Unix - UNIX Pipes vs FIFOs: Design Differences and Practical Use Cases

UNIX provides several mechanisms for allowing processes to exchange data. Two important mechanisms are pipes and FIFOs (First-In, First-Out). Both allow one process to send data that another process can receive, but they differ in how they are created, accessed, and used. Understanding these differences is important when designing UNIX applications that need communication between processes.

1. What Is a UNIX Pipe?

A pipe is a communication channel that allows data to flow from one process to another. A pipe is generally created using the pipe() system call and provides two file descriptors: one for reading and one for writing.

Conceptually, a pipe can be represented as:

Process A
   |
   | writes data
   v
+---------+
|  Pipe   |
+---------+
   |
   | reads data
   v
Process B

The pipe stores data temporarily in a kernel-managed buffer. When one process writes data into the pipe, another process can read that data from the other end.

A typical example is the UNIX command:

ls | grep ".txt"

Here, ls produces output and writes it to the pipe. The grep command reads that output from the pipe and filters the results.

2. What Is a FIFO?

A FIFO, also called a named pipe, is another form of interprocess communication. Unlike an ordinary pipe, a FIFO has a name in the filesystem.

A FIFO can be created using the mkfifo command:

mkfifo mypipe

After creation, processes can open the FIFO using its pathname.

For example, one terminal can execute:

echo "Hello UNIX" > mypipe

Another terminal can execute:

cat < mypipe

The second process receives the data written by the first process.

The important point is that the two processes do not necessarily need to be related through a common parent process.

3. Major Difference Between Pipes and FIFOs

The most important difference is how the communication channel is established.

An ordinary pipe normally exists only as a kernel object associated with the processes that created or inherited its file descriptors. It is commonly used between related processes, such as a parent and child.

A FIFO has a filesystem pathname. Because of this pathname, unrelated processes can locate and open the same communication channel.

For example:

Ordinary Pipe

Parent Process
      |
   pipe()
      |
   +------+
   | Pipe |
   +------+
    /    \
Child A  Child B

With a FIFO:

Process A
    |
    | open()
    v
+-----------+
| /tmp/data |
|   FIFO    |
+-----------+
    ^
    | open()
    |
Process B

The FIFO therefore provides a convenient mechanism for communication between independently started processes.

4. Communication Direction

A traditional UNIX pipe is commonly used for one-way communication. One end is used for writing and the other end for reading.

For example:

Writer
  |
  v
Pipe
  |
  v
Reader

If two processes need bidirectional communication, a common approach is to create two pipes:

Process A
   |       ^
   |       |
 Pipe 1  Pipe 2
   |       |
   v       |
Process B

FIFOs can similarly be used for communication between processes, although two FIFOs may be needed when a clean two-way communication arrangement is required.

For more sophisticated bidirectional communication, mechanisms such as UNIX domain sockets may be more appropriate.

5. Pipes and Process Relationships

Ordinary pipes are particularly useful when processes have a relationship.

For example, a parent process can create a pipe and then call fork():

Parent
  |
pipe()
  |
fork()
 /   \
Parent Child

The child inherits the parent's file descriptors. This makes it possible for the parent and child to communicate through the pipe.

A common UNIX programming pattern is:

Parent Process
      |
      | creates pipe
      |
    fork()
     /  \
    /    \
Parent   Child
   |       |
 write    read

This model is frequently used when implementing command pipelines or custom process-management programs.

6. FIFOs and Unrelated Processes

FIFOs are particularly useful when the communicating processes are not related.

For example, imagine two independent applications:

Application A
      |
      | writes
      v
/tmp/events
   FIFO
      ^
      | reads
      |
Application B

Application A and Application B do not have to share a parent process. They only need access permissions to the FIFO.

This makes FIFOs useful for simple communication between independent programs.

7. Data Handling in Pipes and FIFOs

Both pipes and FIFOs provide byte-stream communication. They do not inherently understand application-level structures such as objects, records, or messages.

Suppose a program writes:

Hello UNIX

The receiving process reads a sequence of bytes.

Therefore, applications that exchange structured data need to define their own communication format.

For example, an application might establish a format such as:

ID:1001
NAME:John
STATUS:ACTIVE

The receiving program can then parse the incoming data according to that format.

8. Blocking Behavior

Both pipes and FIFOs can exhibit blocking behavior.

For example, if a process attempts to read from a pipe when there is currently no data available, the read operation can wait until data becomes available, depending on how the file descriptor is configured.

Similarly, opening a FIFO can block in certain circumstances. For example, opening a FIFO for writing can wait until another process opens the FIFO for reading.

This behavior is important because an application can otherwise appear to stop responding when it is actually waiting for another process.

Non-blocking mode can be used when an application needs greater control over this behavior.

9. Practical Uses of Pipes

Pipes are heavily used in UNIX command-line environments.

A classic example is:

ps aux | grep ssh

The first command generates process information, while the second command processes that output.

Another example is:

cat access.log | grep ERROR | wc -l

Here, several commands are connected through pipes.

The general pattern is:

Command A | Command B | Command C

Each command performs a specific operation, while the pipe transfers the output of one command to the input of the next.

This approach is one of the fundamental ideas behind UNIX's philosophy of combining small tools to perform complex tasks.

10. Practical Uses of FIFOs

FIFOs are useful when independent programs need a simple communication channel.

For example, a logging application could create a FIFO:

mkfifo /tmp/logpipe

One program could write log information:

echo "Application started" > /tmp/logpipe

Another program could read it:

cat /tmp/logpipe

FIFOs can also be useful in simple monitoring systems, producer-consumer applications, test environments, and communication between shell scripts and background processes.

11. Security and Permissions

Because FIFOs have filesystem names, normal filesystem permissions are important.

For example:

ls -l /tmp/mypipe

can show the FIFO's ownership and permissions.

Administrators and developers should ensure that unauthorized users cannot write sensitive information into a FIFO or read information that should remain private.

The location of the FIFO also matters. A FIFO placed in a publicly writable directory may introduce security concerns if permissions and ownership are not properly controlled.

12. Pipes vs FIFOs

Feature Pipe FIFO
Also known as Anonymous pipe Named pipe
Filesystem pathname Normally no Yes
Common creation method pipe() mkfifo()
Communication Byte stream Byte stream
Related processes Commonly used Not required
Unrelated processes Less convenient Well suited
Shell pipelines Frequently used Less common
Persistent filesystem name No Yes
Access controlled through pathname No Yes
Typical purpose Parent-child and command pipelines Independent-process communication

13. When Should You Use a Pipe?

A pipe is generally appropriate when:

  • A parent and child process need simple communication.

  • Commands need to be connected together.

  • Data needs to flow from one process to another.

  • The communication relationship can be established when the processes are created.

  • A simple byte stream is sufficient.

For example:

command1 | command2

is a natural use of a pipe.

14. When Should You Use a FIFO?

A FIFO is generally appropriate when:

  • Independent processes need to communicate.

  • The processes do not have a parent-child relationship.

  • A persistent pathname is convenient.

  • A simple byte stream is sufficient.

  • Communication needs to be established through filesystem access.

For example, two separately started applications can communicate through:

/tmp/application_fifo

without needing to create the relationship through fork().

15. Limitations

Pipes and FIFOs are intentionally simple communication mechanisms. They do not provide all the features required by complex distributed or multi-process applications.

Some limitations include limited buffering, blocking behavior, byte-stream semantics, and the need for applications to establish their own data formats.

When an application requires more advanced capabilities, UNIX programmers may consider alternatives such as message queues, shared memory, semaphores, or UNIX domain sockets, depending on the requirements.

Conclusion

Pipes and FIFOs are fundamental UNIX mechanisms for interprocess communication. A pipe provides a simple communication channel that is especially useful for related processes and command pipelines, while a FIFO provides a named communication channel that can be accessed by independent processes through a filesystem pathname. Although both mechanisms transfer data as byte streams, their different creation and access models make them suitable for different situations. Understanding these distinctions helps developers choose an appropriate IPC mechanism and design reliable UNIX applications.