Introduction to the UNIX Operating System

Table of Contents

Document UNIX 1
July 28, 1995
Abstract
This document is intended to introduce you to the UNIX operating system. It will provide you with a basic understanding of the UNIX operating system, its file and directory structure, basic UNIX commands, and how to get on-line help. The information in this document is primarily designed for (but not limited to) Sun workstations configured and maintained by Information Systems, such as those used in the Owlnet network.

 If you are using a workstation that is configured to run exclusively as an X display server, you may wish to acquire the Information Systems Document, UNIX 2, Introduction to X Windows.
 
 
Product: UNIX
Version: SunOS 4.1.3_U1, Solaris 2.4, IRIX 5.3 
Brief Description: Operating system with multi-user capability 
System: Owlnet, Rice UNIX Facility, Rice Advanced Visualization Lab, and others
Level of Support: Information Systems provides system maintenance and consulting. 
Other References: See the section, Additional Resources, for other ways you can learn more about UNIX. 

Introduction

This document is only a brief introduction to UNIX and does not include information on how to use all of its capabilities.

Since this introduction to UNIX is brief, we encourage you to seek out more detailed introductory information. Information Systems offers free Short Courses on UNIX that you can sign up for. The Computing Resource Area, located in B40 in the basement of Fondren Library, has a number of very good introductory books on UNIX available for checkout. These books go into more detail and cover more of the operating system than will be covered in this document. You can also purchase some introductory UNIX books at the Rice Campus Store or almost any local bookstore with a computing section.

 This document introduces you to the basics of UNIX, including:

There are several variants of UNIX on the Rice campus; the type used most on Information Systems supported networks is from Sun Microsystems. Therefore, although the information concerning the UNIX operating system presented here is not limited to Sun workstations, it will focus on them.

Information Systems maintains groups of machines for different groups of users; these groups of machines are known as domains. The major Information Systems domains are Owlnet, the Rice UNIX Facility (RUF), and Information Systems administration. Other departments or divisions may have their own domains, such as Computer Science or Electrical and Computer Engineering. Accounts in one domain are not valid on machines of other domains. Every workstation is labeled with its name and domain. If you have an Owlnet account, use the workstations or terminals in A121 or B223 Abercrombie, 241 Mechanical Engineering, 102 Ryon Lab, 105 Mudd, 221 Physics, or the workstations at your residential college..
 
 

What is UNIX?

UNIX is a powerful computer operating system originally developed at AT&T Bell Laboratories. It is very popular among the scientific, engineering, and academic communities due to its multi-user and multi-tasking environment, flexibility and portability, electronic mail and networking capabilities, and the numerous programming, text processing and scientific utilities available. It has also gained widespread acceptance in government and business. Over the years, two major forms (with several vendor's variants of each) of UNIX have evolved: AT&T UNIX System V and the University of California at Berkeley's Berkeley Software Distribution (BSD). This document will be based on the SunOS 4.1.3_U1, Sun's combination of BSD UNIX (BSD versions 4.2 and 4.3) and System V because it is the primary version of UNIX available at Rice. Also available are Solaris, a System V-based version, and IRIX, used by Silicon Graphics machines.
 
 

UNIX Layers

When you use UNIX, several layers of interaction are occurring between the computer hardware and you. The first layer is the kernel, which runs on the actual machine hardware and manages all interaction with the hardware. All applications and commands in UNIX interact with the kernel, rather than the hardware directly, and they make up the second layer. On top of the applications and commands is the command-interpreter program, the shell, which manages the interaction between you, your applications, and the available UNIX commands. Most UNIX commands are separate programs, distinct from the kernel. A final layer, which may or may not be present on your system, is a windowing system such as X. The windowing system usually interacts with the shell, but it can also interact directly with applications. The final "layer" is you, the user. You will interact with the entire operating system through just the shell, or through a combination of the shell and the window system. The figure below gives a visual representation of the layers of UNIX.

FIGURE 1. UNIX Layers
 
 

Basic UNIX Elements

You need to be familiar with six basic elements of UNIX. They are: commands, files, directories, your environment, processes, and jobs. Commands are the instructions you give the system to tell it what to do. Files are collections of data that have been given filenames. A file is analogous to a container in which you can store documents, raw data, or programs (it can contain only one kind of data at a time). A single file might contain the text of a research project, statistical data, or an equation processing formula. Files are stored in directories. A directory is similar to a file cabinet drawer that contains many files. A directory can also contain other directories. Every directory has a name, like files. Your environment is a collection of items that describe or modify how your computing session will be carried out. It contains things such as where the commands are located and which printer to send your output to. A command or application running on the computer is called a process. The sequence of instructions given to the computer from the time you initiate a particular task until it ends it is called a job. A job may have one or more processes in it. We will explore each of these elements in a little greater detail later on, but first you need to learn how to get on to your system.

Logging In and Out

Getting the Login Prompt

Before you can start using the system you must login to it. The method that you use to login varies depending on the type of device that you are using to login. Read the section below that is appropriate for you and then read the section, Entering Your Userid and Password.

TTY Terminal

If you are using a TTY terminal (a TTY is line-at-a-time oriented as opposed to page oriented) and the screen is blank, you only need to press RETURN and a login prompt should appear on the screen.

Workstation

If you are using a workstation, you may see a dark screen with a brief message similar to this: 
chub, 4:20 pm, idle 2h 53m (Press ^C)
Press ^C (control-C) and you will see the prompt 
chub login:
Almost every login prompt includes the workstation name before the word "login."

X Terminals

If the display features a box in the center of the screen with text similar to that in the figure below, then you are using a workstation that is configured to run a windowing system called the X Window system. These machines are called X terminals. (For more information on the X Window system, see the Information Systems document, UNIX 2, Introduction to the X Window System.).

FIGURE 2. X Terminal Login Window 

If the screen is entirely black, then a screen-saving program is running automatically to protect the monitor from damage. Moving the mouse or pressing the RETURN key should "wake up" the display. (If you see the words "This screen has been locked..." then someone else is using the workstation, but they are temporarily away from their seat. Look for an unoccupied machine.) Move the mouse until the cursor (a black `X') is on top of the white box.

Entering Your Userid and Password

When you picked up your account information, you were given a userid and a password. This combination of information allows you to access your account. Type your userid using lower-case letters, then press the RETURN key. It is very important that you use lower-case letters for your userid. If you make a typing mistake, you can correct it by pressing the DELETE key once for each character you wish to erase. You must make your corrections before you press the RETURN key. If the text you are typing appears in upper-case, see the section, Troubleshooting.

After you have entered your userid, the system will prompt you for your password (by displaying the word "Password:" if it is not already on the screen, or by moving the cursor behind the word "Password:" already on your screen). Enter your password and press the RETURN key. Notice that the system does not show or "echo" your password as you type it. This prevents other people from learning your password by looking at your screen.

If you receive a message similar to "Login failed, please try again," you may have typed your userid or password incorrectly. Try again, making sure to type in your userid and password correctly. If you are still having problems, go to the Consulting Center in 103 Mudd Lab and ask for help.

When you have successfully logged on, the system will pause for a moment, and then display a few lines telling you when and from which machine you last logged on, and any messages from the system administrator.

On X terminals, you will get a window containing system information. After reading it, use the left mouse button click on either the "Help" or "Go Away" button, depending on what you want. Help puts you into a help system; Go Away allows you to begin your work.

Your new account is provided with a set of command procedures which are executed each time you log in. You can change part of your UNIX environment by changing these setup files (accounts on Information Systems supported systems are set up to produce a default environment). For further information, check the Sun manual SunOS User's Guide: Customizing Your Environment, available for building use only in the Operations Center, 109 Mudd Lab.

 The system will then display the command prompt. The prompt signals that the system is ready for you to enter your next command. The name of the workstation followed by a percent sign (%) forms the command prompt (e.g. chub%). Once you finish typing a command, you must always press RETURN to execute it.

Logging Out

Workstations and TTY Terminals

To end a work session, you must explicitly log out of a UNIX session. To do this, type logout at the command prompt. Once you have logged out, the system will either display the login prompt again or begin executing a screen saver program.

You should never turn a workstation off. Turning off a terminal does not necessarily log you out. If you are having trouble logging out, see the section, Troubleshooting.

X Terminals

To log out of the X Window system from an X terminal, move the cursor into the console window (it is labeled "console"), type the command exit, and press RETURN. If you try to use the logout command in the console window, you will receive the message, "Not in login shell."

If you used the startx command to start the X session (if you are working on a Sun workstation, for example), after you end the X session you will have the option to remain logged on by pressing CTRL-C within three seconds after exiting. If you do so, the X windows and background vanish, returning you to a text-only display. At the prompt, type exit to log out.
 
 

Changing Your Password

You can change your password at any time. You should change it the first time that you log in, and we recommend that you change it on a regular basis. At the command prompt, type passwd.You will be prompted to enter your old password and be asked twice to enter your new password. Neither your old nor new password will appear on the screen as you type. In order to be accepted, your password must meet the following conditions:
  1. It must be at least seven characters long. 
  2. It must not match anything in your UNIX account information, such as your login name, or an item from your account information data entry. 
  3. It must not be found in the system's spelling dictionary unless a character other than the first is capitalized. It must not have three or more consecutively repeated characters or words in the dictionary contained within it. 
For example, changing your password from Kat899 to B00z00e will look similar to the following example, except that the keystrokes for you old and new password will not be echoed on the screen.
 
  On many systems, the password change does not take effect immediately, even though you have finished with the passwd command. It can take upwards of an hour for the system to install the new password, due to the scheduling of the password changing process. Thus you should be prepared to use your old password to login again shortly after changing it.

If you should ever forget your password, you can go to the Information Desk in 103 Mudd Lab and request that a new password be generated for you. You will need to bring your Rice ID card with you to pick up your new password.

UNIX Commands

The UNIX Shell

Once you are logged in, you are ready to start using UNIX. As mentioned earlier, you interact with the system through a command interpreter program called the shell. Most UNIX systems have two different shells, although you will only use one or the other almost all of the time. The shell you will find on Information Systems supported networks is the C shell. It is called the C shell because it has syntax and constructs similar to those in the C programming language. The C shell command prompt often includes the name of the computer that you are using and usually ends with a special character, most often the percent sign (%). Another common shell is the Bourne shell, named for its author. The default prompt for the Bourne shell is the dollar sign ($). (If the prompt is neither one of these, a quick way to check which shell you are using is to type the C shell command alias; if a list appears, then you are using the C shell; if the message, "Command not found" appears, then you are using the Bourne shell). Modified versions of these shells are also available. TC shell (tcsh) is C shell with file name completion and command line editing (default prompt: >). The GNU Bourne-Again shell (bash) is basically the Bourne shell with the same features added (default prompt: bash$).

 In addition to processing your command requests, UNIX shells have their own syntax and control constructs. You can use these shell commands to make your processing more efficient, or to automate repetitive tasks. You can even store a sequence of shell commands in a file, called a shell script, and run it just like an ordinary program. Writing shell scripts is a topic discussed in the class notes for the UNIX III-Scripts Short Course.
 
 

About UNIX Commands

UNIX has a wide range of commands that allow you to manipulate not only your files and data, but also your environment. This section explains the general syntax of UNIX commands to get you started.

A UNIX command line consists of the name of the UNIX command followed by its arguments (options, filenames and/or other expressions) and ends with a RETURN. In function, UNIX commands are similar to verbs in English. The option flags act like adverbs by modifying the action of the command, and filenames and expressions act like objects of the verb. The general syntax for a UNIX command is:

command [-flag options] file/expression 
The brackets around the flags and options are a shorthand way to indicate that they are often optional, and only need to be invoked when you want to use that option. Also, flags need not always be specified separately, each with their own preceding dash. Many times, the flags can be listed one after the other after a single dash. Some examples later on will illustrate this concept.

You should follow several rules with UNIX commands:

  1. UNIX commands are case-sensitive, but most are lowercase.
  2. UNIX commands can only be entered at the shell prompt.
  3. UNIX command lines must end with a RETURN.
  4. UNIX options often begin with a "-" (minus sign).
  5. More than one option can be included with many commands.

Redirecting Input and Output

UNIX maintains a couple of conventions regarding where input to a program or command comes from and output from that program or command goes. In UNIX, the standard input is normally the keyboard, and the standard output is normally the screen. UNIX is very flexible, and it allows you to change or redirect where the input comes from and where the output goes. First, any command that would normally give results on the screen can be directed instead to send the output to a file with the ">" (output redirection) symbol. Thus, directs the system to put the output from the date command, which merely reports the time and date as the system knows it, into the file namedfile rather than printing it to your screen. One thing to keep in mind about ">" is that each successive redirection to a particular file may overwrite all of the previously existing data in that file, though this is unlikely.

 Another redirection is "<," which tells the command to take its input from a file rather than from the keyboard. For example, if you have a program that requires data input from the keyboard, you may find that you have to type the same data a large number of times in the debugging stage of program development. If you put that data in a file and direct the command to read it from there you will only have to type the data once, when you make the data file.

If you do this, you would see the same response from program as if you had typed the data in from the keyboard when requested.

You can also combine both kinds of redirection as in,

The data in the file datafile will then be used as input for program and all output will be stored in outputfile.

If you want to accumulate output from different sources in a single file, the symbol ">>" directs output to be appended to the end of a file rather than replacing the previous (if any) contents, which the single ">" redirection will do.

A final I/O redirection is the pipe symbol, "|." The "|" tells the computer to take the output created by the command to the left of it and use that as the input for the command on the right. For example, we could type:
 
 

This would use the output of the date command as input to another program.

Getting On-Line Help with Commands

The standard on-line help facility available with UNIX is electronic reference manuals, known as the man pages, and you access them with the man command. The man pages provide an in-depth description of command-name, with an explanation of its options, examples, and further references. The information is an electronic duplicate of the paper reference manual pages. Use the man command for explicit information about how to use a particular command. Use the -k option to search for a keyword among the one line descriptions in the help files. The command apropos serves exactly the same function as man -k and is used in the same way.

 You can read about the man command itself using man. Type man man at the prompt. The UNIX reference manual is divided into eight numbered sections:

  1. General User Commands 
  2. System Calls
  3. User-level Library Functions
  4. Device Drivers, Protocols
  5. File Formats
  6. Games (rarely available)
  7. Document Preparation
  8. System Administration
You can see the command summary for each section by typing: where # is one of the eight section numbers.

In addition, other applications that reside on your system may have man pages. These pages can often be called up in the same manner as the operating system man pages.

Case Sensitivity

UNIX is a case sensitive operating system. It treats lower-case characters differently than upper-case characters. For example, the files readme, Readme, and README would be treated as three different files. Most command names and files are entirely in lower-case. Therefore, you should generally plan to type in lower-case for most commands, command line arguments, and option letters.

Special Keys and Control Characters

UNIX recognizes special keys and control-character key strokes and assigns them special functions. A special key such as the DELETE key is usually mapped to the ERASE function, which erases the most recent character that you typed on the current line. A control-keystroke such as CTRL-C is invoked by holding down the key labeled CONTROL and pressing the "c" key (in the same manner that you hold down the SHIFT key and press the "c" key to generate a capital C). The notation for control characters is usually ^C or CTRL-C. Some standard special keys and control characters are summarized below.

TABLE 1. Special Keys and Control Characters  

SPECIAL KEY             FUNCTION/DESCRIPTION                                    


DELETE          Acts as a rubout or erase key. Pressing DELETE once will backup                 and erase one character, allowing you to correct and retype 
                mistakes.
                                                     
BACKSPACE       This key is sometimes used as the rubout key instead of the                     DELETE key. Otherwise, it is mapped as a backspace key,                         which generates a ^H on the display.                                 
CTRL-U          ^U erases the entire command line. It is also called the line
                 kill character.                                                           
CTRL-W          ^W erases the last word on the command line.                         
CTRL-S          ^S stops the flow of output on the display.                          
CTRL-Q          ^Q resumes the flow of output stopped by CTRL-S.                     
CTRL-C          ^C interrupts a command or process in progress and returns to                   the command line. This will usually work; if it doesn't, try typ                ing several ^C's in a row. If it still doesn't work, try typing                 ^\, q (for quit), exit, ^D, or ^Z.                                         
CTRL-Z          ^Z suspends a command or process in progress.                        
CTRL-D          ^D generates an end-of-file character. It can be used to 
                terminate input to a program, or to end a session with a shell.                
CTRL-\          ^\ quits a program and saves an image of the program in a file                  called core for later debugging.                                     

A Selected Command List

The next few pages summarize many of the basic UNIX commands you need to get started. For further details on UNIX commands not discussed (or that are beyond the scope of this introductory document), consult the system manuals available in the Operations Center, 109 Mudd Lab, and in the labs, or get some practice using the man command.

Setup and Status Commands

COMMAND         PURPOSE

logout          end your UNIX session 
passwd          change password by prompting for old and new passwords
stty            set terminal options
date            display or set the date
finger          display information about users 
ps              display information about processes
env             display or change current environment 
set             C shell command to set shell variables
alias           C shell command to define command abbreviations
history         C shell command to display recent commands

File and Directory Commands

COMMAND         PURPOSE

cat             concatenate and display file(s) 
more            paginator - allows you to browse through a text file
most            more versatile paginator than more
mv              move or rename files
cp              copy files 
rm              remove files
ls              list contents of directory
mkdir           make a directory 
rmdir           remove a directory
cd              change working directory 
pwd             print working directory name
du              summarize disk usage
chmod           change mode (access permissions) of a file or directory 
file            determine the type of file

Editing Tools

COMMAND         PURPOSE

vi              screen oriented (visual) display editor
diff            show differences between the contents of files
grep            search a file for a pattern
sort            sort and collate lines of a file (only works on one file at a time)
wc              count lines, words, and characters in a file
look            look up specified words in the system dictionary
awk             pattern scanning and processing language

Formatting and Printing Commands

COMMAND         PURPOSE

lpq             view printer queue 
lpr             send file to printer queue to be printed 
lprm            remove job from printer spooling queue
enscript        converts text files to POSTSCRIPT format for printing

Program Controls, Pipes, and Filters

COMMAND         PURPOSE

CTRL-C          interrupt current process or command 
CTRL-D          generate end-of-file character 
CTRL-S          stop flow of output to screen
CTRL-Q          resume flow of output to screen 
CTRL-Z          suspend current process or command 
jobs            lists background jobs
sleep           suspend execution for an interval
kill            terminate a process
nice            run a command at low priority
renice          alter priority of running process
&               run process in background when placed at end of command line
>               redirect the output of a command into a file   
>>              redirect and append the output of a command to the end of a file     
<               redirect a file to the input of a command
>&              redirect standard output and standard error of a command into a file (C shell only)   
|               pipe the output of one command into another

THE FOLLOWING COMMANDS ARE NOT AVAILABLE WITH THE BOURNE SHELL

bg              run a current or specified job in the background
fg              bring the current or specified job to the foreground
!!              repeat entire last command line
!$              repeat last word of last command line

Other Tools and Applications

COMMAND         PURPOSE

mail            electronic mail 
bc              desk calculator 
man             print UNIX manual page to screen 

Files

About UNIX Files

Now that you understand UNIX commands, let's discuss the objects manipulated by most commands: files. As we said before, all files have a filename, and UNIX imposes few restrictions on filenames. This makes it easy for you to name your files so that you can easily recognize their contents. You will find it useful to adopt names and classes of names that indicate how important each file is and what connection it has with other files. For example, temporary files used to test commands and options could all begin with a "t." A filename can be up to 256 characters long, consisting of any alphanumeric character on the keyboard except the "/". In general, you should keep your filenames relatively short (to reduce typing effort) and use normal lower-case characters such as letters, numbers, periods and underscores. For instance, if your program calculates employee paychecks, you might call it payroll, or if your file is a research paper on Frank Lloyd Wright, you might call it wright. Do not include blanks in your filenames as they will make it very difficult for you to work with the file. If you do wish to separate letters in a filename, use the underscore ("_") character (as in wright_paper) or the hyphen ("-") character. Remember that UNIX is case sensitive, which means it recognizes the difference between upper-case and lower-case letters. For instance, Wright and wright would refer to two different files.

When you place a single period in the middle of a filename, the part after the period is commonly referred to as an extension or suffix and usually indicates what type of information is stored in the file. You may use any extension desired; a text file might have the extension .txt or .text; a note may have the extension .note, and so forth. UNIX does not require extensions; they are intended to help identify similar types of files. Since some UNIX programs (especially compilers) look for certain standard extensions, it is common practice to use the following conventions: .h for header files, .c for C source files, .f for FORTRAN, .p for Pascal, and .s for assembler source files. So the file wright.txt indicates a text file whereas the file payroll.c indicates a C program called payroll. For more information on programming conventions, see the section, Additional Resources.

Some UNIX files begin with a period, for example, .cshrc or .login. Files that begin with a period will not appear in a normal directory listing and are usually UNIX environment and application setup files.

A large grouping of files and directories is referred to as a file system. File systems are related to the disk size and structure, and to the internal structure of UNIX. What you should remember is that users' files and directories are usually on a different file system than the system's files and directories. If the number of users is large, as on Owlnet, the user files and directories may be on more than one file system.

Creating Files

Many files are created using a text editor. A text editor is a program that allows you to enter and save text. You can also use a text editor to manipulate saved text through corrections, deletions, or insertions. The main text editors on Information Systems managed networks are vi, GNU Emacs, Pico, and aXe. (Note: vi is included with every UNIX system, but GNU Emacs is commonly installed separately by system managers. aXe is only available if you are using the X Window system.) You should learn how to use at least one of these tools. Information Systems has tutorial documents on each of these editors. Please see the section, Additional Resources, for information on the tutorials.

You can create a file without a text editor by using the cat command (short for concatenate) and the ">" (redirect output) symbol. To create a file using the cat command, type:

where new-filename is the name you wish to give the file. The command cat generally reads in a file and displays it to standard output. When there is no filename directly following the command, cat treats standard input as a file. The ">" symbol will redirect the output from cat into the new filename you specify. cat will keep reading and writing each line you type until it encounters an end-of-file character. By typing CTRL-D on a line by itself, you generate an end-of-file character. It will stop when it sees this character. Try it, using this example as a guide: When you reach the end of each line, press the RETURN key. You can only correct mistakes on the line you are currently typing. Use the DELETE key to move the cursor back to the mistake and then retype the rest of the line correctly. When you have completed the last line, press RETURN and type CTRL-D.

Displaying Files

Now that you have created a file, you can display it one of several ways. You could use the cat command. Just type cat followed by the name of the file that you want to see. Sometimes the files you want to view are very long. When using the cat command, the text will scroll by very quickly. You can control the flow of text by using CTRL-S and CTRL-Q. CTRL-S stops the flow of text and CTRL-Q restarts it. If you use CTRL-S, stopping the flow of text, and so on, you must remember to type CTRL-Q or the computer will not respond.

more is a program that displays only one screen of information at a time; it waits for you to tell it to continue. Type more followed by a filename.

The computer will display one screen of text and then wait for you to press the space bar before it displays the next page of text, until you reach the end of the file. Pressing the "?" character will show help for more. A utility of greater power called most is available on many systems; it allows reverse scrolling of files and other enhancements. It is invoked the same way as more.

Listing Files

The ls command will list the files in the current directory that do not begin with a period.

Below is a list of options you can tack on to ls: 

ls -a           lists all the contents of the current directory, including files                with initial periods, which are not usually listed.

ls -l           lists the contents of the current directory in long format,
                including file permissions, size, and date information.

ls -s           lists contents and file sizes in kilobytes of the current 
                directory.





If you have many files, your directory list might be longer than one screen. You can use the programs more or most with the "|" (vertical bar or pipe) symbol to pipe the directory list generated as output by the ls command into the more program. more or most will display the output from ls one page at a time.

Copying Files

To make a copy of a file, use the cp (copy) command. where filename is the file you wish to copy and newfilename is the file you are creating. The example created a new file called sample that has the same contents as practice. If sample already exists, the cp command will overwrite the previous contents. New accounts are often set up so that cp will prompt for confirmation before it overwrites an existing file. If your account is not set up in this manner, use the -i option (cp -i) to get the confirmation prompt.

Renaming Files

To rename one of your files, use the mv (move) command.
 
  where oldfilename is the original filename and newfilename is the new filename. For instance, to rename sample as workfile type:
 
  This moves the contents of sample into the new file workfile. (Note: Moving a file into an existing file overwrites the data in the existing file.) New accounts are often set up so that mv will prompt for confirmation before doing this. If your account is not set up in this manner, use the -i option (mv -i) to get the confirmation prompt.

Deleting Files

To delete files, use the rm (remove) command. For instance, to delete workfile, type: Important: rm can be very dangerous. Once a file has been removed you cannot get it back, except, possibly, from system backups (which may or may not contain the file). It may take the system administrators several days to to recover your deleted file, so use a great deal of caution when deleting files. New accounts are often set up so that rm will prompt for confirmation. If your account is not set up in this manner, use the -i option to get the confirmation prompt.

Creating Links Between Files

You can refer to one particular file by different names in different directories. The ln command creates a link, which "points" to the file. Note that links are simply alternative names for a single file; ln does not rename the file (as does mv) nor does it make a copy of the file (as does cp). It allows you to access the file from multiple directories. Since only one copy of the file actually exists, any changes that you make through one of its links will be reflected when you access it through another of its links, yet if you delete the link, you do not delete what it points to.

Links are useful for cross-referencing files. If you know that you will need to access a file from different directories, creating links is a better alternative to making a copy of the file for each directory (and then having to alter each one every time a change is made to the original). It is also more convenient than having to use the file's full pathname every time you need to access it. Another use for linking a file is to allow another user access to that particular file without also allowing entry into the directory that actually contains the file. The kind of link you will want to create is called a symbolic link. A symbolic link contains the pathname of the file you wish to create a link to. Symbolic links can tie into any file in the file structure; they are not limited to files within a file system. Symbolic links may also refer to directories as well as individual files. To create a symbolic link to a file within the same directory, type:
 
 

where originalFile is the file that you want to link to and linkName is the link to that file. To create a link in a directory other than that of the original file, type:
 
  If you create a link within the same directory as the original file, you cannot give it the same name as the original file. There is no restriction on a file's additional names outside of its own directory. Links do not change anything about a file, no matter what the link is named. If someone makes a link to one of your files, and you then delete that file, that link will no longer point to anything and may cause problems for the other user.

Printing Files

To print a file, use the lpr command: or  To get a list of the printers available to your machine, type:
 
  lprloc lists all of the printers that your system knows about, by name, along with their type and location. To get some status information on the printers, use the command lpstat -p.

NOTE: Line printers are used for text-only files. Laser printers are needed to handle graphics or PostScript files. PostScript is a page-description language developed by Adobe Systems, Inc. and was specially designed for creating graphics and typography on a printed page. The option flag -Pprintername specifies which laser printer to use and is optional (as indicated by the brackets). When no printer is given, the print command uses the system default printer. However, on some systems such as Owlnet, you must always specify a laser printer with the -P flag. For more information on printing commands, use the man command to consult the manual pages on lpq, lpr, and lprm.

Directories

About UNIX Directories

UNIX directories are similar to regular files; they both have names and both contain information. Directories, however, contain other files and directories. Many of the same rules and commands that apply to files also apply to directories.

All files and directories in the UNIX system are stored in a hierarchical tree structure. Envision it as an upside-down tree, as in the figure below.

FIGURE 3. UNIX Directory Structure

 At the top of the tree is the root directory. Its directory name is simply / (a slash character). Below the root directory is a set of major subdirectories that usually include bin, dev, etc, lib, pub, tmp, and usr. For example, the /bin directory is a subdirectory, or "child," of / (the root directory). The root directory, in this case, is also the parent directory of the bin directory. Each path leading down, away from the root, ends in a file or directory. Other paths can branch out from directories, but not from files.

Many directories on a UNIX system have traditional names and traditional contents. For example, directories named bin contain binary files, which are the executable command and application files. A lib directory contains library files, which are often collections of routines that can be included in programs by a compiler. dev contains device files, which are the software components of terminals, printers, disks, etc. tmp directories are for temporary storage, such as when a program creates a file for something and then deletes it when it is done. The etc directory is used for miscellaneous administrative files and commands. pub is for public files that anyone can use, and usr has traditionally been reserved for user directories, but on large systems it usually contains other bin, tmp, and lib directories.

Your home directory is the directory that you start out from when you first login. It is the top level directory of your account. Your home directory name is almost always the same as your userid.

Every directory and file on the system has a path by which it is accessed, starting from the root directory. The path to the directory is called its pathname. You can refer to any point in the directory hierarchy in two different ways: using its full (or absolute) pathname or its relative pathname. The full pathname traces the absolute position of a file or directory back to the root directory, using slashes (/) to connect every point in the path. For example, in the figure above, the full pathname of file2 would be /usr/bin/file2. Relative pathnames begin with the current directory (also called the working directory, the one you are in). If /usr were your current directory, then the relative pathname for file2 would be bin/file2.

If you are using C shell, TC shell, or the Bourne-Again shell, UNIX provides some abbreviations for a few special directories. The character "~" (tilde) refers to your home directory. The home directory of any user (including you, if you want) can be abbreviated from /parent-directories/userid to ~userid. Likewise, you can abbreviate /parent-directories/youruserid/file to ~/file. The current directory has the abbreviation . (period). The parent of the current working directory uses .. (two consecutive periods) as its abbreviation.

Displaying Directories

When you initially log in, the UNIX system places you in your home directory. The pwd command will display the full pathname of the current directory you are in. By typing the ls -a command, you can see every file and directory in the current directory, regardless of whether it is your home directory. To display the contents of your home directory when it is not your current directory, enter the ls command followed by the full pathname of your home directory.
 
  If you are using a shell other than the Bourne shell, instead of typing the full pathname for your directory, you can also use the tilde symbol with the ls command to display the contents of your home directory.
 
  To help you distinguish between files and directories in a listing, the ls command has a
-F option, which appends a distinguishing mark to the entry name showing the kind of data it contains: no mark for regular files; "/" for directories; "@" for links; "*" for executable programs.

Changing Directories

To change your current directory to another directory in the directory tree, use the cd command. For example, to move from your home directory to your projects directory, type:
 
  or, or, Using pwd will show you your new current directory.
 
  To get back to the parent directory of projects, you can use the special ".." directory abbreviation.
 
  If you get lost, issuing the cd command without any arguments will place you in your home directory. It is equivalent to cd ~.

Moving Files Between Directories

You can move a file into another directory using the following syntax for the mv command:
 
  For example, moves the file sample.txt into the projects directory. Since the mv command is capable of overwriting files, it would be prudent to use the -i option (confirmation prompt). You can also move a file into a another directory and rename it at the same time by merely specifying the new name after the directory path, as follows:

Copying Files to Other Directories

As with the mv command, you can copy files to other directories:
 
  As with mv, the new file will have the same name as the old one unless you change it while copying it.
 
 

Renaming Directories

You can rename an existing directory with the mv command:
 
  The new directory name must not exist before you use the command. The new directory need not be in the current directory. You can move a directory anywhere within a file system.

Removing Directories

To remove a directory, first be sure that you are in the parent of that directory. Then use the command rmdir along with the directory's name. You cannot remove a directory with rmdir unless all the files and subdirectories contained in it have been erased. This prevents you from accidentally erasing important subdirectories. You could erase all the files in a directory by first going to that directory (use cd) and then using rm to remove all the files in that directory. The quickest way to remove a directory and all of its files and subdirectories (and their contents) is to use the rm -r command along with the directory's name. For example, to empty and remove your projects directory (assuming that you are in that directory), type:
 
 

File and Directory Permissions

It is important to protect your UNIX files against accidental (or intentional) removal or alteration by yourself or other users. The UNIX operating system maintains information, known as permissions, for every file and directory on the system. This section describes how to inspect and change these permissions.

UNIX was designed and implemented by computer scientists working on operating system research. Many of the fundamentals of UNIX reflect this origin in academia. A low concern for security is one of the hallmarks of UNIX operating systems. Therefore, unless you act to restrict access to your files, chances are high that other users can read them.

Every file or directory in a UNIX file system has three types of permissions (or protections) that define whether certain actions can be carried out. The permissions are:


For each file and directory, the read, write, and execute permissions may be set separately for each of the following classes of users:


The primary command that displays information about files and directories is ls. The -l option will display the information in a long format. You can get information about a single UNIX file by using ls -l filename.

 Each file or subdirectory entry in a directory listing obtained with the -l option consists of seven fields: permission mode, link count, owner name, group name, file size in bytes, time of last modification, and the filename (the group name appears only if the "g" flag is also specified, as in ls -lg).

The first 10 characters make up the mode field. If the first character is a "d" then the item listed is a directory; if it is a "-" then the item is a file; if it is an "l" then it is a link to another file. Characters 2 through 4 refer to the owner's permissions, characters 5 through 7 to the group's permissions (groups are defined by the system administrator), and the last three to the general public's permissions. (You can type id to verify your userid and group membership.) If a particular permission is set, the appropriate letter appears in the corresponding position; otherwise, a dash indicates that the permission is not given.

The second field in the output from ls -l is the number of links to the file. In most cases it is one, but other users may make links to your files, thus increasing the link count. A special warning to people using links to other people's files: your "copies" of their files can be counted against them by the file quota system available on certain UNIX variants. The third field gives the userid of the owner of the file. The group name follows in the fourth field (if the -g option is used in conjunction with -l). The next two fields give the size of the file (in bytes) and the date and time at which the file was last modified. The last field gives the name of the file.
 
 

A file's owner can change any or all of the permissions with the chmod (change mode) command. The chmod command allows you to dictate the type of access permission that you want each file to have. In the previous example the current permissions for myfile are read for everybody, write for the owner, and execute by no one.

The arguments supplied to chmod are a symbolic specification of the changes required, followed by one or more filenames. The specification consists of whose permissions are to be changed: u for user (owner), g for group, o for others, or some combination thereof (a (all) has the same effect as ugo), how they are to be changed (+ adds a permission, - removes a permission, and = sets the specified permissions, removing the other ones) and which permission to add or remove (r for read, w for write, and x for execute). For example, to remove all the permissions from myfile:
 
 


(Note: chmod a= myfile achieves the same effect.)

To allow read and write permissions for all users:
 
 


To remove write permission for your groups and other users:
 
 


Finally, to allow only read permission to all users:
 
 


Now the file is protected by allowing only read access; it cannot be written to or executed by anyone, including you. Protecting a file against writing by its owner is a safeguard against accidental overwriting, although not against accidental deletion.

chmod will also accept a permission setting expressed as a 3-digit octal number. To determine this octal number, you first write a 1 if the permission is to be set and a 0 otherwise. This produces a binary number which can be converted into octal by grouping the digits in threes and replacing each group by the corresponding octal digit according to the table below.

 TABLE 2. Symbolic to Octal Conversions  

SYMBOLIC        BINARY          OCTAL 

---             000             0      
--x             001             1      
-w-             010             2      
-wx             011             3      
r--             100             4      
r-x             101             5      
rw-             110             6      
rwx             111             7      





Thus, if the setting you want is rw-r--r--, determine the octal number with the following method:

 This shows that the octal equivalent of rw-r--r-- is 644. The following example illustrates that the permissions for myfile have been reset to the values with which we began.
 
 


To change the permissions back to read only, you can execute chmod as follows:


As with files, directories may also have permissions assigned. When listing directories, you may use the -d option to keep from descending into the directories you list. Otherwise, the contents of the directories will be displayed as well as their names. Below is an example of permissions assigned to a directory:
 
 


The directory home and the files and directories under it may be read and executed by anyone, but written to only by the owner and users in the masc223 group. Assuming you are the owner of this directory, you may decide to change the permission to allow only yourself and the masc223 group to read and execute files in the home directory. You would set the permissions accordingly:
 
 


You may decide that only you should be able to alter the contents of the directory. You must remove the write permission for the group.
 
 


An alternative to the previous command is chmod g-w.

When you create a file the system gives it a default set of permissions. These are controlled by the system administrator and will vary from installation to installation. If you would like to change the default which is in effect for you, choose your own with the umask command. Note that the permission specified by the umask setting will be applied to the file, unlike that specified in the chmod command, which normally only adds or deletes (few people use the = operator to chmod).

 First, issue the command without arguments to cause the current settings to be echoed as an octal number:
 
 


If you convert these digits to binary, you will obtain a bit pattern of 1's and 0's. A 1 indicates that the corresponding permission is to be turned off, a 0, that it is to be turned on. (Notice that the bit patterns for chmod and umask are reversed.) Hence, the mask output above is 000010010, which produces a permission setting of rwxr-xr-x (i.e., write permission is turned off for group and other).

Suppose you decide that the default setting you prefer is rwxr-x---. This corresponds to the masking bit pattern 000010111, so the required mask is 027:
 
 


Now, if you create a new file during this session, the permissions assigned to the file will be the ones allowed by the mask value.
 
 

Wildcard Characters

Using wildcard characters that allow you to copy, list, move, remove, etc. items with similar names is a great help in manipulating files and directories.
  1. The symbol ? will match any single character in that position in the file name.
  2. The symbol * will match zero or more characters in the name.
  3. Characters enclosed in brackets [and] will match any one of the given characters in the given position in the name. A consecutive sequence of characters can be designated by [char char]. 
Examples of each follow:
  1. ?ab2 would match a name that starts with any single character and ends with ab2. ?ab? would match all names that begin and end with any character and have ab in between. 
  2. ab* would match all names that start with ab, including ab itself. 
    3. a*b would match all names that start with a and end with b, including ab.
  3. s[aqz] would match sa, sq, and sz. 
    4. s[2-7] would match s2, s3, s4, s5, s6 and s7.
These wildcard symbols help in dealing with groups of files, but you should remember that the instruction:
 
  would erase all files in your current directory (although by default, you would be prompted to okay each deletion). The wildcard * should be used carefully.

Processes

Every command or program running under UNIX is called a process. A sequence of related processes is called a job. Your applications and even your shell itself are processes. The windowing system is also a process, or a collection of processes. The UNIX kernel manages the processes on the system, usually without distinguishing among them. UNIX is a multi-tasking system---it allows you to continue to work in the foreground while running one or more jobs in the background. It also runs the processes of many users simultaneously. You could even log off and come back later if the background jobs do not require interaction with you.

Viewing Your Processes

The command ps will show you the status of your processes.
 
  ps displays the process ID, under PID; the control terminal (if any), under TT; the state of the process, under STAT; the cpu time used by the process so far (including both user and system time), under TIME; and finally, an indication of the COMMAND that is running.

The state of the process is indicated by a sequence of letters. The man pages for ps explain what the letters mean if you want to know. For most purposes, you won't really need to know what the letters mean.

Running Background Jobs

Putting a program into an unattended state where it continues to execute is referred to as putting it (the process or job) into the background. (Running a program on one machine and displaying its output on another via a windowing system like X is not considered backgrounding the job.)

Adding an & (ampersand) at the end of the command line instructs UNIX to run the job in the background.
 
 

The response you receive will be something like this:
 
  This particular response means that you have one job running in the background (and its job number is 1), and its process identification number (PID) is 5432. You will need to know the PID if you want to stop the job. This is known as killing a job. To kill the job in the above example, you would type:
 
  In the C shell, the job number can be used to control which jobs run in background or foreground. The job number is used when switching a job that is processing in the foreground to the background, and one that is processing in the background to the foreground. To do the former, first press CTRL-Z to suspend the job. Then type:
 
  To switch the job to the foreground, simply type:
 
  If you have forgotten the job number, type the command jobs to see a list of the jobs that are running in the background at the moment.

Note: The rules imposed by system administrators about where and how to run background jobs varies from network to network and changes over time. It is important to stay current with the background job policy of your network.

Process Scheduling Priority

The nice command is used to set the processing priority of a command. The priority of a process determines how much attention the system will devote to completing that job. The higher the priority, the more attention a job gets, which implies that it will take less time to complete than the same job run at a lower priority. There are two versions of nice. In the C shell, the syntax is:
 
 


In the Bourne shell, the syntax is:
 
 

The available priority numbers for users ranges from 1 to 19 with 19 being the lowest priority. In other words, the higher the nice value, the lower the processing priority. (Note: It is important to check the network policy for the required nice value for background jobs on your system; they are usually required to be niced and your job may be downgraded in priority if it was niced at the wrong value.) Set your command at the required nice value or higher. If you do not include a number argument, the value will default to 4 for the C shell and 10 for the Bourne shell.

For example, if you wanted to run a long non-interactive job, and you didn't have to have the results of this job right away, you should run it in the background and set a high nice value. Using the C shell, you would type:
 
 

Remote Login

Sometimes, while you are logged into one workstation, you will find that you would like to be logged in to another workstation, file server, or other UNIX system. The command rlogin allows you to do so provided that you have an account on the other system. Type:
 
  You may then have to supply your password. You should also get the messages about logging in that are used on newSystem. If your userid is different on newSystem you will have to use the form:
 
 

Troubleshooting

Logging In

Problem: Nothing happens when you type CTRL-C to begin your session. 
Try typing CTRL-C several times in a row at the prompt. If that doesn't work, type CTRL-\. It's a more powerful interrupt. 
Problem: The system prints "login incorrect."
Check to make sure that you are typing your userid and password exactly as they appear on your information sheet (make sure you're using the proper case for all characters). If you still cannot log in, go to the Consulting Center in 103 Mudd Lab and ask for help. 
Problem: The computer types everything in upper-case with slashes in it. Your userid was entered in uppercase letters, so the system thinks that your terminal can only understand uppercase letters. You need to type logout and then login again making sure that you enter your userid in lowercase letters only. (If the screen displays uppercase as you type your userid, you may have to turn the CAPS key "off" by pressing it once and try logging in once more.) 

Logging Out

If you experience difficulty logging out, check to see if you are having one of the following problems. Do not turn off the workstation or terminal. This will not disconnect you from UNIX.

Problem: The system replies "There are stopped jobs" when you try to log out.

Type jobs to see what they are, then type kill %% to terminate all stopped background jobs. After that, try logging out again.

Problem: The system replies, "Not in login shell."

First, type exit, press RETURN, and then type logout at the prompt, if necessary.

Problem: Nothing seems to work and you can't log out.

You might be stuck in a program or shell other than the login shell. Press RETURN to clear any previous commands; then try typing CTRL-C, q, :q, :q!, exit, CTRL-D, CTRL-Q, or CTRL-Z to get back into the login shell. Then try logout again. The following sequence might also work if your terminal is NOT connected directly to your real host machine. Press RETURN, then type "~." followed by RETURN. 
 
 

Additional Resources

You now have enough information to get started using UNIX. To learn more about UNIX and specific applications, you can refer to a number of resources. You can use the man command to get on-line help. This is the quickest and easiest way to get help during a UNIX session. Virtually every UNIX operation and command has an entry. UNIX documentation written by the vendor for their workstations is available in the document racks of Information Systems maintained networks.

Below are some of the UNIX-related documents.