1. Introduction

A named pipe is a special type of file that serves as a communication channel between two processes. Unlike regular pipes, named pipes exist persistently in the filesystem, making them suitable for long-term communication.

In certain scenarios, we may need to merge or concatenate the data flowing through multiple named pipes into a single input stream. This could be useful when we want to consolidate information from different sources or when we need to feed a single stream of data into a subsequent process.

In this tutorial, we’ll explore various methods for merging data from multiple named pipes into a single input stream in Linux.

Notably, the sequence of data in the merged output can vary due to the timing of writes to named pipes. Hence, asynchronous processes may impact the order of content in the consolidated output.

2. Preparing Named Pipes

Let’s consider a scenario where we have two processes, each writing data to a named pipe. We want to merge the data from both pipes into a single stream that another application can process.

2.1. Create Named Pipes

First, we create the named pipes via mkfifo:

$ mkfifo pipe1
$ mkfifo pipe2

In this instance, we create two named pipes, pipe1 and pipe2, to serve as our communication channels. Now, we have the two pipes available for use.

2.2. Writing to Named Pipes

Then, we start two processes that write data to pipe1 and pipe2 via the echo command:

$ echo "This is Process 1" > pipe1 &
$ echo "This is Process 2" > pipe2 &

Here, the & at the end of each command runs the processes in the background.

Further, both writes are pending reads.

3. Using cat

In Linux, the cat command is commonly used to concatenate and display the content of files. When combined with shell redirection >, it becomes a powerful tool for merging the content of multiple named pipes into a single input stream.

3.1. Join Named Pipes

Indeed, we can concatenate the content of pipe1 and pipe2:

$ cat pipe1 pipe2 > merged_pipe &

In this case, we use the cat command and shell redirection > to concatenate the content of both pipes into a single input stream merged_pipe. Of course, merged_pipe can be a regular file. However, unless pipe1 and pipe2 only experience one write, we usually employ another named pipe for merged_pipe.

3.2. Read From the Merged Pipe

Finally, we can read from the merged pipe using any command or application that expects input from a stream:

$ cat merged_pipe
This is Process 1
This is Process 2

The output shows the successful merging of data from two processes, each writing to its own named pipe.

4. Using tee

The tee command in Linux is a utility to read from standard input and write to standard output and files simultaneously.

In the context of joining two named pipes into a single input stream, the tee command can be employed to read from multiple pipes and write to a single pipe.

Let’s utilize the tee command to read from pipes:

$ tee merged_pipe < pipe1 < pipe2 &

In this instance, the tee command duplicates its input, directing it to the specified file merged_pipe. This way, we have more control over the redirection.

Then, we read from the merged pipe:

$ cat merged_pipe
This is Process 1
This is Process 2

As we can see, the tee command duplicates the input stream from multiple sources and redirects it into a single-named pipe.

5. Using awk

The awk command in Linux is a powerful text-processing tool that can be utilized to process and manipulate data from multiple sources. Hence, we can employ awk to join two named pipes into a single input stream.

First, we create a pipe:

$ mkfifo merged_pipe

Here, we use mkfifo to create a named pipe merged_pipe.

Then, we print the content of each line into the named pipe:

$ awk '{print > "merged_pipe"}' pipe1 pipe2 &

In this case, we use awk to print the content of each line from pipe1 and pipe2 into the merged pipe. Moreover, the > operator redirects the output to the named pipe.

Finally, we read from the merged_pipe:

$ cat merged_pipe
This is Process 1
This is Process 2

As a result, awk processed and merged data from multiple sources into a single input stream.

6. Using paste

paste is primarily used to merge lines from multiple files. However, we can also utilize paste to display the lines from each input stream.

Notably, this method is suitable for displaying the lines from two named pipes side by side.

First, let’s use paste to merge the content of pipes:

$ paste <(cat pipe1) <(cat pipe2) > merged_output

The <() is a process substitution that enables paste to treat the output of cat pipe1 and cat pipe2 as input files.

Again, we can print the content of the merged output to the terminal via cat.

7. Using tail

Using tail for joining two named pipes into a single input stream in Linux involves monitoring the content of each pipe and redirecting it to a common destination.

First, let’s use tail to begin tracking the data from each named pipe:

$ tail -f pipe1 pipe2 > merged_pipe &

In this case, we monitor the content of pipe1 and pipe2 and redirect it to the destination merged_pipe. Moreover, the -f option enables continuous monitoring, ensuring that tail stays active and updates in real time as new data is written to the pipes.

Thus, we can use cat to read from merged_pipe and observe the expected combined output from multiple processes writing to named pipes. This way, we can view the contents as they get updated, thereby unblocking and keeping track of all writes.

8. Conclusion

In this article, we explored various methods for merging or concatenating data from multiple named pipes into a single input stream. Named pipes, serving as persistent communication channels between processes, offer flexibility in handling long-term communication scenarios.

We employed different commands such as cat, tee, awk, paste, and tail to demonstrate the merging process. Each method has its strengths and use cases, providing versatility in handling data consolidation from different sources.

In summary, the choice of merging method depends on the specific requirements of the application and the characteristics of the data being processed.

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