The Most Often Used Commands

The most often used commands:

Here we introduce critical commands in three basic groups: commands for files, directories, and printing. Note that UNIX is case-sensitive, and all commands are lower case. In the following, italics refer to the argument of the command.

Commands for files:

ls lists all the files and subdirectories in the current directory

cat file shows the contents of file

mv file1 file2 moves (or renames) file1 to file2

cp file1 file2 copies file1 to file2

rm file removes (or deletes) file

Flags. Most commands also accept flags, which modify the command or its output. For example, ls usually doesn’t list ‘hidden’ files, which are files with names that start with a period. To make ls list all files, including the hidden ones, use the -a flag: ls -a. Using ls -l shows extra information about each file as it lists them. Flags can often be combined; ls -al lists each file and the extra information for all files including hidden files. The order of the flags is sometimes important, and sometimes the flags take arguments.

Wildcards. To manipulate a large number of files at once, wildcards can be used. An asterik is a wildcard that stands for any string of characters; for example, ls m* will list all files and subdirectories that begin with the letter m. A question mark stands for any single character: ls mat? lists all files or directories that are four letters long and start with mat. Wildcards are very powerful; hopefully you will realize this before typing rm*. Be careful!

The man command:

man command displays the manual page for command

This is one of the most important commands in UNIX when you are just starting. The man command gives a description of each command, its arguments, and its flags. The output of man is called a ‘man page’.

Commands for directories:

cd directory change from the current directory to directory

mkdir directory make a directory called directory

rmdir directory remove directory if it contains no files

Directories. Directories are used to organize your files. Your ‘home’ directory is where you are when you login to a machine. Your home directory is at the ‘top’ of your account, and subdirectories you create using the mkdir command are ‘below’. To go to directories one level below, you just use cd directory1. To go two levels below, use cddirectory1/directory2. There are names for two special directories; the directory immediately above is referred to by two periods, while the current directory is referred to by a single period. If you type ls-a these will be the first two things listed. Use cd .. or cd../ to go up one directory.

The mv command can also move files to other directories. If file2 is a directory, file1 is moved but retains its name. Alternatively, file2 can have the form directory1/newname; then file1 is moved to directory1 and renamed newname. In fact, you can always use commands for files when the files are in other directories as long as you refer to the file using the format directory/file.

Your home directory is actually a subdirectory, and the levels above it contain everything else stored on the machine, like various software packages, the operating system (UNIX), stuff for the printer, scanner, your monitor, etc. Actually, a lot of the stuff is not even on the particular machine you are logged in to, but the directory structure is essentially seamless so you don’t notice you are looking at files stored on a different machine.

Printing commands:

lpr file send file to the default printer queue

lpq show the entries in the default printer queue

lprm number delete job number from the printer queue

Printing. The lpr command sends files to the printer. The other commands are useful for canceling mistakes you make. If you try to print a file in a format that the printer doesn’t recognize (i.e. a binary file) it will consume lots of paper and you will have gibberish to show for it. Avoid this, because everyone behind you in the print queue has to wait for your job, and because paper and toner are expensive. If you lpr something you don’t want printed, you need to find its job number with the lpq command, then remove it from the queue using the lprm command. It is important that you take responsibility for your own print jobs, because other users cannot cancel them; lprm only allows you to dequeue jobs you have submitted, not others.

The default printer is in AkerH 305 and is called lp_305. For other printers, you can print, check the queue, and cancel jobs using the -Pprinter flag, for example lpqlp_107 checks the print queue on the printer called lp_107.

Some other useful commands:

date gives the current time and date

who lists the users currently logged onto the machine

finger name gives information about the user name

pwd lists the current directory

quota lists allotted and used disk space

du disk usage by the files in the current and lower directories

more file displays the contents of a file, one page at a time

less file a newer version of more

passwd, yppasswd used to change your password

enscript prints plain text files using different page layouts

ispell file a spell-check program for plain text files

chmod file changes the protections (who can access) of file

For more information on these commands see their man pages.

Editors. Editors are used to create and modify plain text (or ascii) files. Popular editors include jot, emacs, vi, and nedit. jot, which runs only on Silicon Graphics machines, and nedit are simple and easy to learn. emacs is more powerful, but it takes a little longer to master all the added features. vi is not as easy to learn as the others, but it is always available on UNIX machines.

The Network and the Internet

When you sit at a workstation in the departmental computer lab, you have access to many other computers. A workstation consists of a monitor, a keyboard, and a rectangular box with the actual computer inside. Each particular workstation has a name; in the AEM department they are various words for winds in several languages, such as typhoon, tornado, hurricane, zephyr, crivat and minuano. While workstations can stand alone, they are almost always connected to each other electronically through the ‘network.’ On the network each workstation has an address, for example, typhoon.aem.umn.edu. When you log in to one of the workstations by typing your login name and password at the prompt, the workstation accesses your ‘account,’ a certain amount of space in memory which contains any files you have. To get your login name and password, see Ray Muno, the the department’s system administrator. One of the first things you should do is change your password (using yppasswd) to one of your own choosing. Don’t pick anything obvious, which includes anything in a dictionary, any real or fictional names, or any part of your address, phone number, birthday or other personal information. Your login name is a particularly bad choice.

You have several other accounts at the University of Minnesota. All students have an account on tc.umn.edu. Your login name is the first four letters of your last name and four numerical digits. When you log in to tc, you are presented with menu options which allow you to choose from the above services; the UNIX commands are not accessible unless you follow the menu choices to “Shell access”. The departmental workstations are more powerful, so most of your work will be done through your AEM account.

You may also have an account on the Institute of Technology’s Computers (IT account.) Graduate students who have completed fewer than 36 quarter or 24 semester credit hours or are registered for a course that requires IT computing facilities are automatically charged the IT computing fee for the semester ($165) and have an IT account. Any IT graduate student that isn’t automatically charged can voluntarily open or maintain an IT account by paying the fee. IT workstations are located in several computer labs around campus; the one in ME 308 is the closest to Akerman Hall. To activate your IT account, go to one of the labs. While the IT workstations do have some software not available on the department machines, they generally duplicate AEM computing facilities. To access the files in your IT account from an AEM machine, you will need to use the network.

telnet address log in to another machine

ftp address get or put files on another machine file

The ftp command is used to transfer files between accounts, while with telnet you are actually logged in to the other machine. Both of these commands establish a connection to another machine; other commands are used after the connection is established. In the case of telnet, the other commands are just the UNIX commands we have been discussing. To close a telnet connection, type exit. See the man pages for more information about ftp.

Email. Each of your accounts has an associated email address, namely, name@address, where name is your login name for your account at address. Two popular programs for managing your email are pine and elm, which are both easy to use. To read your email on tc, there are menu options to choose and invoke a mail program. On your AEM account, type the mail program name.

It is possible to have all your email directed to a single account using forwarding. To forward your mail from your IT account to your AEM account, create a file in the home directory of your IT account named .forward. Mail sent to your IT account will then be redirected to the address contained in the .forward file, in this case . There are menu options to allow forwarding from your tc account.

The Web. By now everyone has heard of the World Wide Web, the vast collection of data organized into ‘pages’ which are viewed with a ‘browser’ such as netscape or mosaic. If you have never explored the web, the easiest way to learn is by doing. Netscape will start a browser (it may take a few seconds to come up.) Pages are accessed by clicking the mouse on ‘links,’ which are generally underlined text, but can also be images or icons. Pages can also be accessed by typing in the address of the page, preceeded by http://, in the browser location window.

Web pages are just files at a particular internet address, written in a language (HyperText Markup Language, or html) that a browser can read. If you invoke netscape from your AEM account, you will start at the AEM home page, www.aem.umn.edu; this page has basic departmental information. More specific information about courses, departmental computing and general computing help, career services and job hunting, and links to University services are available at the AEM student page, www.aem.umn.edu/aero/.

Despite its image as a trendy, flashy toy without substance, the web can actually be used for more than entertainment. The following brief list of sites may be helpful:

U of M Twin Cities campus / www.umn.edu
Council of Graduate Students / gapsa.stu.umn.edu/cogs/cogs.html
Maps, Busses, and Getting Around Campus / onestop.umn.edu/Access/index.html
Register for class online / onestop.umn.edu/registrar/registration/index.html
Student access system / onestop.umn.edu/registrar/Grades/index.html
University library / www.lib.umn.edu
U of M Science and Engineering Library / sciweb.lib.umn.edu
*RECON / www.sti.nasa.gov
*COMPENDEX / online.lib.umn.edu/ovidweb/login.html

*RECON and COMPENDIX are useful for literature searches. RECON was developed by NASA and performs searches most frequently on aerospace related journals and NASA sources. COMPENDIX searches through science and engineering journals.