This document was orginally developed by the Campus Computing Organization at Caltech, edited by the Oceanography department at U. Washington, and edited locally to provide a better match to the computing environment at our site.
Sending Messages and Files to Other Users
The Department of Atmospheric and Environmental Sciences has a
network of Unix workstations, running mostly Linux, although a few
run Sun Solaris. Any of these accept secure shell (ssh)
logins over the department network as well. Our principal
remote login server is ash.atmos.albany.edu.
Here are
some instructions for downloading and
installing a secure-shell client to connect to our remote login
server.
If you are directly logged into a workstation, you will be
prompted to enter your department-assigned (NOT, in most cases, your
UAlbany NetID) user id and password. If you have forgotten
either of these, see either Kevin Tyle in ES235 or David Knight in
ES228. Both your user id and passwords are case-sensitive;
in other words, these parameters, like everything else in UNIX,
distinguishes between lower- and upper-case.
Open a terminal
window if one does not appear. In Linux, open one by clicking
on Applications --> Accessories --> Terminal. On Sun
Solaris, right click in an empty area of the desktop and select
"Terminal Window".
Your initial password is set to be your first and last initials, in lower case, followed by the last four digits of your UAlbany ID. Thus, John Smith, UAlbany ID 044234681 would have an initial password of "js4681". You must immediately change your password, by typing the "passwd" command in the terminal window, and following the prompts.
Your password should be longer than six characters. Currently, passwords are limited to eight characters. They should consist of a mixture of letters, numbers, and special characters.
In the interests of self-preservation, don't set your password to your username, to ``password'' or to any information which people are likely to know about you (your real name, your nickname, your pet dog's name).
If you mistype your username or password you will be notified of an incorrect password entry, and prompted to try again.
If you type your username and password correctly, the computer will begin running the login program. It starts by displaying a special ``message of the day''---contained in the /etc/motd file, if one has been set up by the system managers (typically this is not the case in our Department). If this file were to exist, it will usually contain information about the computer you are logging onto, maybe a basic message about getting help, and any important system messages from the system managers.
When you log in, the Unix login program finally starts up a command ``shell.'' Users do not deal with the operating system directly. Instead they interact with a shell, which is initialized with several pieces of information (such as your username, login directory and ``path''). By default all users use a shell called the "tcsh" shell (an "enhanced C Shell", the program /usr/local/bin/tcsh) and interact with it.
There are a couple of files read by this shell when your login session starts up. These are the .cshrc file and the .login file. These files are created when your account is created. As you learn more about how Unix and the tcsh shell work, you may want to customize these files.
If your files get corrupted for some reason, copies of the system defaults are available in /usr/local/skel/.
Finally you are logged in! You will see a prompt like this:
smith@ash:/home/smith%
Note that this prompt is of the form "user id" at
"machine name", followed by your "home"
directory. This is the directory where your session will
always place you upon log in.
The computer is now just waiting
for you to type something. Throughout the Unix Tutorial section we
will use % to indicate the computer's ``ready'' prompt.
Okay, let's try a simple command. Type ls and press Enter (<CR>). ls is the program to list files in a directory. Right now you may or may not see any files-not seeing any files doesn't mean you don't have any! Just plain ls won't list hidden files (files whose names start with ``.'', like .login). Now try typing:
% ls -a
Don't actually type the % symbol! Remember, that's the
computer's prompt which indicates it is ready to accept input. The
spacing should be exactly as shown. ls followed by a space,
followed by a -a. The -a is a ``flag'' which tells
the ls program to list all files.
Now, try
typing:
% ls -al
Here we add the
"-l" flag. This will produce more information about
the files in your directory, such as the owner and group name, and
the date the files were created.
For more about command flags see below.
Next, let's look at the contents of another directory, one with lots of weather-related files. Directory names in Unix are straightforward. They are all arranged in a tree structure from the root directory ``/''.
For now, use cd to change your directory to the /data1 directory. Type:
% cd /data1
and press <CR>. Now type ls again. You should see
a long list of files-many of them followed by "/" and some
with "@". The trailing "/" denotes that
the "file" is actually another directory (a subdirectory).
The trailing "@" is a special type of file, called a
"link" or an "alias"--it is actually a pointer
to where the actual file is.
albany/
decoded/ gempak/ gopher-data/
iamesonet/ logs/ nmc/
severe/ upperair/ archive@ difax/
gfsanl/ hds/
lastfile mcidas@ radar/ surface/
climate/ forecasts/ gif@
hydro/ local.note misc/ recon/
txmesonet/
Just for the absolute thrill and joy of it,
type "ls -al" and view the longer listing. Now, for
the ultimate experience, add one more flag: "t":
viz: "ls -alt" and hit return.
What
happened? The "t" flag told the computer to list the
files with the most recent appearing first. This is such a
handy command that everyone has a "shortcut" to it: just
type "lslt" and you will see the same result. (There's
actually one additional feature added to the shortcut, which we'll
mention later).
Note that the /data1 in the
command we typed above was not a flag to cd.
It was a ``parameter.'' Flags tell commands how to act, parameters
tell them what to act on.
Now return to your login directory with:
% cd
Always: Entering cd with no parameter returns you to your home directory. You can check to make sure that it worked by entering:
% pwd
which prints your current (or ``working'') directory. The computer should return a line of words separated by ``/'' symbols which should look something like:
/home/username
Whatever it returns, the list should end in your username.
Now,
here are a couple of handy additional tricks using cd. Go back
to the /data1 directory by typing "cd /data1". Now,
cd into the "albany" directory by typing "cd albany".
Now let's say you
want to move back up one directory, back
to /data1. Of course, you could type "cd /data1",
but in this case, if you want to go up one directory, you can type
"cd ..". You will see that you are back up one
directory in the hierarchy.
Let's dig even deeper: change
directories to /data1/forecasts/mod. What do you think would
be the quick way to go back up two directories, back to
/data1??
You've now seen that the ".." is shorthand
for backing up one directory. You can also try "cd .".
What does the single period do? Well, it is just a
shorthand for your current directory! It seems kind of silly
to have this feature, but it can actually be quite useful when
specifying full directory paths, say when running a script.
Most Unix commands have very short and sometimes cryptic names like ls. This can make remembering them difficult. Fortunately there are on-line manual pages which allow you to display information on a specific program (to list all the flags of ls, for example) or list all the information available on a certain topic.
To investigate other flags to the ls command (such as which flags will display file size and ownership) you would type man ls.
The second way of using the on-line manual pages is with man -k. In this case you use a word you expect to be in a one-line description of the command you wish to find. To find a program which ``lists directory contents'' you might type man -k dir. Partial words can be used and this is one of the few places in Unix where upper and lower case are allowed to match each other.
Try it now. Use man ls to find out how to make the ls program print the sizes of your files as well as their names. After typing man ls and pressing , note how man displays a screenful of text and then waits with a prompt --More-- at the bottom of the screen.
What man is doing is sending everything it wants to display to the screen through a program known as a ``pager'' The pager program is called more. When you see --More-- (in inverse video) at the bottom of the screen, just press the space-bar to see the next screenful. Press <CR> to scroll a line at a time.
Have you found the flag yet? The -s flag should display the size in kilobytes. You don't need to continue paging once you have found the information you need. Press q and more will exit.
Now type ls -as. You can stack flags together like this-this tells ls to list all files, even hidden files, and list their sizes in kilobytes.
It turns out that on our systems, we "secretly" provide a "shortcut", more accurately an "alias", to ls in everyone's tcsh shell environment. We are actually appending the "-F" flag of ls. This flag, as discussed in the "man" page, allows one to distinguish between whether a file is a "plain old file", with nothing after the file name, a "directory", with a "/" after the file name, an "executable program", with a "*" afterwards, et al.
How would you run ls if you did not want our "aliased"
version? If you specify the full directory path to the ls
program, which in this case is /bin/ls, it will run the program
ignoring any aliases. Try it and notice that you do NOT see
the trailing characters as noticed previously.
You
can also force the system to use the "vanilla" ls by
prepending a backslash "\" to it. Try it. The
backslash is yet another way to tell the system to ignore aliases.
This is just one of many
illustrations of the maxim that "There is always another way to
do something in UNIX!"
When you are finished you should be sure to logout! The
way you do this depends on whether you are remotely logged into a
computer using ssh, or directly logged into one with a full window
environment (called "X-Windows" in UNIX). If the
latter, the procedure differs depending on whether you are logged
into a Linux workstation or a Solaris one. If Linux, go to the
taskbar which is likely at the top of your screen, click on System
and then click on "Log out <your user id>". If
on Solaris, there is an "Exit" button near the bottom of
your screen that you would click on.
From a terminal
window, type either "exit" or "logout". You
need to be careful that you've typed "exit" or
"logout" correctly. The Unix operating system is not
forgiving of mis-typed commands. Mis-typing logout as
``logotu'', pressing return and then leaving without glancing at the
screen can leave your files at anyone's mercy.
When you list files in Unix, it is very hard to tell what kind of files they are. The default behavior of the ls program is to list the names of all the files in the current directory without giving any additional information about whether they are text files, executable files or directories! This is because the ``meaning'' of the contents of each file is imposed on it by how you use the file. To the operating system a file is just a collection of bytes.
There is a program file which will tell you information about a file (such as whether it contains binary data) and make a good guess about what created the file and what kind of file it is.
Unlike, say the Windows operating systems, filenames are not broken into a name part and a type part. Names can be many characters long and can contain most characters. Some characters such as * and ! have special meaning to the shell. They should not be used in filenames. If you ever do need to use such a symbol from the shell, they must be specified sneakily, by ``escaping'' them with a backslash, for example \!.
Directories in Unix start at the root directory ``/''. Files are ``fully specified'' when you list each directory branch needed to get to them. E.g.:
/bin/ls
/unidata/gempak/os/linux/bin/nmap2
/spare11/atm350/smith/homework1.txt
Usually disks are ``partitioned'' into smaller sized sections called partitions If one partition of the disk fills up the other partitions won't be affected.
Only certain large directory points are partitions and the choice of these points can vary among system managers. Partitions are like the larger branches of a tree. Partitions will contain many smaller branches (directories) and leaves (files).
To examine what disks and partitions exist and are mounted, you can type the df command at the % prompt. This should display partitions which have names like /dev/sda3---a for disk "a", 3 for partition 3. It will also display the space used and available in kilobytes and the ``mount point'' or directory of the partition.
It's important to keep track of how much disk space you are using. The command du displays the disk usage of the current directory and all of its subdirectories. It displays the usage, in kilobytes, for each directory-including any subdirectories it contains-and ends by displaying the total.
% du
% du -s
% du -s -k
Let's try it for our "data1" directory:
cd
/data1
du
We'll only include the first several lines, plus
the last line, for brevity:
29632 ./logs
4
./hds/logs
8 ./hds
966628
./misc
1951908 ./surface/syn
638688
./surface/sao
168176 ./surface/boy
2488
./surface/rad
268 ./surface/front
2761532
./surface
40929760
.
According to the display above, the "logs"
subdirectory within /data1 is using nearly 30 megabytes (MB) of
space, and the /data1 directory as a whole is using a whopping 40+
GB!
Were you to have typed "du -s", you would have
just seen the last line in the column above, giving the total disk
usage for the current directory, "."
Users have home directories for storing permanent files.
These directories are backed up nightly by our automatic tape backup
software. Each user is assigned a quota, defining how much
storage that you can use in your home directory. Typically,
undergraduate students are allotted 50 MB of space. You can
determine how much space you are using in your home directory by
typing "quota -v <userid>" at a shell prompt.
e.g. "quota -v smith"
Disk
quotas for user smith (uid 8888):
Filesystem blocks quota
limit grace files quota
limit grace
cypress:/home5/smith
36171 50000
55000
1862 10000 11000
This
shows that user "smith" has used about 72% of his/her
available storage. (There is also a limit to the # of files in
the user's directory, here set to 10,000).
If you exceed the
quota, you have 7 days to clean out room, or you will be locked out
of further sessions. Also, you are allocated 10% "emergency
overhead" if you fill your initial quota. Should you
exceed this emergency overhead (a total of 55 MB for undergrads),
you will also be "locked out", and you will need to see
one of the system managers for assistance.
Note:
if things "seem strange" when you
are in the middle of a session, check your quota first! Once
your quota is maxed out, NOTHING will work right.
A login directory can always be specified with ~username (~ is commonly called ``twiddle,'' derived from proper term ``tilde.'') If you needed to list files in someone else's login directory, you could do so by issuing the command:
% ls ~username
substituting in their username. You can do the same with your own directory if you've cd'd elsewhere. Please note-many people would consider looking at their files an invasion of their privacy; even if the files are not protected! Just as some people leave their doors unlocked but do not expect random bypassers to walk in, other people leave their files unprotected.
If you have many files or multiple things to work on, you probably want to create subdirectories in your login directory. This allows you to place files which belong together in one distinct place.
The program to make a subdirectory is mkdir. If you are in your login directory and wish to create a directory, type the command:
% mkdir directory-name
Once this directory has been created you can copy or move files to it (with the cp or mv programs) or you can cd to the directory and start creating files there.
Copy a file from the current directory into the new subdirectory by typing:
cp filename directory-name/new-filename
cp filename directory-name
Or cd into the new directory and move the file from elsewhere:
% cd directory-name % cp ../filename .
copies the file from the directory above giving it the same filename: ``.'' means ``the current directory''
There are two ways you can specify files. Fully, in which case the name of the file includes all of the root directories and starts with ``/'', or relatively, in which case the filename starts with the name of a subdirectory or consists solely of its own name.
When Charlotte Smith (username smith) created her directory tropical, and placed a file called "gustav" therein, within her home directory, /home/smith, all of the following sets of commands could be used to display the same file:
% more ~smith/tropical/gustav
or
% cd ~smith
% more tropical/gustav
or
% cd /home/smith/tropical
% more gustavThe full file specification, beginning with a ``/'' is very system dependent. On our department's machines, all user directories are in a network-attached partition, called "/home"..
/home/smith/tropical/gustav
When created, all files have an owner and group associated with them. The owner is the same as the username of the person who created the files and the group is the name of the creator's default login group, such as users, system etc. Most users do not belong to a shared group on our systems. If the creator of the file belongs to more than one group (you can display the groups to which you belong with the groups command) then the creator can change the group of the file between these groups. Otherwise, only the root account can change the group of a file.
Only the root account can change the ownership of a file.
The command ls -l filename will list the long directory list entry (which includes owner and protection bits) and the group of a file.
The display looks something like:
protection owner group filename -rw-r----- smith ugrad gustav_track.gif
The first position (which is not set) specifies what type of file this is. If it were set, it would probably be a d (for directory) or l (for link). The next nine positions are divided into three sets of binary numbers and determine protection to three different sets of people.
u g o rw- r-- --- 6 4 0
The file has ``mode'' 640. The first bits, set to ``r + w'' (4+2) in our example, specify the protection for the user who owns the files (u). The user who owns the file can read or write (which includes delete) the file.
The next trio of bits, set to 4, or ``r,'' in our example, specify access to the file for other users in the same group as the group of the file. In this case the group is ugrad-all members of the ugrad group can read the file (print it out, copy it, or display it using more).
Finally, all other users are given no access to the file.
From 0 to 7, here is what each "bit setting" represents: note that settings "1,2, and 3" are very rarely used.
0: no access
1: execute permission only
2: write permission only
3: write and execute permission only
4: read permission only
5: read and execute permissions only
6: read and write permissions only
7: read, write, and execute permissions
The one form of access which no one is given, even the owner, is ``x'' (for execute). This is because the file is not a program to be executed-it is probably a text file which would have no meaning to the computer. The x would appear in the 3rd position and have a value of 1.
The group of a file can be changed with the chgrp command. Again, you can only change the group of a file to a group to which you belong. You would type as follows:
% chgrp groupname filename
Since users tend to stay in the same group during their tenure in
the department, this is normally not a command you would run here.
Were you to become a grad student or a faculty/staff member at
some point, your account data would be changed by the system
managers, automatically changing your default group.
You can change the protection mode of a file with the chmod command. This can be done using the “bitmask” method as described above, or, more literally and intutitvely, by adding/subtracting access rights according to user, group, or other.
Note that in UNIX, everything, including a directory, is considered a “file” by the operating system, so <filename> can not only refer to a file but also a directory.
e.g. to take away read permission from the group: chmod g-r <filename>
e.g. to take away execute permission from the group: chmod g-x <filename>
to combine the above two actions: chmod g-rx <filename>
e.g. if you wanted to add write and execute permission to everyone (aka “other”): chmod o+wx <filename>
Using bitmasks, the gustav_track.gif file above had the mode 640. If you wanted to make the file readable to all other users, you could type:
% chmod 644 filename or
% chmod o+r filename
or
% chmod +4 filename (since the current mode of the file was 640)
In order to access a directory, you must give it "execute" permission. A diirectory that you have read permission for, but not write permission, would have a protection of "5". A directory that you have read AND write permission for would have a permission of 7.
If you were to have a non-executable file (thus, not a directory) readable and writeable only by yourself, you would make it so by typing "chmod 600 <filename>".
If you were to have an executable file (thus, including directories) readable and writeable only by yourself, you would make it so by typing "chmod 700 <file or dirname>".
If you wanted to have a directory writeable by yourself, everyone in your "group", and everyone else, you would make it so by typing "chmod 777 <file or dirname>".
The last item above is NOT recommended for the safety of your files!!!
For more information see the man page for chmod.
All files get assigned an initial protection. To set the default initial protection you must set the value of the variable umask. umask must be defined once per login (usually in the .cshrc file). Common umask values include 022, giving read and directory search but not write permission to the group and others and 077 giving no access to group or other users for all new files you create.
As mentioned earlier, user commands are parsed by the shell they run. There are many shells other than the tcsh shell which allow different types of shortcuts. We will only discuss the tcsh shell here, but some alternate shells include the Bourne shell ( /bin/sh), the Bourne-Again Shell ( bash), zsh and the "classic" C Shell (csh).
One of the most important elements of the shell is the path. Whenever you type something at the % prompt, the shell first checks to see if this is an ``alias'' you have defined, and if not, searches all the directories in your path to determine the program to run.
The path is just a list of directories, searched in order. Your default .cshrc will have a path defined for you. If you want other directories (such as a directory of your own programs) to be searched for commands, add them to your path by editing your .cshrc file. This list of directories is stored in the PATH environment variable. We will discuss how to manipulate enviroment variables later.
Most commands expect or allow parameters (usually files or directories for the command to operate on) and many provide option flags. A ``flag'' as we saw before, is a character or string with a - before it-like the -s we used with the ls command.
Some commands, such as cp and mv require file parameters. Not surprisingly, cp and mv (the copy and move commands) each require two! One for the original file and one for the new file or location.
It would seem logical that if ls by itself just lists the current directory then cp filename should copy a file to the current directory. This is logical-but wrong! Instead you must enter cp filename . where the ``.'' tells cp to place the file in the current directory. filename in this case would be a long filename with a complete directory specification.
Not surprisingly ls . and ls are almost the same.
cat is one of the most versatile commands. The simplest use of cat will just dump the contents of a file to the screen:
% cat /data2/gopher-data/local/albany/extgfs.alb
displays the contents of the above file to the screen. Unix allows you to redirect output which would otherwise go to the screen by using a > and a filename. You could copy this file, for example, by typing:
(first change back to your home directory by simply typing "cd")
% cat /data2/gopher-data/local/albany/extgfs.alb > alb_gfs.txt
This would have the same effect as:
% cp /data2/gopher-data/local/albany/extgfs.alb alb_gfs.txt
More usefully cat will append multiple files together.
% cat /data2/gopher-data/local/albany/zone /data2/gopher-data/local/albany/FPUS03 > albstuff
will place copies of the first two files into a single file. Warning! Be careful not to cat a file onto an existing file! The command:
% cat albstuff > albstuff
will destroy the file albstuff if it succeeds.
If you fail to give cat a filename to operate on, cat expects you to type in a file from the keyboard. You must end this with a <Ctrl>-D on a line by itself. <Ctrl>-D is the end-of-file character.
By combining these two-leaving off the name of a file to input to cat and telling cat to direct its output to a file with > filename, you can create files.
For example:
% cat > temp ;klajs;dfkjaskj alskdj;kjdfskjdf <Ctrl>-D %
This will create a new file temp, containing the lines of garbage shown above. Note that this creates a new file-if you want to add things on to the end of an existing file you must use cat slightly differently. Instead of > you'd use >> which tells the shell to append any output to an already existing file. If you wanted to add a line onto your .cshrc, you could type
% cat >> .cshrc echo "blah blah blah" <Ctrl>-D %
This would append the line echo "blah blah blah" onto your .cshrc. Using > here would be a bad idea-it might obliterate your original .cshrc file.
cat is fine for files which are small and never need to have real changes made to them, but a full fledged editor is necessary for typing in papers, programs and mail messages. Among the editors available pico, vi and emacs.
Be careful! Not all Unix editors keep backup copies of files when you edit them.
The first two programs, gedit and nedit, will work only when you are directly logged onto one of the computers. This is because they depend on having a desktop environment that supports a graphical interface (I.e, they are running X-windows). If you are remotely logged in using a text-based terminal access program, such as ssh or putty, you will need to use one of the other three text editors.
gedit is a simple, easy-to-use graphical text editor, similar to "notepad" on Windows. It can be accessed by typing "gedit" on the command line or by going to Applications-->Acessories-->Text Editor on the Linux taskbar.
nedit is another simple, easy-to-use graphical text editor. It can be accessed by typing "gedit" on the command line
pico is a simple, friendly terminal window editor--the same editor as used in pine. Type pico filename to start it and type man pico for more information about how to use it. This is only available on our Linux systems.
vi is an editor which has a command mode and a typing mode. When you first startup vi (with the command vi filename) it expects you to enter commands. If you actually want to enter text into your file, you must type the insert command i. When you need to switch back to command mode, hit the escape key, usually in the upper left corner of your keyboard.
To move around you must be in command mode. You can use the arrow keys or use j, k, h, l to move down, up, left and right.
For more information type man vi. More user-friendly documentation can be found on the web, or use this vi "cheat sheet"
Emacs is a large, powerful editing system.
To use emacs, type:
% emacs For more information, check out this emacs "cheat sheet"
Ordinarily there are two types of output from commands: output to standard output (stdout) and to standard error (stderr). The > and >> examples above directed only standard output from programs into files. To send both the standard output and error to a file when using the tcsh shell, you should type >& :
% command >& filename
The basic commands for comparing files are:
cmp
diff
comm
See the man pages on these for more information.
The grep program can be used to search a file for lines
containing a certain string:
(why "grep"? It
actually stands for, in true Geek-ese, "generalized regular
expression parser". For now, let's just consider it a
nifty search tool.
% grep string filename % grep -i string filename (case insensitive match)
or not containing a certain string:
% grep -v string filename
See the man page for grep---it has many useful options.
more and the vi editor can also find strings in files. The command is the same in both-type a /string when at the --More-- prompt or in vi command mode. This will scroll through the file so that the line with ``string'' in it is placed at the top of the screen in more or move the cursor to the string desired in vi. Although vi is a text editor there is a version of vi, view, which lets you read through files but does not allow you to change them.
Most Unix commands which return information about how much CPU-time you've used and how long you've been logged in use the following meanings for the words ``job'' and ``process.''
When you log in, you start an interactive ``job'' which lasts until you end it with the logout command. Using a shell like the tcsh or C shell which has ``job-control'', you can actually start jobs in addition to your login job. But for the purposes of the most information returning programs, job (as in the ``JCPU'' column) refers to your login session.
Processes, on the other hand, are much shorter-lived. Almost every time you type a command a new process is started. These processes stay ``attached'' to your terminal displaying output to the screen and, in some cases (interactive programs like text editors and mailers) accepting input from your keyboard.
Some processes last a very long time-for example the /bin/tcsh (tcsh shell) process, which gets started when you login, lasts until you logout.
You can get information about your processes by typing the ps command.
PID TT STAT TIME COMMAND 9980 s9 S 0:06 -tcsh (tcsh) 12380 s9 R 0:01 ps
The processes executing above are the tcsh shell process and the ps command. Note that both commands are attached to the same terminal (TT), have different process identification numbers (PID), and have different amounts of CPU-time (TIME), accumulated.
The simplest and quickest information you can get about other people is a list of which users are logged in and at which ``terminals'' (terminal here is either a terminal device line or telnet or rlogin session). The command to do this is who and it responds quickest of all the commands discussed here because it simply examines a file which gets updated everytime someone logs in or out.
Be careful though! This file, utmp, can get out of date if someone's processes die unexpectedly on the system. Any program which uses utmp to report information may list users who are not really logged in!
The w command is slower than the who command because it returns more information such as details about what programs people are running. It also returns a line containing the number of users and the system load average. The load average is the average number of processes ready to be run by the CPU and is a rough way of estimating how busy a system is.
w also uses the utmp file mentioned above. It takes longer than who because it then looks around and collects more information about the users it finds in the utmp file.
The ps command used earlier to list your own processes can be used to list other users' processes as well. who and w list logins-but not individual processes on the system. They don't list any of the running operating system processes which start when the computer is booted and which don't have logins.
Since ps doesn't use utmp it is the program to use when you really want to find out what processes you might have accidentally left on the system or if another user is running any processes. Note that although ps might report processes for a user, it might be because that user has left a ``background job'' executing. In this case you should see a ``?'' in the TT field and the user won't really be logged in.
To get this fuller listing, give the flags -aux to ps. For more information on the uses of ps, type man ps.
The finger program returns information about other users on the system who may or may not be logged in. finger by itself returns yet another variation of the list of currently logged in users. finger followed by a username or an e-mail -style address will return information about one or more users, the last time they logged into the system where you are fingering them, their full name, whether or not they have unread mail and, finally, the contents of two files they may have created: .plan and .project
For more information about using finger or ways to provide information about yourself to others, type man finger.
Electronic mail programs run on almost all the computers in the DEAS and usually have two parts: a user interface which lets users read and send messages and a system mailer which talks to mailers on other computers. This mailer receives outgoing messages from the user interface programs and delivers incoming messages to the user mailbox (which the interface program reads). Most of you, though, have "webmail" boxes ("xx123456@albany.edu") assigned to you by UAlbany, and you should already be comfortable using a web browser to read and write mail. That said, here are a few commonly-used mail tools on the UNIX systems here:
There are many user interfaces available on the Unix computers, all of which provide similar functionality. The program supplied with most Unix computers is /usr/ucb/mail (or Mail). To read messages type Mail, to send messages type:
% Mail address
Mail has been changed to mailx.
You should next see a Subject: prompt. If you don't see a prompt, don't worry, just type in your one line subject anyway and press return. You may start typing your message (but you will be unable to correct errors on lines after you have pressed <CR> to move to the next line) or you may may specify a file to include with r filename.
You may invoke a text editor like vi by typing v. If you wish regularly to use an editor other than vi you should see the information later in this section about enviroment variables.
There are many other commands you may enter at this point-see the Mail man page for all of them. When you are finished typing in your message (if you have used v to run a text editor, you should exit from it) press <Ctrl>-D on a line by itself. Most likely you will now see a CC: prompt. If you wish to send copies of your message to someone besides the recipient you would enter the address or addresses (separated by ``,'') and press return. Otherwise press return without entering an address.
PINE is a full-screen interactive mailer, developed at UW, that is very straightforward to use. To use it type pine.
The write program can be used to send messages to other users logged onto the system. It's not a great way of having a conversation, but it's simple to use. Enter:
% write username
and you can start writing lines to the terminal of the person you want to send messages to. The person must be logged in, and, if they are logged in more than once, you must specify the terminal to write to-for example write melville ttyh1.
If you use certain command flags regularly ( -lga for ls) you can alias them to shorter commands. You can use wildcard symbols to refer to files with very long names. You can easily repeat commands you have already executed or modify them slightly and re-execute them.
As mentioned above, you can alias longer commands to shorter strings. For example, as mentioned earlier, ls -F will list all the files in the current directory followed by a trailing symbol which indicates if they are executable commands (a *) or directories (a /). We have already added the following command to your .cshrc:
% alias ls ls -F
To list the aliases which are set for your current process, type:
% alias
without any parameters.
Everyone in the ATM350 class has an alias already defined in their .cshrc:
alias atm350 “cd /spare11/atm350/$LOGNAME”
which will place you in your class directory when you type “atm350” at a command prompt. The $LOGNAME is an environment variable which automatically is set to your userid.
Wildcards are special symbols which allow you to specify matches to letters or letter sequences as part of a filename.
Some examples:
rm *!!
ls *.dat
.dat
ls r*
r
ls ?.dat
5.dat, u.dat, but not 70.dat
ls *.[ch]
.h and .c files
more [Rr][Ee][Aa][Dd][Mm][Ee]
mores the files README, readme,ReadMe,
and Readme, among others
Environment variables are pieces of information used by the shell and by other programs. One very important one is the PATH variable mentioned earlier. Other important variables you can set include:
EDITOR
TERM
To see what environment variables are set and what they are set to, type the command printenv. To set a variable, use the setenv command as in the example below.
% setenv TERM xterm % setenv EDITOR emacs
Many programs mention environment variables you may want to set for them in their man pages. Look at the csh man page for some of the standard ones.
Environment variables are also useful to define commonly used files or directories. For example, the environment variable GEMTBL points to the directory of station and parameter tables in GEMPAK. If you wanted to quickly change directories to that specified by GEMTBL, you would type "cd $GEMTBL".
Most shells allow ``command line editing'' of some form or another-editing one of the previous few lines you've typed in and executing the changed line. You can set a history ``environment variable'' to determine how many previous command lines you will have access to with set history=40
The simplest form of command line editing is to repeat the last command entered or repeat the last command entered with more text appended.
If the last command you typed was:
% ls /data1
Then you can repeat this command by typing:
% !!
This will return a list of files. If you saw a directory albany in the list returned and you wanted to list the files it contained, you could do so by typing:
% !!/albany
If you mistype albany as ablany you can correct it with the following command:
% ^abla^alba
This substitutes leaven for leave in the most recently executed command. Beware! This substitutes for the first occurrence of abla only!
You can type history at any time to get a list of all the commands remembered. This list is numbered and you can type ! number to repeat the command associated with number. Alternately you can type ! and a couple of letters of the command to repeat the last line starting with the characters you specify. !ls to repeat ls -lg albany, for example.
The .cshrc file is run whenever a tcsh or C shell process is started. Then, if this is a login process, the .login file is executed.
If you are using a Sun or Linux console and you have the default setup, any terminal windows ("xterm") which you start up will not execute the .login.
It is very easy to do many things at once with the Unix operating system. Since programs and commands execute as independent processes you can run them in the ``background'' and continue on in the foreground with more important tasks or tasks which require keyboard entry.
For example, you could set a program running in the background while you edit a file in the foreground.
When you type <Ctrl>-Z whatever you were doing will pause. If you want the job to go away without finishing, then you should kill it with the command kill %. If you don't want it paused but want it to continue in the foreground-that is, if you want it to be the primary process to which all the characters you type get delivered-type fg. If you want it to continue processing in the background while you work on something else, type bg.
You should not use bg on things which accept input such as text editors or on things which display copious output like more or ps.
If you've got several processes stopped-perhaps you are editing two files or you have multiple telnet or rlogin sessions to remote computers-you'll need some way of telling fg which job you want brought to the foreground.
By default fg will return you to the process you most recently suspended. If you wanted to switch processes you would have to identify it by its job number. This number can be displayed with the jobs command. For example:
% jobs [1] Stopped vi .login [2] + Stopped rn [3] Running cc -O -g test.c %
The most recently suspended job is marked with a + symbol. If you wanted to return to job one instead, you would type:
% fg %1
You can type %1 as a shortcut.
Some jobs should start in the background and stay there-long running compilations or programs, for example. In this case you can direct them to the background when you start them rather than after they have already begun. To start a job in the background rather than the foreground, append an & symbol to the end of your command.
You should always run background processes at a lower priority by using the nice command. Non-interactive jobs are usually very good at getting all the resources they need. Running them at a lower priority doesn't hurt them much-but it really helps the interactive users-people running programs that display to terminal screens or that require input from the keyboard.
If you need to run CPU-intensive background jobs, learn about how to control the priority of your jobs by reading the manual pages (man nice and man renice).
The cp command allows you to create a new file from an existing file. The command line format is:
% cp input-file-spec output-file-spec
where input-file-spec and output-file-spec are valid Unix file specifications. The file specifications indicate the file(s) to copy from and the file or directory to copy to (output). Any part of the filename may be replaced by a wildcard symbol (*) and you may specify either a filename or a directory for the output-file-spec. If you do not specify a directory, you should be careful that any wildcard used in the input-file-spec does not cause more than one file to get copied.
% cp new.c old.c % cp new.* OLD (where OLD is a directory)
command allows the user to get a list of files in the current default directory. The command line format is:
% ls file-spec-list
where file-spec-list is an optional parameter of zero or more Unix file specifications (separated by spaces). The file specification supplied (if any) indicates which directory is to be listed and the files within the directory to list.
The lp command tells the system that one or more files are to be printed on the default printer. If the printer is busy with another user's file, an entry will be made in the printer queue and the file will be printed after other lp requests have been satisfied. The command line format is:
% lp myfile-list
where myfile-list is one or more Unix files to be printed on the default printer.
The man command is a tool that gives the user brief descriptions of Unix commands along with a list of all of the command flags that the command can use. To use man, try one of the following formats:
% man command % man -k topic
The more command will print the contents of one or more files on the user's terminal. The command line format is:
% more file-spec-list
more displays a page at a time, waiting for you to press the space-bar at the end of each screen. At any time you may type q to quit or h to get a list of other commands that more understands.
Aren't UNIX developers funny!! The less command is actually an "improved" version of "more". The command line format is:
% less file-spec-list
more displays a page at a time, waiting for you to press the space-bar at the end of each screen. At any time you may type q to quit or h to get a list of other commands that more understands.
The mv command is used to move, or rename files to different names or directories. The command line syntax is:
% mv input-file-spec output-file-spec
where input-file-spec is the file or files to be renamed or moved. As with cp, if you specify multiple input files, the output file should be a directory. Otherwise output-file-spec may specify the new name of the file. Any or all of the filename may be replaced by a wild card to abbreviate it or to allow more than one file to be moved. For example:
% mv data.dat ./research/datadat.old
will change the name of the file data.dat to datadat.old and place it in the subdirectory research. Be very careful when copying or moving multiple files.
The rm command allows you to delete one or more files from a disk. The command line format is:
% rm file-spec-list
where file-spec-list is one or more Unix file specifications, separated by spaces, listing which files are to be deleted. Beware of rm *! For example:
% rm *.dat able.txt
will delete the file able.txt and all files in your current working directory which end in .dat. Getting rid of unwanted subdirectories is a little more difficult. You can delete an empty directory with the command rmdir directory-name but you cannot use rmdir to delete a directory that still has files in it.
To delete a directory with files in it, use rm with the -r flag (for recursive). And be very careful!!