Filters – commands which use both standard input and standard output (like bc)

Explanation: We can redirect bc’s standard input to come from file001 and save the output in another file named result.txt. bc obtained the expressions from the redirected standard input, processed them and sent out the results to a redirected output stream. Here bc command is used as a filter.

Explanation: There are two special files named /dev/null and /dev/tty which is used in UNIX for special purposes. For example, if we want to check whether the program runs successfully without seeing its output on the screen or may not want to save output in a file either. For this purpose, a special file /dev/null is used. It accepts any stream without growing in size. The second file /dev/tty is the one used for indicating one’s terminal.

Explanation: UNIX has a special file that simply accepts any stream without growing in size. This file is /dev/null. This file is used for redirecting error messages away from the terminal so that they don’t appear on the screen. This file is pseudo-device because like other device files, it’s not associated with any physical device.

/dev/tty can be accessed by multiple users without conflict.

Explanation: We can make the command ignorant of its source using the pipeline. Above command is also an example of the same. It will simply display the total size of those files having a .c extension in the current directory.

Explanation: There is a restriction which is applied while using a pipeline. In a pipeline, the command on the left of | must use standard output (stream) and one on the right must use standard input (stream). Otherwise, an error will be generated or the output will be erroneous.

Above command will the save the output of who command in a file named user.txt as well as it will also display the output of who command on the terminal.

Explanation: tee is an external command and not a feature of the shell. tee command handles a character stream by duplicating its input. It saves one copy in a file and writes the other to standard output.

Explanation: tee command is not a feature of the shell. tee command handles a character stream by duplicating its input. It saves one copy in a file and writes the other to standard output. It is an external command.

In the above command, tee saved the output of who in /dev/tty, thus effectively showing it on the screen and wc -l counted the number of lines from another stream.