Bachelor of Computer Applications: Unix

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 Most UNIX commands expect input to come from a file(s), and
produce output to anther file(s).  “One
of these files is the standard input, and is the place from which a program
expects to read its input.  Another is
called the standard output, and it is the file to which the program writes its
results.  The third file is the
diagnostic output (also called Standard error), and it is a file to which
program writes any error responses. 
Generally, the standard input is taken to be the keyboard (input is
typed by the user), the standard output is the terminal screen.<o:p></o:p></div>
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<b><span style="font-family: 'Times New Roman', serif;"><span style="font-size: large;"><span style="color: #0b5394;">Concept of</span> </span></span></b><b><span style="font-family: 'Times New Roman', serif;"><span style="color: #0b5394; font-size: large;">Redirection</span></span></b></h2>
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<b>(1) Redirection Input:</b></h3>
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            There are
two possible sources of input for UNIX commands.  Programs such as ls and find get their input
from the command line in the form of options and filenames.  Other programs, such as cat, can get their
data from the standard input as well as from the command line.  Try the cat command with no options on the
command line:<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$cat</span><o:p></o:p></b></div>
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            There is no
response.  Because n files are specified
with the command, cat waits to get its input from your keyboard, the standard
input file.  The program will accept input
lines from the keyboard until it sees a line which begins with Ctrl+D, which is
the end of file signal for standard input.<o:p></o:p></div>
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            To redirect
the standard input, you use the<span style="color: #0b5394;"><b> <span style="font-family: Courier New, Courier, monospace;"><</span></b></span> operator. 
For example, if you wanted cat to get its input from output.txt you could
use the command.<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$cat<output.txt</span><o:p></o:p></b></div>
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            The
difference between this command and<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$cat output.txt</span><o:p></o:p></b></div>
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            is a subtle
one.  In filenames provided as options to
a command, you can use filename substitution. 
When redirecting input, you must use the name of an existing file or
device.<o:p></o:p></div>
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<b>(2) Redirection
Output:</b></h3>
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            Now, let’s
say that you want to send the output of cat to a file, to save our shopping
list on disk.  The shell lets you redirect
standard output to a file name, by using the “<span style="color: #0b5394; font-family: Courier New, Courier, monospace;"><b>></b></span>” symbol.  Here’s how it works:<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$cat
amg.txt>output.txt<o:p></o:p></span></b></div>
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            The<b> </b><b style="color: #0b5394; font-family: 'Courier New', Courier, monospace;">></b> operator causes a new file to be created. 
If you had already created a file named output.txt, it would be deleted
and replaced with the new data.  If you
wanted to add the new data to the old output, you could use the <span style="color: #0b5394; font-family: Courier New, Courier, monospace;"><b>>></b></span>
operator.  For example.</div>
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<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$cat
amg.txt>>output.txt<o:p></o:p></span></b></div>
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            This will
add content of amg.txt to output.txt file both file must be exists at specified
path.<o:p></o:p></div>
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<b><span style="color: #0b5394;">Concept of Piping</span></b></h2>
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Suppose that you wanted a directory listing that was sorted
by the mode file type plus permissions. 
To accomplish this, you might redirect the output from ls to a data file
and then sort that data file.  For
example.<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$ls –l>tempfile</span><o:p></o:p></b></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$sort<tempfile</span><o:p></o:p></b></div>
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<span style="color: #7f6000; font-family: Georgia, Times New Roman, serif;">-rw-rw-r-         1          marsha             adept   1024    Jan
20 <st1:time hour="14" minute="14" w:st="on">14:14</st1:time> Lines.dat<o:p></o:p></span></div>
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<span style="color: #7f6000; font-family: Georgia, Times New Roman, serif;">-rw-rw-r-         1          marsha             adept   3072    Jan
20 <st1:time hour="14" minute="14" w:st="on">14:14</st1:time> Lines.idx<o:p></o:p></span></div>
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<span style="color: #7f6000; font-family: Georgia, Times New Roman, serif;">-rw-rw-r-         1          marsha             adept   256      Jan
20 <st1:time hour="14" minute="14" w:st="on">14:14</st1:time> Pages.dat</span><span style="color: #bf9000;"><o:p></o:p></span></div>
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            Although
you get the result that you wanted, there are three drawback to this method:<o:p></o:p></div>
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            You might
end up with a lot of temporary file in your directory.  You would have to go back and remove
them.  The sort program does not begins
its work until the first command is complete. 
This is not too significant with the small amount of data used in this
example, but it can make a considerable difference with larger files.  The final output contains the name of your
temp file, which might not be what you had in mind.<o:p></o:p></div>
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            Fortunately,
there is a better way.<o:p></o:p></div>
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            The pipe
symbol <b><span style="color: #0b5394;">(|)</span></b> causes the standard output of the program on the left side of the
pipe to be passed directly to the standard input of the program on the right
side of the pipe symbol.  Therefore, to
get the same results as before, you can use the pipe symbol.  For example<o:p></o:p></div>
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<b><span style="color: red; font-family: Courier New, Courier, monospace;">$ls –l|sort</span><o:p></o:p></b></div>
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<span style="color: #7f6000; font-family: Georgia, Times New Roman, serif;">-rw-rw-r-         1          marsha             adept   1024    Jan
20 <st1:time hour="14" minute="14" w:st="on">14:14</st1:time> Lines.dat<o:p></o:p></span></div>
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<span style="color: #7f6000; font-family: Georgia, Times New Roman, serif;">-rw-rw-r-         1          marsha             adept   3072    Jan
20 <st1:time hour="14" minute="14" w:st="on">14:14</st1:time> Lines.idx<o:p></o:p></span></div>
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<span style="color: #7f6000; font-family: Georgia, Times New Roman, serif;">-rw-rw-r-         1          marsha             adept   256      Jan
20 <st1:time hour="14" minute="14" w:st="on">14:14</st1:time> Pages.dat</span><o:p></o:p></div>
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            To connect
two or more operation within the same stream at that time pipe sign is used to
perform such a operation in unix.<o:p></o:p></div>
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Concept of Redirection and Piping in Unix

Most UNIX commands expect input to come from a file(s), and produce output to anther file(s). “One of these files is the standard input,...