The pipe denoted by the operator | connects the standard output of a process in the left to the standard input in the right process by inter process communication mechanism. In other words, the | (pipe) connects commands by providing the output of a command as the input to another command.

Consider the following example:

$ cat /proc/cpuinfo | less

Here, the cat command, instead of displaying the content of the /proc/cpuinfo file on stdout, passes its output as an input to the less command. The less command takes the input from cat and displays on the stdout per page.

Another example using pipe is as follows:

$ ps -aux | wc -l    # Showing number of currently running processes in system
254

Pipeline is a sequence of programs/commands separated by the operator ' | ' where the output of execution of each command is given as an input to the next command. Each command in a pipeline is executed in a new subshell. The syntax will be as follows:

command1 | command2 | command3 …

Examples showing pipeline are as follows:

$ ls /usr/lib64/*.so | grep libc | wc -l
13

Here, we are first getting a list of files from the /usr/lib64 directory that has the .so extension. The output obtained is passed as an input to the next grep command to look for the libc string. The output is further given to the wc command to count the number of lines.