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Friday, May 27, 2011

General Unix Information

General Unix Information

There are numerous flavours of Unix; AIX (IBM), Solaris (SUN), Xenix, Linux etc. all of which conform to the Posix standard. Unix often comes with a number of command shells, e.g. Bourne Shell ($ prompt) or C shell (% prompt with enhancements on the Bourne Shell) or Korn shell ($ prompt) which have slightly differing command syntax, although principally they are the same. The shell interprets the commands that you type. The 'Tea Sea Shell' (tcsh) is often used in the Linux environment and contains useful attributes such as 'up-arrow' and 'down-arrow' recall of previous command entries, and the use of the TAB key to complete commands (much like the Cisco IOS!). The shell interprets commands with the operating system kernel. The beauty of Unix is that hundreds of people can access one box at once and each one can run a number of programs, a separate shell opens for each log on that occurs, and each user can have a completely different 'environment' setup, different colours, priviledges, file and directory access and different shell.

Unix file names can be up to 14 characters long and include the _ and the . characters.

Every Unix command or filename is case sensitive, unlike DOS, this is the most common error to be aware of. Commands leave 'notes' for programs (such as printing) that are 'buried' in the Unix system. This is so that one user does not hog one program, many people can access it. Such a program is called a 'daemon'.

Below is a diagram illustrating a typical Unix file system structure on a box:


unix file system

An Absolute Pathname starts from the root directory e.g. /user/bin. The Relative pathname points to a file or directory that is relative to the position that you are in within the directory tree and this does not start with a /.

Information about users is kept in the passwd file which sits in the /etc directory along with the other configuration files. For each user there are seven fields separated by colons:
  • User name (first part of e-mail address).
  • Encrypted password.
  • UID (Users ID) needed by the Unix system. User names can change without changing user permissions.
  • GID (Group ID). A user may be a member of several groups each having different permissions.
  • Comment containing more detail on the user if desired.
  • User's home directory.
  • The shell to be used by the particular user.
Nowadays, when you are confronted with a Unix box you will come across an X-windows interface. To get to a command line interface, grab the three button mouse click the right button and select programs clicking on it with the left mouse button. In the list of programs that appear select either shell or command to open a command line window (much like a DOS box in Microsoft Windows). You normally need to click on the title bar or border before you can type in the window, sometimes the X-window interface has been set up such that the mouse moving over the window is enough to highlight the box. Any number of these command line windows can be opened. Resizing them is achieved by selecting the bottom right hand corner with the left mouse button (a circle appears) and dragging the window edges to the required size. Minimising a window is achieved by 'left-clicking' on the top left corner of the menu bar and selecting close. Selecting quit closes the window.

Keystrokes


Ctrl-c


Stops a command or program that is currently being executed.

Ctrl-d


Removes you from the current environment, this will log you out of the system if you are at a shell prompt.

Ctrl-h, Del


Deletes the last character typed and moves back one space. Unlike DOS, the backspace key does not work!

Ctrl-q


Resumes the command that was halted by Ctrl-s.

Ctrl-s


Temporarily halts the current command being executed, e.g. scrolling of text on the screen.

Ctrl-u, Ctrl-x, @


Cancels what you have just typed, so that you can start again.

Ctrl-x


Deletes the current line of text being entered.

Directory Commands


cd


Stands for change directory, e.g. 
cd /user/dave
takes you to dave's personal directory. The first / refers to root. Root is the equivalent of / in DOS. Typing cd without a path takes you back to your home directory, i.e. where you arrive when you first log on. Typing cd.. takes you up one directory, whereas typing cd ../user/dave, takes you up one directory and then right down to the '/user/dave' directory.

df -k


Stands for disk free, gives you the amount of space available on the disk that you are currently on.

mkdir


Means 'make directory', e.g. 
mkdir user
creates a directory called user in the directory you are in when you issue the command.

pwd


Stands for print working directory and prints the directory that you are in to the screen.

rmdir


Means remove directory e.g. 
rmdir user
removes the directory user provided that it is empty!

System Commands


-&


Using this switch after a command causes it to operate in the background, allowing you to continue using the same command line window without having to open another one.

*


This wildcard character matches any number of characters and is useful in searches, e.g. 
g*
matches all files beginning with 'g'.

?


This wildcard character matches any single character, e.g. 
g??
matches all three character files beginning with 'g'.

>


Redirect output from a program to a file, e.g. 
ls -l > listing
redirects the listing of ls -l into a file called 'listing'.

<


Redirect output from a file into a program, e.g. 
mail john < hello
redirects the greeting letter called 'hello' to John, rather than you having to type it.

|


Pipe output from one program to another, e.g. 
who | wc -l
gives a count of the users on the system.

>>


This append adds the input to an existing file without overwriting the original, e.g. 
postcript >> letter
adds the contents of 'postscript' to an existing file called 'letter'.

chsh


Means change shell and changes the shell that the user is using. The user will be prompted for a password since the 'passwd' file is being changed, then the user will need to type the path to the shell e.g. /bin/bash.

echo


This 'echoes' arguments to the screen, e.g. 
echo $SHELL
displays the value of the environment variable SHELL. This could return /bin/tcsh (Linux often uses this shell) or /bin/bash. echo $PATH displays the current path.

env


The environment gives you the variables set up for the particular user that issues the command.

exit, Ctrl-d


Logs you out.

jobs


This lists the jobs running under the current shell in 'job ID' order. You can type bg %jobid to put a particular job running in the background. Ctrl Z also suspends a job. Typing fg %jobid brings the job back to the foreground.

kill


This kills a process e.g. 
kill 5173
kill the process which has been given the temporary number 5173. This process number is found by using the ps command. Do not use kill 1 as this kills the system scheduler! If a process refuses to die you can type kill -KILL [PID] to stop a process immediately without any tidying up on exitting. Finally, kill -HUP [PID] tells the process that an event has occurred, or a configuration file change has occurred and needs to br reread.

man


The manual command is very useful for finding out comprehensive information on an individual command e.g. 
man cd
gives all the information on the command cd. Typing man -k mail lists the Unix commands that relate to the word mail.

passwd


Allows you or the administrator to change passwords.

printenv


The 'prints the environment' variables to the screen.

ps


The process status command shows the programs currently running. ps -a shows all the processes being run by all users. An example is the following: 
ps -ef | grep erpcd
where '-ef' gets the process number and pipes it to grep which filters on the following word, in this case for the program 'erpcd'.

The following information is shown:
  • PID Process ID.
  • TTY Each shell opened has a 'character special' called a 'tty' (held in '/dev').
  • STAT State, either 'S', sleeping, or 'R', running.
  • TIME CPU time that the process is taking up.
  • COMMAND The command running.
Typing ps x shows all the processes relating to X windows, whereas ps ax shows all the processes being run by everybody. Typing ps ux gives even more information such as the user.

setenv


The command set environment variable, sets aside a small amount of memory to hold paths etc. e.g. 
setenv GUI /usr/utility/gui_r4
sets a variable 'GUI' with the path that follows to the actual program. This program can now be run by typing 'GUI'. 
setenv DISPLAY :0.0
sets an X window session locally.
These settings are commonly setup permanently in the user's .profile (located in the '/etc' directory). This can be edited with any text editor.

The following are common environment variables:
  • SHELL The current shell.
  • HOME The current user's home directory.
  • HOSTNAME The name of the computer.
  • DISPLAY The X display that the applications are to use.
  • LD_LIBRARY_PATH The search path for libraries.
  • PATH The search path for applications.
If you wish to append directories to the path then type setenv PATH ${PATH} : /search/here. In order to use it then you need to cache the new path by typing rehash.

The DISPLAY variable is made up of three parts 'hostname : displaynumber : screennumber'. The hostname is the computer, whilst the other variables are '0' unless several machines are connected. X windows looks to this variable to find out where to send the X Windows traffic.

set path


Sets a path where regularly used programs or data are found e.g. 
set path=($path /usr/utility/gui_r4/bin)
sets the path '/usr/utility/gui_r4/bin'.

Some commands used to set the environment come from the C shell. In order to check which shell you are running type echo $SHELL, if this does not return '/sbin/csh' then you type /bin/csh.

su


The command switch user switches the login user to another user, e.g. 
su root
switches to the 'root' login.

top


Gives a constantly updating view of the top 20 processes (a real time version of 'ps'), i.e. those that are using the CPU the most.

who


This displays the users currently logged on the system.

whoami


Displays who you are currently logged on as. (e.g. 'root', a user etc.)

xhost +


Opens an X window for a program to run in. After issuing this you would then run the program (e.g. Netscape).

File Commands


.


The dot is not a command as such. If a file is spelled with a dot at the beginning, Unix treats it as a hidden file. Configuration files are often preceded with a dot.

cat


The concatenate command displays a file, e.g. 
cat bankletter
displays the contents of 'bankletter' on the screen. 
cat > newletter
takes whatever you type and redirects it into the file 'newletter', Ctrl-d gets you out of it. 
cat >> existing
takes whatever you type and appends it to an existing file called 'existing' funnily enough.

chmod


The command change mode changes the mode or permissions, of a file or a directory. When you do an ls -l you will see in the first column, a line of 10 characters looking something like 'drwx-w-rw-'. The 'd' means 'directory' (you could have '-' for file, 'l' for link to a file, 'b' for a 'block special', 'c' for a 'character special', 'p' for a 'named pipe', or 's' for 'socket'). The next three characters refer to the permissions of the login user, in this case the user has read, write and execute access to the directory. The next three characters refer to the permissions of the group and the final three characters refer to the permissions of all users. The chmod command can be used in various ways as shown by the following examples:
  • chmod go-rwx newletter removes read, write and execute permissions for users in the group (g), and all other users, for the file 'newletter'. Using a '+' instead of '-' adds the permissions. You can also use 'o' for others, or 'u' for user.
  • chmod 766 newletter causes the file 'newletter' to have read, write and execute permissions for the user, read and write permissions for the group members and read and write permissions for all other users. Why? Well, the 7 represents 111(binary) and 6 represents 110(binary) for each set of three 'rwx's. 'r' being set is given binary 1, 'x' being not set is given binary 0. Read permission is '4', write permission is '2', execute permission is '1' and no permissions is given with '0'.
  • chmod 700 dirname results in drwx------ for the directory which restricts access to everyone bar the owner.
  • chmod 664 filename gives -rw-rw-r-- that allows you and your group to read and edit the file but all others can only read the file.
  • chmod 600 filename gives -rwx------ creates a private file that only you can see and edit.
You can change permissions for groups of files with one command by using wildcards such as *.

chown


Use this to change ownership of a file e.g. 
chown dave myfile
changes the ownership of the file 'myfile' to dave. This can only be carried out by the owner of the original file. A way around this is for the recipient to copy the file, then the copied file becomes their own.

chgrp


Use this to change group ownership of a file.

compress


This compresses a file e.g. 
compress myfile
results in a file called 'myfile.Z'. The command uncompress can be used to uncompress the file.

cp


The copy command copies files from one directory to another, or to the same directory with a different name, e.g. 
cp bankletter /user/dave/bankletter1
copies the file 'bankletter' from the directory that you are currently in, to the '/user/dave' directory with a new name 'bankletter1'.

file


This returns information on the content of a file, e.g. 
file myletter
might return 'ASCII' to say that Unix guesses that 'myletter' contains ASCII.

find


This finds a file or directory, e.g. 
find / -name na -print &
this finds a file with name 'na' starting the search from the 'root' and printing the result to the shell window, whilst still allowing you to carry on using it.

grep


This stands for global regular expression and print and is a search utility, e.g. 
grep "325 Victory"
searches the current directory for files containing the text '325 Victory'.

gzip


GNU zip compresses files to create a '**.gz' file.

head


This command followed by a filename, displays the first ten lines of that file.

less


This is a way of displaying a file, it will give a percentage of file so far displayed at the bottom of the screen, and you can progress through reading the file by pressing the space bar.

ln


The command link, links files and directories, e.g. 
ln -s/export/home/fred usr/fred
creates a copy of 'fred' in the '/export/home/' directory in the 'usr/fred' directory. A 'hard link' is like a Windows 'shortcut', there can be a number of them, with different names and they take up little space. A 'soft link' is identified with the '-s' switch and creates a copy of the file elsewhere.

lp (for System V) or lpr (for BSD)


The command line printer, prints a file, e.g. 
lp newletter
prints 'newletter'.

lpstat -a all


The line printer stats command checks the printer queue in System V Unix.

ls


This lists the contents of the current directory, e.g. 
ls /etc
lists the files and sub-directories of the current directory.
  • ls -l gives a long list of directories including file sizes, permissions, type etc. Using the -a switch causes 'all' files to be listed including those hidden files starting with ..
  • ls -c lists files by creation time.
  • ls -p marks directories with a slash at the end of the name.
  • ls -x displays the list in rows across the screen.
  • Using ls | more is useful for large directories as it stops the screen scrolling, you press the 'Return' key to advance one line at a time, or press the space bar to advance one page at a time.
You can type 'ls' and then define one or more directories for it to list.

more


This is another way of displaying a file, it will give a percentage of file so far displayed at the bottom of the screen, and you can progress through reading the file by pressing the space bar. Whilst in more, if you type v you will be taken straight to the vi editor.

mv


This moves a file from one directory to another or renames it in the same directory, e.g. 
mv bankletter bankletter1
renames 'bankletter' to 'bankletter1'.

pg filename


Displays the content of the file one page at a time. You advance pages by pressing 'Return'. Option -l displays one more line, option n moves you to the page number specified by n and options +n and -n moves you forward or backward the number of pages specified by n.

rm


'remove' a file, e.g. 
rm oldletter
removes the file 'oldletter'. Using rm -rf recursively removes all files and directories below the one that you are in. Using rm -i gives you the option of cancelling or confirming the command.

sort


Sorts the contents of a file, e.g.: 
sort -o outfile infile

The contents of 'infile' are sorted in alphabetical order and fed into a new file called 'outfile', as defined by the switch '-o'.

tail


This means the tail end, this dynamically displays the file that is being written to in real time, e.g. 
tail -f logfile
shows the file 'logfile' which is being written to.

tar


The command tape archive is an file archiving command. It creates a single uncompressed archive file from several, ideal for sending data over networks. Often files are archived, and then compressed using 'gzip'. E.g. 
tar -tvf tarfile
displays the contents of a tarfile. 
tar -xvf tarfile
extracts the contents of a tarfile. 'x' is extract, 'v' means 'verbose' and 'f' means the file. 
tar -xvf tarfile target
extracts the file target from the tarfile.

touch


This just creates an empty file for appending to later on e.g. 
touch log
creates an empty file called 'log' that needs to be available for another program to write to it perhaps.

uncompress


This command uncompresses a 'gzip' file, e.g. 
uncompress myfile.gz
uncompresses the file 'myfile.gz'.

wc


The command word count counts the words in a particular file, e.g. 
wc letter
counts the number of words in the file 'letter'

Simple Scripting


A Unix script is the equivalent of the DOS batch file. Using vi, the following could be typed into a file called 'new_script': 
echo These users are on the system
who
echo Here is a detailed listing of the directory you are in
ls -al
The command chmod u+x new_script makes the script file executable by the logged in user.

Networking Commands


arp


Displays the 'Address Resolution Protocol' table e.g. 
arp -a
displays all arp entries for all connected devices. 
arp -d <ip address>
deletes the arp entry for that particular IP address.

ftp


The command file transfer protocol attaches you to another IP device e.g. 
ftp 141.205.15.154
attaches you to the device with address 141.205.15.154. You are normally presented with a login and password screen.

Commands that are used in FTP are:
  • dir - directory listing.
  • quit - quit from ftp.
  • cd - change directory.
  • get or mget - get a file (or multiple files).
  • put or mput - put a file (or multiple files).
  • bin - sets up your system to receive binary files.
  • hash - displays hashes whilst files are being transferred.
  • lcd - local change directory changes the directory on your local machine to which you are sending and receiving files. This is useful as it saves you having to quit ftp to carry out the directory change.
The Hosts file can be found in the directory '/etc'.

netstat


This stands for network statistics, e.g. 
netstat -r
displays the routing table of the Unix box. 
netstat -a
displays alll network information.

Unix uses routed to listen to RIP in order to discover the Default Gateway.

ping


Ping an IP device e.g. 
ping 141.205.51.26

rlogin


This works like telnet, e.g. 
rlogin 141.205.52.16
takes you to another Unix machine only. To quit you press 'return', '~', .' and 'return' again.

telnet


Ctrl-6 and then Ctrl-] gets you to the telnet> prompt where typing close gets you out of telnet.

vstat


This displays CPU utilisation and gives a list of processes and their share of CPU utilisation, e.g. 
vstat 10
displays the CPU utilisation every 10 seconds.

ifconfig


This displays the IP configuration of the box, e.g. 
ifconfig -a
displays all IP configuration.

If you want to look at the routing process you can type: 
ps -ef type grep routed
to send the 'routed' information to a file.

snoop


This command captures the network packets in a readable format, e.g. 
snoop -p 23
captures all IP traffic using port 23 (Telnet). use Ctrl-C to stop the snoop.

Useful vi commands


Each command needs to be preceded by pressing the escape key!

i insert mode.
<esc> leave insert mode and go into command mode.
a append characters to the end of the line.
o open a line below your cursor.
O open a line above.
<shift>g go to the bottom of the file.
r replace the letter that you are on with the one you type next.
x erase the character that you are on.
dd delete the line that you are on. A number before dd deletes that number of lines.
yy copy the line you are on. A number before yy copies that number of lines.
p paste the line you are on below you.
P paste the line you are on above you.
:wq write and quit the file that you are editing.
:wq! write and quit the file that you are editing, even if it is designated as read only!
:w! write to a read only file.
:q quit.
:q! discard any editing and quit.
/ this takes you to the bottom of the window where you can type a string and return to perform a search in the file.

(The character ! is often referred to as pling)

You can use vedit which is vi with more user friendly additions and also ed, or emacs.

Thanks to : http://www.rhyshaden.com/unix.htm

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