Piping in Unix or Linux

A pipe is a form of redirection (transfer of standard output to some other destination) that is used in Linux and other Unix-like operating systems to send the output of one command/program/process to another command/program/process for further processing. The Unix/Linux systems allow the stdout of a command to be connected to the stdin of another command. You can make it do so by using the pipe character ‘|’. 

The pipe is used to combine two or more commands, and in this, the output of one command acts as input to another command, and this command’s output may act as input to the next command, and so on. It can also be visualized as a temporary connection between two or more commands/ programs/ processes. The command line programs that do the further processing are referred to as filters. 

This direct connection between commands/ programs/ processes allows them to operate simultaneously and permits data to be transferred between them continuously rather than having to pass it through temporary text files or through the display screen. 
Pipes are unidirectional i.e., data flows from left to right through the pipeline. 

Syntax:  

command_1 | command_2 | command_3 | .... | command_N 

1. List all files and directories and give them as input to `grep` command using piping in Linux

ls | grep file.txt

In this first we are using `ls` to list all file and directories in the current directory, then passing its output to `grep` command and searching for file name `file.txt`. The output of the ls command is sent to the input of the grep command, and the result is a list of files that match the search term.

2. List all files and directories and give them as input to `more` commands using piping in Linux.

$ ls -l | more 

The more command takes the output of $ ls -l as its input. The net effect of this command is that the output of ls -l is displayed one screen at a time. The pipe acts as a container which takes the output of ls -l and gives it to more as input. This command does not use a disk to connect standard output of ls -l to the standard input of more because pipe is implemented in the main memory. 
In terms of I/O redirection operators, the above command is equivalent to the following command sequence. 
 

$ ls -l -> temp
more -> temp (or more temp)
[contents of temp]
rm temp 

Output of the above two commands is same.