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Unix Accounts
18/04/23 2
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.
18/04/23 35
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.
18/04/23 36
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
18/04/23 39
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
18/04/23 41
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.
18/04/23 42
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.
18/04/23 43
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
18/04/23 45
ls -l and permissions
-rwxrwxrwx Owner Group Others
Type of file:- means plain filed means directory
18/04/23 46
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
18/04/23 47
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
18/04/23 48
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.
18/04/23 49
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.
18/04/23 51
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)
18/04/23 53
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
18/04/23 54
vi
Remember there are two modes insert and command ESC will always get you back to command
18/04/23 55
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, …
18/04/23 58
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] …
18/04/23 59
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
18/04/23 61
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
18/04/23 62
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