Difference between revisions of "BOSS-Linux/C2/Redirection-Pipes/English"

When we have to execute a command, we generally type through the keyboard.

"date"

"cat aaa"

Now let us see how redirection is done in the 3 streams.

we redirect stdin from a file, using the < (left angled bracket) operator.

Let us see how.

"wc"

Type wc on the terminal window. Now press enter. What happens?? we have a blinking cursor. It means that we need to enter through the keyboard.

“This tutorial is very important"

press Enter </

Ctrl+d </

Now press enter. Now press Ctrl and d keys together.

Type at the command prompt

"wc < test1.txt"</

"wc space 'left-angled bracket" space test1 dot txt"

what happens is that wc will tell us the number of lines,words and charecters in the file test1.txt

"wc test1.txt"

"wc space test1 dot txt", we see the same result. So what is the difference?

Now let us see how to redirect standard output and standard error. There are two ways to redirect output or error to a file:

n> [narration- n 'single right-angled bracket']</

redirects output from file descriptor n to a file. You must have write authority to the file.

also redirects output from file descriptor n to a file. Again, you must have write authority to the file.

From the last example we have seen that the result of the wc command on a file or stdin is displayed at the terminal window.

"wc test1.txt"

wc_results.txt"

"cat wc_results.txt"

"wc space test1 dot txt 'right-angled bracket' wc_results dot txt" . Press enter.

Now to see whether this has actually happened we can display the contents of wc_results.txt by the cat command. Yes it has.

"wc test2.txt > wc_results.txt"

"cat wc_results.txt"

Now we again execute the command with test2 file. We type

"wc space test2 dot txt 'right-angled bracket' wc_results dot txt"

So,the contents of the file wc_results would be overwritten.

Let's see this .

"cat wc_results.txt"

the new contents will not overwrite the already present contents of the file wc_results.txt, it would be appended.

Let's see this too.

"wc aaa"

"wc space aaa"

the shell will give the error “No such file or directory”.

"wc space aaa space 2 'right-anged bracket' errorlog.txt"

now the error will not show on the terminal, rather it will be written in the file errorlog.txt

"cat space errorlog dot txt"

"cat space bbb space 2 'right-angled bracket' errorlog.txt".

the previous error would be overwritten and the new error will show.

simple we would run the command

"wc space aaa space 2 'right-angled bracket' twice errorlog.txt"

command1 | command2 option1 | command3 option1 -option2 | command4

"ls -l"

"ls space minus l" will list all files and directories of the present directory.

"ls space minus l 'right-angled bracket' files.txt"

This we can do using the command "wc space minus l files dot txt"

First we need an intermediate file , here files.txt . If the first command produces a lot of data,it may unneccesarily eat away the disk memory. Also if we want to chain several commands, this method is slow.

"ls space minus l 'vertical bar' wc space minus l"

and we can get the same result with much more ease.

One common use of pipes is for reading multipage displays.

Type "cd space /usr/bin". So, we are now in bin directory.

"ls -l | more"

Press "q" to come out of it.

More information on the same is available at the following link http://spoken-tutorial.org/NMEICT-Intro