Unix Tutorial

Software

Basic Commands

vi Commands

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File Commands

Different types of file

File Redirection

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Pipelines / Filters

File Redirection

File Redirection is the special commands in Unix that instruct the computer to read from a file, write to a file, or even append information to an existing file.When you issue a Unix command like date, the output of the command goes to what is called Standard output.

Type at the prompt

%date

output will look like

Thu Oct 11 16:03:06 EDT 2001

Each of these acts can be accomplished by placing a file-redirection command in a regular command line:

< redirects input : For example to run prog1 but read data from a file input_file instead of the keyboard, you would type

prog1<input_file

> redirects output : For example to run prog1 and write data to output_file instead of the screen, you would type

prog1>output_file

Above two commands can be combined as follows

prog1 < input_file > output_file

>> redirects output and appends the information to the existing file

To show application of above commands, here are the few examples :

%touch testme
First, use this empty file to learn how to redirect output.Use ls to list the files in your directory, saving them all to the newly created file.

%ls -l testme

-rw-rw-r-- 1 cs1234 0 Nov 15 09:11 testme

%ls -l > testme

%ls -l testme

-rw-rw-r-- 1 cs1234 120 Nov 15 09:12 testme

Here when we redirected the output, nothing was displayed on the screen; there was no visual confirmation that it worked. But it did, as you can see by the increased size of the new file.
Instead of using cat or more to view this file, try using file redirection:

%cat < testme

total 126

drwx------ 2 cs1234 512 Nov 6 14:20 Archives/

drwx------ 2 cs1234 512 Nov 16 21:55 InfoWorld/

likewise here will be all the list of files that are shown in testme.

The results are the same as if you had used ls command, but the output file is saved too.You now can easily print the file or go back to it later to compare the way it looks with the way your files look in the future.
Use the ls command to add some further information at the bottom of the testme file by using >>, the append double-arrow notation:

%ls -FC >> testme

Removing the duplicate lines using function uniq

Create a file that has duplicate lines for example

%tail -1 testme > lastline

%cat lastline lastline lastline lastline > newtest2

%cat newtest2

News/ abc.txt testme

Now you can see what uniq does

%uniq newtest2

News/ abc.txt testme

Obtain a count of the number of occurrences of each line in the file.The -c flag does that job.

%uniq -c newtest2

This shows that this line occurs four times in the file. Lines that are unique have no number preface.