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This article provides a thorough discussion of pipes and double pipes. It will discuss the usage, notation, and effects of pipes in Bash command scripts.

In real life, pipes carry fluids or gases from one place to another. But in Bash, pipes do not carry fluids, but rather pass the output of one command as input to the next.

Pipes in Bash

A pipe is a redirection operator that transfers the output of a command to the input of the command that immediately follows it. |The symbol is used as a pipe in Bash.

We can also say that in Linux/UNIX systems, a pipe connects the standard output of one command to the standard input of another command.

Pipeline is also used to combine two or more commands, where the output of the previous command becomes the input of the next command. This thing eliminates the overhead of file usage.

Usually, when we want to extract some relevant information from a file, we first save the previous output to a file, and then read from the file to convert the file stream into the required output.

However, in a pipeline, the output of one command is directly given as input to another command. The basic notation of commands is shown below.

Comand-1 | command-2 | .... | command-N

Add a pipe between ls and grep

Now we will see an example of piping to connect the output of one command to the input of the next command. In this example, we will select the required file by name from a bunch of files in a given directory.

Just open the terminal and go to the directory where you have placed the files and write ls command on it. The ls -l command will list the details of the selected directory files.

However, our goal is to select the output of ls -l, add a pipe, and search for specific file names so that the output may contain only the desired files.

Following the code, first list the names of all the files in a given directory.

ls -l

There are three files under the current directory; their details are shown in the screenshot. However, we are interested in the details of the bash_double_pipe.md file.

We will use a pipe and grep the file and its name.

The following code will display the details of bash_double_pipe.md.

ls -l | grep "bash_double_pipe.md"

Avoid using pipes for I\O redirection

Pipes do not use standard input or output during redirection. Pipes are implemented in main memory.

The following examples show stream redirection with and without pipes.

I\O redirection without pipes

ls -l > temp

more < temp

The code given above redirects the output of ls -l to a temporary file and then displays the contents of the temporary file on the screen. The following screenshot shows the output of running the above code.

Now, we will explore IO redirection of standard input or output without using pipes. The above example redirects the output of the first command to the input of the second command.

The following code provides the same IO redirection operation, but uses pipes.

ls -l | more

The above code will perform the same redirection operation without disk IO. The following screenshot shows the result of running the above command.

The screenshot above shows that we can use pipes for redirection without disk IO.

Using pipes with cat, grep, tee, and wc commands

Pipes are also very useful in the cat, grep, , teeand wccommands. Suppose we want to see how many times a record appears in a file.

Then, instead of creating many variables, we need to use pipes between these commands and get the desired output.

The following code shows the use of cat, grep, tee, and wc commands and pipes.

cat abc.txt | grep "temp" | tee file2.txt | wc -l

catThe command passes all the contents of abc.txt to the grep command. The grep command selects only the lines that have a string that matches temp.

The line containing the temporary string is then passed to the tee command, which writes the output to file2.txt.

This output is then passed to the wc command, which counts the number of records in it. file2.txt has three lines; the output of the above command is 3.

To summarize this article, pipes are very useful for transferring the output of one command to the input of the next command.

The difference between single pipe and double pipe

A single pipe |sends the output of one command to the input of the next command, but a double pipe ||is different from a single pipe. The double vertical bar represents the OR operator, which is generally used in conditional statements.

Syntax and short-circuit evaluation of the OR operator

The following code shows the basic notation for a double pipe.

command-1 || command-2

The above code shows ||the basic notation and usage of the operator. Bash supports short-circuit evaluation.

Short-circuit evaluation means that the evaluation of a condition stops and returns a final decision as soon as it becomes unambiguous. For example, a compound condition with multiple expressions combined with the OR operator will stop as soon as one of the expressions evaluates to true.

It does not evaluate other expressions further, since their results no longer affect the final decision.

Therefore, the Bash interpreter will try to execute the first command in the above Bash script. If the first command is executed successfully, command-2 will not be executed.

The second command will be executed only if the first command does not execute successfully.

Let's explore the dual-pipe example.

ls -a || echo "hello world"

In the above script, in command-1, we list the elements of the current directory. In command-2, we print the message hello world.

We have used a double pipe between these two commands. When we run the above command, command-1 successfully lists the elements of the current directory, and therefore, command-2 is ignored.

Now let us consider the scenario where command-1 does not execute successfully.

ls a || echo "hello world"

The above command shows the scenario of executing the second command. In the first command, we did not use a hyphen (-) before the a flag; therefore, the command will not run successfully.

Therefore, the OR operator will execute the echo command, printing the hello world message.

The following screenshot shows the output of the above two commands.

The idea above can be extended to chain any number of commands. For example, the following script will ||chain four commands using the operator, and the evaluation will start from left to right.

Once any command is successfully executed, all remaining commands on the right will be skipped.

Command-1 || Command-2 || Command-3 || command-4

Summarize

This article discusses single and double pipe symbols and their functions. The single pipe symbol represents a Linux pipe, which puts the output of one command into the input of another command.

On the other hand, the double vertical bar symbol is the logical OR operator which facilitates logical operations.