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Emergency Information
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UNIX TutorialBy Heather Sherman, Roger MurrayContents:
A Sample Login SessionThe IMSS UNIX Cluster is available for logins from the Student Lab (see the Getting Started on the IMSS UNIX Cluster and the Sun Console Guide) or over the campus network or dial-up lines. The Cluster has several computer which can be logged into from the network of the dial-up lines. We recommend that people connect to its.caltech.edu, using either ssh or telnet, rather than using the name of a specific server. The name its rotates between the servers to distribute the load.
Logging OnWhen you first connect to one of the UNIX computers you will see the prompt:
login:
If you see only the prompt Password: you probably used
rlogin. rlogin assumes that your username is the same on
all computers and enters it for you. If your username is different, don't
worry, just press
At the login: prompt, type in your username.
Be careful to type only lower case! The UNIX operating system is "case
sensitive." If you type your username in mixed case (Rarmour
rather than rarmour, for example) the computer will not recognize
it.
Once you have typed in your username you will be prompted to type in your
password. Type carefully! It won't be displayed on the screen.
When you first log in, you should change your password.
In the interests of self-preservation, don't set your password to any
information which people are likely to know about you (your real name, your
nickname, your pet dog's name). Because there are programs that people can run
to guess at your password, you should not make it relate to any word or name.
For more information about choosing a good password, as well as how to
change your password using the new IMSS Web Utilities System, see the IMSS reference Password Security Tips.
If you mistype your username or password you will get a suspicious message
from the computer and see the login: prompt 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. This
file 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 manager.
Next the announce program will display any
brief one-time messages which might be of interest to cluster users.
announce is run by your .login.
When you log in, the UNIX login program 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, home directory and "path." By
default all users use the C-shell (the program
/bin/csh) 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 C-shell
work, you may want to customize these files.
If your files get corrupted for some reason, you can run the
ccoconfig program to get new copies of the
default files given to all new accounts.
Finally you are logged in! You will see a prompt like
one of the following three:
{coil:1}
%
just waiting for you to type something. Throughout the UNIX Tutorial we will
use % to indicate the computer's "ready" prompt.
Okay, let's try a simple command. Type ls
and press
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.
The -a is a "flag" which tells the
ls program to list all files.
For more about command flags see below.
Just for fun, let's look at the contents of another directory, one with lots
of 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 /bin directory. Type:
and press
Now return to your home directory with:
Entering cd with no parameter returns you to your home directory.
You can check to make sure that it worked by entering:
which prints your current (or "working") directory. The computer will return
a line of words separated by "/" symbols which should look something like:
Whatever it returns, the list should end in your username.
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 command you are interested in type:
For example, 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:
In this case you use a word to describe the topic you are interested in or that you might expect to be in a one-line description of the command you wish to find. For example, 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
What man is doing is sending everything it wants to display on 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
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. The command
ls -as lists all files, and lists their sizes in kilobytes.
When you are finished you should be sure to log out! You need to be careful
that you've typed logout correctly. The UNIX operating system is
not forgiving of mistyped commands. Mistyping logout as "logotu",
pressing return and then leaving without glancing at the screen can leave
your files at anyone's mercy.
As mentioned earlier, user commands are parsed by the shell. There are many
shells other than the the C-shell which allow different types of shortcuts.
We will only discuss the C-shell here, but some alternate
shells installed on the IMSS UNIX Cluster include the Bourne
shell (/bin/sh), the
Bourne-Again Shell
(bash),
zsh and
tcsh (a C-shell variant). The last three
all reside in /usr/local/bin. While you are welcome to experiment
with any of these shells, realize that C-shell is our default shell and that
some IMSS UNIX Cluster utilities assume you are using it (and may not work
if you're not).
To change your current shell to another use the chsh command (for change shell). You will then be prompted for your password and the new shell's path (e.g. /usr/local/bin/tcsh for the tcsh shell). After enter
ing these you will be informed that the login shell has been changed. To change to this shell logout and then re-login. For more information on changing your shell see the Changing Shell FAQ page.
One of the most important elements of the shell is the path. Whenever you
type something at the % prompt, the C-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, delimited by colons, which are 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 environment 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 such as 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. Instead you must enter
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.
Directories in UNIX start at the root directory "/".
Files are fully specified when you list each directory branch needed to get
to them.
/home/pamela/src/file.c
A home directory can always be specified with
~username (~ is commonly called "twiddle,"
derived from the proper term "tilde.") If you needed to list files in someone
else's home directory, you could do so by issuing the command:
substituting in their username. You can do the same with your own directory
if you've cd'd elsewhere. Please note: many people consider looking
at their files an invasion of privacy, even if the files are not protected.
Just as some people leave their doors unlocked but do not expect random
passers-by to walk in, other people leave their files unprotected without
intending to invite browsers.
If you have many files or multiple things to work on, you probably want to
create subdirectories in your home 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 home directory
and wish to create a directory, type the command:
Once this directory has been created you can copy or move files to it (with
the cp or mv commands) or you can cd to the directory
and start creating files there.
To copy a file use the cp command, specifying the name of the file you wish to be copied and the filename you wish to give the copied file:
You can copy a file from the current directory into another subdirectory by entering:
Or cd into the new directory and move the file from elsewhere:
% cp ../filename .
copies the file from the directory above (represented by "..") to the current
directory (represented by "."), giving it the same filename.
Files can be renamed or moved to different subdirectories using the mv command. The mv command works in a similar fashion to the cp command:
To remove files use the rm command, specifying the name of the file you want to remove (and the path if it is in another directory):
To remove a directory the command rmdir is used. NB: This command will only work if the directory speficied is empty. To remove a directory with files in it use the command rm -r directory-name (for recursive).
To avoid unwanted deletion of precious files the mv and rm commands can be made interactive using the -i flag. For example rm -i filename would return a prompt asking if you are certain you want to delete tha
t file. We recommend that you alias these commands to be interactive in your .cshrc file. For more help on aliasing see the aliases section later.
You can search for files using the find command, using the -name flag. The directory you wish to be searched must also be specified.
Unlike other 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:
C-shell would have interpreted rm !badfile differently. In that
case, !badfile would have been replaced with the last command beginning
with "badfile." Chances are no such command would have existed, resulting
in the error message badfile: Event not found. See the section on
history for more information.
There are two ways to specify files:
When Charlotte Lennox (username lennox) created her directory
arabella, all of the following sets of commands could be used to
display the same file:
or
% cd ~lennox
% more arabella/chapter1
or
% cd ~lennox/arabella
% more chapter1
The full file specification, beginning with a "/" is very system dependent.
On the IMSS UNIX Cluster, all user directories are on the /home
partition. This means that ~lennox on the IMSS UNIX Cluster would be
the same as /home/lennox and that chapter1 would be fully
specified by:
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 512-byte units, for each directory, including any subdirectories
it contains, and ends by displaying the total.
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/sd3g: 3 for disk 3, g
for partition g. It will also display the space used and available in kilobytes
and the "mount point" or directory of the partition.
Users have home directories for storing permanent files. At various busy times
of the year there may be shortages of disk space on the IMSS UNIX Cluster. You
should use the du command to stay aware of how much space you are using
and not try to exceed the system limits. For more information on user disk quotas
see the FAQ - Quotas document.
Temporary scratch space is available in the event of
a disk crunch, however, and can be used to store files which are extremely
large or relatively unimportant. The scratch space is located in the
/ccovol/suntmp directory. You should use
the mkdir program to create a directory for yourself on either of
these partitions.
Remember, because the scratch space is for temporary
storage, you should delete the files as soon as you are done with them.
This is particularly important if your files are large. If you forget
to remove your files, they will be removed for you, but not until seven or
more days after you last access them.
The command ls -l filename will list the long directory
list entry (which includes owner and permission bits) and the group of a
file.
The display looks something like:
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 faculty, grads, ug,
etc.
Most users belong to one group on the IMSS UNIX computers, such as ug
or faculty. If the owner of the file belongs to more than one group
(you can display the groups to which you belong with the
groups command) then the owner 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 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 permissions for three different sets of people.
The file has "mode" 640. The first bits, set to
"r + w" (4+2=6) in our example, specify the permissions
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 "r" (4) in our example, specify access
to the file for other users in the same group (g) as the group of the file.
In this case the group is ug -- all members of the ug group
can read the file (print it out, copy it, or display it using more).
Finally, all other users (o) are given no access to the file.
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 third position if it was an excutable file.
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:
You can change the protection mode of a file with the
chmod command. This can be done relatively
or absolutely. The file in the example above had the mode 640. If you wanted
to make the file readable to all other users, you could type:
or
% chmod o+r filename
or
% chmod +4 filename
For more information see the man page for chmod.
All files are assigned a default set of permissions. To set the default,
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 execute (or 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. Note that the umask bits represent
permissions not to be given (i.e. the opposite of what ls -l
would show).
cat is one of most versatile commands. The
simplest use of cat:
displays your .cshrc 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 your .cshrc,
for example, by typing:
This would have the same effect as:
More usefully, cat will append multiple files
together.
will place copies of your .cshrc and .login into the same
file. Warning! Be careful not to cat a file onto an existing file! The command:
may destroy the file .cshrc.
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
By combining the above two concepts, 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:
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
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.
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 C-shell, you should type >&:
Sometimes you may wish to log the output of a login session to a file so
that you can show it to somebody or print it out. You can do this with the
script command. When you wish to end the
session logging, type exit.
When you start up you should see a message saying script started, file
is typescript and when you finish the script, you should see the message
script done. You may want to edit the typescript file: visible ^M's
get placed at the end of each line because linebreaks require two control
sequences for a terminal screen but only one in a 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 on the IMSS UNIX computers are pico, vi and
emacs.
Be careful: not all UNIX editors keep backup copies of files when you edit
them.
pico is a simple, friendly editor. Type
setup pine to set it up, pico filename to start
it and man pine for more information about how to use it.
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, respectively.
For more information type man vi.
Emacs is a large editing system.
To use emacs, type:
The grep program can be used to
search a file for lines containing a certain string:
% grep -i string filename (case insensitive match)
or not containing a certain string:
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 /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 called view, which lets you read through files but does
not allow you to change them.
The basic commands for comparing files are:
See the man pages on these for more information.
When you list files in UNIX, it can be 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.
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
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" 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/csh
(C-shell) process, which gets started when you log in, lasts until you log
out.
You can get information about your processes by typing the
ps command.
The columns indicate terminal (TTY) on which the process is running, the
process identification numbers (PID), and the amount of CPU time the process
has accumulated (TIME).
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 every time someone logs in or out.
Be careful though! This file, /etc/utmp,
can get out of date if someone's processes die unexpectedly on the system.
Any program which uses utmp to report information might occasionally
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.
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; the user is
not really logged in. In this case you should see a "?" in the TTY field.
To get this fuller listing, use ps -ef. For more information on the uses of ps, type man ps.
top is an interactive command that displays and periodically updates the top cpu processes, ranked by raw cpu percentage. The default number of processes displayed is 15 but you can specify the number of processes with top number (e.g. to specify the top 10 processes enter top 10). You should see something like the following:
As top is an interactive command the cursor will remain blinking above the PID column, waiting for the next command. If you only want to see a list of jobs by a specific user type the letter u. You will then be asked which username to you wish to show. To only show non-idle jobs use the -I flag.
To quit or cancel a job that is still running the kill command can be used within top. Simply enter the letter k and the cursor will prompt you with kill . Then enter the PID of the job you wish to cancel and
To quit top simply type the letter q.
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 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
at Caltech 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).
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
mailx). To read messages type mailx,
to send messages type:
For information about valid mail addresses, please see the
Electronic Mail Guide.
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. 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
commas) and press return. Otherwise press return without entering an address.
You can now start typing your message (but you will be unable to correct errors
on lines after you have pressed
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 the section about environment
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) either press
Pine is a full-screen interactive mailer that is very
straightforward to use. It is currently installed on the IMSS UNIX Cluster
Suns. If you have the command setup pine in your .cshrc
then you should type pine. If the pine command fails you
probably need to type setup pine first. For more information type
man pine.
Other mailers include ELM and
MH. Both require setup commands. The command
to start ELM is elm. MH is actually a set of several separate programs.
See man mh for more information. Documents about using Pine, ELM
and MH are available from IMSS. To get a copy, send mail to help@caltech.edu.
The write program can be used to send messages
to other users logged onto the system. It's not the best way of having a
conversation, but it's simple to use. Enter:
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; write
melville ttyh1, for example.
ytalk is a program which allows two users
to hold a conversation. Unlike write, it can be used between different
computers; and, unlike write, it divides the screen so that the
things you type appear in the top half and the things written to you appear
in the bottom half.
To ytalk to users on the same computer:
To ytalk to users on another computer use the address format of
username@nodename:
ytalk can only be used to other Internet nodes: usually computers
which have ending names such as .edu, .com, .org, .gov, or .mil. Not all
computers with these names are attached directly to the Internet.
finger and ytalk won't work with computers which are only
attached by mail gateways.
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, ls -F will list all the
files in the current directory followed by a trailing symbol which indicates
if they are executable commands (*) or directories (/). If you wanted this
to be the default behavior of ls you could add the following command
to your .cshrc:
This command will then be aliased the next time you login. To make the changes you have made effective in your current shell you must first source your .cshrc file by typing:
NB: You must be in the directory that contains your .cshrc file or else specify the correct path to your .cshrc file.
To list the aliases which are set for your current process, type:
without any parameters.
Wildcards are special symbols which allow you to specify
matches to letters or letter sequences as part of a filename.
Some examples:
Beware of the
You've already met the ~ shortcut. The two other important directory symbols
are "." for the current directory and ".." for the previous
(parent) directory.
moves you out of a subdirectory and into its parent directory.
Environment variables are pieces of information used
by the shell and other programs. A very important one is the
PATH variable mentioned earlier. Other important
variables you can set include:
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 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.
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 the history variable to determine how
many previous command lines you will have access to. set history=40
will let you search the last 40 commands.
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:
Then you can repeat this command by typing:
This will return a list of files. If you saw a directory leavenworth
in the list returned and you wanted to list the files it contained, you could
do so by typing:
If you mistype leavenworth as leaveworth you can correct
it with the following command:
This substitutes leaven for leave in the most recently
executed command. Beware: this substitutes for the first occurrence
of leave 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. Alternatively
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 your
last ls command, for example.
The .cshrc file is run whenever a C shell
process is started. Then, if this is a login process, the .login
file is executed. If you are using a Sun console and you have the default
setup, any xterm windows 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
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:
The most recently suspended job is marked with a + symbol. If you wanted
to return to job one instead, you would type:
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.
The IMSS UNIX Cluster has an explicit policy about the proper running of
CPU-intensive background jobs. Be sure to read over this information in the
UNIX Cluster Policies.
You should man nice and man renice for more information
about how to alter the priority of your processes.
Some programs provide you with special ways of suspending them. If you started
another shell by using the csh command, you would have to use the
suspend command to suspend it.
If you wish to suspend a telnet or rlogin session you must
first get past the current login to get the attention of the telnet
or rlogin program.
Use ~ (immediately after pressing a return) to get rlogin's attention.
Use
If you have a general computing question please send mail to help@caltech.edu
For more information about UNIX you could consult:
This book is now a few of years old -- other books might be more up to
date, but the above remains useful for informational purposes.
Another good place to start is the O'Reilly series of computing
guidebooks.
Online launching points include:
The cp command allows you to create a new
file from an existing file. The command line format is:
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. Any
part of input-file-spec may be replaced by a wildcard symbol (*
or ?) 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.
The ls command allows the user to get a
list of files in the current directory. The command line format is:
where file-spec-list is an optional parameter of zero or more UNIX
file specifications (separated by spaces). The
file specifications supplied (if any) indicates which directories are to
be listed and the files within the directories 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:
where file-spec-list is one or more UNIX files to be printed on
the default printer. Any part of the filenames may be replaced by a wild
card.
For more information about where the printers actually are and what kind
of printers are available, please see our listing of
IMSS
Printers.
The man command is a tool that gives the
user brief descriptions of UNIX along with a list of all of the command flags
that the command can use. To use man, try one of the following formats:
The more command will print the contents
of one or more files on the user's terminal. The command line format is:
more displays a page at a time, waiting for you to either press the
space bar at the end of each screen or
The mv command is used to move files to
different names or directories. The command line syntax is:
where old-file-spec is the file or files to be renamed or moved.
As with cp, if you specify multiple files, the
new-file-spec file should be a directory. Otherwise
new-file-spec may specify the new name of the file. Any or all of
the old filenames may be replaced by a wild card to abbreviate it or to allow
more than one file to be moved.
For example:
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:
where file-spec-list is one or more UNIX file specifications, separated
by spaces, listing which files are to be deleted. For example:
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).
Beware of the rm * command!
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