ronan and replace with
  Robert

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Open, save, quit

• vi filename1 filename2
• n will move to the next file you want to edit
• w write the file
• q - quit vi, assumes no changes since last save
• q! quit and lose changes
• e - close the current file and open another
• e! as e but lose changes

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Some options

• set all
• Lists all the options
• set tabstopx
• Set the number of spaces for each tab
• set number
• Show line numbers
• set nonumber
• Hide line numbers

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