In Linux, there are three standard streams; standard input ( stdin ), standard output ( stdout ), and standard error ( stderr ). Each stream is represented by a file descriptor. The standard input is represented by a 0 , the standard output by a 1 , and the standard error by a 2.

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Stdout(Standard Output)

Now suppose I have a directory named "project1" having a.txt , b.txt, c.txt and "project2" having d.txt , e.txt , f.txt , g.txt respectively. Now I want to store the output received by $ ls project1 into an output.txt file via command $ ls > ~/output.txt, will look something like this :

Now if we do $ ls project2 , and store it's output in the same output.txt file with command $ ls > output.txt , it will store the new output. Rather than rewriting even the previous outputs , it will write the new ones. But we want everything to be written i.e the output of project1 as well as of project2, just do $ ls >> ~/output.txt

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Stderr(Standard error)

Since we have stored ls command's output in "output.txt" . But lets do $ lg > output.txt , as we know this is a wrong command , but an empty output.txt would be created.

But what if we want to store the error also. Command for that is $ lg 2> output.txt

What if we want to nullify the error we are receiving upon doing lg rather than ls command , that can be done using $ lg 2> /dev/null command.

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Suppose we do $ ls -la /etc command , we get a lot of information about the "/etc" folder and it messes up our terminal. To deal with this , we have $ less command and press "q" to exit that mode.
"Pipe" : Here the output of the first command becomes the input for the second
ex: $ ls -la /etc | less and then press "q".

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Environment Variables

The environment variables are dynamic values that are stored within a system and used by applications launched in shells or sub-shells. These variables have a name and their respected value. The environment variable customizes the system performance and the behavior of an application.

$ echo $PATH : contains all the paths that a system will search whenever we execute a command.

$ head: command prints first 10 lines of any file.
$ tail : command prints the last 10 lines of any file.
$ sort : arrange the file according to the letters.
$ uniq : display all singly repeated values inside a file.
$ grep : “global regular expression print”, is a command used in searching and matching text files contained in the regular expressions. Furthermore, the command comes pre-installed in every Linux.

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That's all for this one , share it with your friends and like it. In the next one we will go through the "Linux System administration and the Permissions ". Thanks once again everyone.
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