As you have seen, it is easy to run programs
(or utilities) on UNIX systems. Essentially, you just type the name
of the command at the command line at the program runs.
In this section we will discuss managing
those programs that you run. However, before we do that, we
should say a few more things about commands and how to run them
correctly. As we said, you execute a command by typing the name
of the command at the command line. But what exactly is the name?
Well the name refers not only to the name of the utility, but also
to its location on the system. A command's name is really its
For example, when you run the "ls" utility,
you are actually running the "usr/bin/ls" program. Fortunately,
UNIX typically already knows to look for programs in "/usr/bin",
so you do not need to specify the directory explicitly.
(We will talk more about how UNIX knows about "/usr/bin" later).
But you can also try running "ls" by calling
it explicitly from "/usr/bin" by typing something like:
at the command line. But what if the
program is not located in the "/usr/bin" directory? What happens
if UNIX cannot find the program by itself.
Well, the first thing you should do is
find the program. Typically you will use the "which" utility
or the "whereis" utility to find the location of the program.
Then you can execute it explicitly with the full pathname.
Another useful thing to know is that
you can always stop an application from running by hitting
CONTROL-C (or CONTROL-Z to suspend).
Further, if you want to run an application
in the background, you need only append a "&" to the end of the
command when you execute it. When you do this, UNIX will execute
the command by itself and allow you to continue working from the
This is very useful when you have a utility or application that
needs a lot of time to complete. You do not want to have to sit there and wait
for the application to finish while you could be doing other work.
Sometimes it is easier to simply open another shell with telnet or
rlogin, but often, you don't want to clutter up your desktop and it is
reasonable just to run the command in the background.
In the following example, we run a lengthy spell check
on a large file in the background.
Finally, the "nohup" utility is very useful when
you want a utility to continue to run even after you have logged out.
Without the "nohup" utility, your applications would all quit when you log out.
If you use the nohup utility, they will run until complete. Consider the
following example in which we put a sort operation on "nohup" and log out.
When you log back in, the sort will have finished without you.
(Don't forget that you need to run "nohup" applications in the
Once you are running an application, or
several applications, however, you will need to be aware of several
other utilities that help you manage running applications.
The first such utility is the "ps" utility
that allows you to get a listing of processes currently being
executed by the UNIX system. By default the utility displays
four columns. Essentially, this information let's you know
exactly what UNIX is tasked with.
- PID = ID number of the process that the kernel uses
to keep track of it.
- TTY = Associated terminal
- TIME = CPU time spent running the process.
- CMD = Name of the command.
The "ps" utility comes with several
useful options, some of which are listed below.
||Displays ALL processes
||Gives the full process list including the command line arguments
||Provides a long listing
||Reports activity of given users.
Check out the examples of "ps" below:
One thing to keep in mind is that
"ps -ef" usually provides a lot more info than you want
since it reports on all users. Thus, it is usual to see the
utility used with grep such as in the following example
Once you know what processes are
currently running, you can stop them by using the "kill" utility.
The "kill" utility takes a PID value and terminates the
associated application. Look at the following example in which
we accidentally start a find operation which will take forever and
kill it from another window:
There are actually several other utilities to
manage processes, but we will not cover them here since they are
primarily sys admin tools and not things you will be doing. The "ps"
and the "kill" should provide you with enough tools to do the jobs
you will be interested in doing.
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