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.

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

/

bin etc home tmp usr

bob fred bin etc

netprog unix Xwho

ls

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The Filesystem/

bin etc home tmp usr

bob fred bin etc

netprog unix Xwho

ls

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

/

bin etc users tmp usr

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

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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 it’s 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

> ls -l foo

-rw-rw---- 1 bob grads 13 Jan 10 23:05 foo

permissionsowner group

size

time

name

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ls -l and permissions

-rwxrwxrwx Owner Group Others

Type of file:- means plain filed 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]

u=user g=group o=other a=all

+ 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

> ls -al foo

rwxrwx--x 1 hollingd grads …

> chmod g-wx foo

> ls -al foo

-rwxrw---- 1 hollingd grads

>chmod u-r .

>ls -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 doesn’t 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

Managing users on Linux

Linux and Windows

User management

Adding/Removing Users UNIX: Traditionally, user account details are

stored in the /etc/passwd file (/etc/group, /etc/shadow)

This file has the format:username:password:uid:gid:comment:home

dir:shell one way to get a list of usernames on a system: cut –d: -f1 /etc/passwd

Do not edit this file directly, use useradd / userdel / groupadd / groupdel commands

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useradd (linux) Create a new user or update default new user

information useradd [-c comment] [-d home_dir] [-e expire_date] [-f inactive_time] [-g initial_group] [-G group[,...]] [-m [-k skeleton_dir] | -M] [-n] [-o] [-p passwd] [-r] [-s shell] [-u uid] login

useradd -D [-g default_group] [-b default_home] [-e default_expire_date] [-f default_inactive] [-s default_shell] The –D option causes system defaults specified in

/etc/login.defs to be used for this account creation

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userdel Delete a user account and related files

userdel [-r] login

The userdel command modifies the system account files, deleting all entries that refer to login. The named user must exist. The options which apply to the userdel command are:

-r Files in the user’s home directory will be removed along with the home directory itself and the user’s mail spool. Files located in other file systems will have to be searched for and deleted manually.

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groupadd (linux)… Create a new group

groupadd [-g gid [-o]] [-r] [-f] group

Creates a new group account using the values specified on the command line and the default values from the system.

The new group will be entered into the system files as needed.

Options: -g gid The numerical value of the group’s ID. This value must be

unique, unless the -o option is used. The value must be non-negative. The default is to use the smallest ID value greater than 500 and greater than every other group. Values between 0 and 499 are typically reserved for system accounts.

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…groupadd (linux) -r instructs groupadd to add a system account. The first

available gid lower than 499 will be automatically selected unless the -g option is also given on the command line.

-f the force flag. This will cause groupadd to exit with an error when the group about to be added already exists on the system. If that is the case, the group won’t be altered (or added again).

Also modifies the way -g option works. When you request a gid that it is not unique and you don’t specify the –o option too, the group creation will fall back to the standard behavior (adding a group as if neither -g or –o options were specified).

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groupdel (linux)

Delete a group groupdel group The groupdel command modifies the

system account files, deleting all entries that refer to group. The named group must exist.

You must manually check all filesystems to insure that no files remain with the named group as the file group ID.

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groupdel (linux)

Delete a group groupdel group The groupdel command modifies the

system account files, deleting all entries that refer to group. The named group must exist.

You must manually check all filesystems to insure that no files remain with the named group as the file group ID.

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Managing Users

Whether in the UNIX or Windows (or other) environment, it is best practice to manage users, where possible, on a per-group basis

Wherever possible, settings and configurations should be managed on a per-group basis

For this reason, the management of group settings is as important in any multi-user environment as is the management of individual settings

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Managing Groups

One approach to the management of groups is a hierarchical approach, whereby the user community is divided into a number of groups based on roles within the organisation

In this way, an individual may belong to several groups, each of which may have a different combination of security settings etc.

If we can be assured users have only setting which are assigned on a per-group basis, it becomes easier for us to ensure that all users with similar or identical roles are subject to the same rules

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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

/Br*ley, /Br[0-9], /B[rR] …

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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 tabstop=x Set the number of spaces for each tab

:set number Show line numbers

:set nonumber Hide line numbers