Share a file descriptor between parent and child processes using `coproc`

How to Share a File Descriptor between Parent and Child Processes Using `coproc` in Bash

Q1: What is coproc in the context of Bash scripting?

A1: The coproc keyword in Bash introduces a coprocess, which is a shell command preceded by the coproc keyword. A coprocess is essentially another Bash process that runs concurrently with the original (parent) Bash script. It allows you to execute a command or script in the background while continuing the execution of the main script. This is ideal for situations where you need two scripts to communicate or share data asynchronously.

Q2: How does coproc help in sharing file descriptors?

A2: When you create a coprocess in Bash, it automatically sets up a two-way communication path between the parent process and the coprocess. This is implemented using a pair of file descriptors: one for reading from the coprocess and another for writing to it. By default, these file descriptors are available in the parent process, allowing data to be sent to and read from the coprocess.

Q3: Can you illustrate a simple use case of coproc?

A3: Sure, imagine you want the parent script to send some data to a child process, which will then process the data and send back results. For instance, you might have a coprocess that reads strings from its standard input, converts them to uppercase, and sends them back. Using coproc, you can perform this operation asynchronously.

Background and Further Explanation

To expand on the concept of coprocesses in Bash, consider a situation where you need continuous interaction between scripts. For instance, you might have a monitoring script as the parent, and a logging script as a coprocess. The coproc mechanism is beneficial here because it keeps the two scripts engaged in a persistent, ongoing communication channel.

Here's a very simple example to demonstrate how coproc works:

# Define a coprocess that reads a string from its input, converts it to uppercase, and outputs it
coproc UPPER { tr '[:lower:]' '[:upper:]'; }
# Write to the coprocess
echo "hello world" >&${UPPER[1]}
# Read the result from the coprocess
read -u ${UPPER[0]} result
# Close the file descriptors
exec {UPPER[0]}<&-
exec {UPPER[1]}>&-
echo "Result: $result"

Executable Script Example

Let's create a script that demonstrates sharing a more meaningful task between a parent and a child process using coproc.

#!/bin/bash

echo "Starting the coprocess to perform addition of two numbers..."

# Define a coprocess that reads two numbers, adds them, and sends the result back
coproc ADDER {
    while read num1 num2; do
        echo "$((num1 + num2))"
    done
}

# Function to send numbers to the coprocess and receive results
add_numbers() {
    local num1=$1
    local num2=$2
    echo "$num1 $num2" >&${ADDER[1]}
    read -u ${ADDER[0]} summation
    echo "Sum of $num1 and $num2 is $summation"
}

# Using the function to perform calculations
add_numbers 5 20
add_numbers 10 15

# Cleanup: close coprocess file descriptors
exec {ADDER[0]}<&-
exec {ADDER[1]}>&-

echo "Completed calculations and closed the coprocess."

This script defines a coprocess that can continuously receive pairs of numbers, add them, and return results until the file descriptors are closed.

Conclusion

Using coproc in Bash scripts can dramatically empower scripts by enabling asynchronous processing and facilitating inter-process communication. It is especially useful in scenarios requiring ongoing data exchange between scripts, like data processing pipelines, monitoring tasks, or where the performance could be enhanced by concurrent execution. Unlock the full potential of your Bash scripts with this powerful yet underutilized feature.

How to Share a File Descriptor between Parent and Child Processes Using coproc in Bash

Background and Further Explanation

Executable Script Example

Conclusion

Further Reading

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How to Share a File Descriptor between Parent and Child Processes Using `coproc` in Bash