How to use the command "coproc" (with examples)

The “coproc” command is a built-in command in Bash that allows you to create interactive asynchronous subshells. It is useful for running commands simultaneously or interacting with them through input and output streams. This article will provide examples of various use cases of the “coproc” command to illustrate its functionality.

Use case 1: Run a subshell asynchronously

Code:

coproc { command1; command2; ...; }

Motivation: Running a subshell asynchronously allows you to execute multiple commands simultaneously, which can improve performance and efficiency. This can be particularly useful when executing time-consuming or intensive tasks.

Explanation: The “coproc” command creates a new background process or subshell and executes the specified commands within it. The commands to be executed are enclosed within curly braces. The coprocess runs asynchronously, meaning that it allows other commands to continue executing in the main shell without waiting for it to finish.

Example output:

• command1 output
• command2 output

Use case 2: Create a coprocess with a specific name

Code:

coproc name { command1; command2; ...; }

Motivation: Assigning a name to a coprocess allows you to easily refer to it later when interacting with its input and output streams. It provides a more structured way to manage and control the coprocess.

Explanation: In this example, “name” is the name you want to assign to the coprocess. The coprocess is created using the “coproc” command followed by the desired name, and then the commands to be executed within the coprocess are enclosed within curly braces.

Example output:

• command1 output
• command2 output

Use case 3: Write to a specific coprocess stdin

Code:

echo "input" >&"${{{name}}[1]}"

Motivation: Writing to the stdin of a specific coprocess allows you to send input to it and control its behavior based on the input provided. This is useful when you want to interact with the coprocess dynamically.

Explanation: In this example, “name” refers to the name of the coprocess created earlier. The command “>&"${{name}}[1]” redirects the output of the “echo” command to the stdin of the coprocess. The [1] indicates the stdin of the coprocess, which allows you to pass input to it.

Example output: No output is typically produced when writing to a coprocess stdin. The output depends on the specific coprocess and the commands it executes using the input provided.

Use case 4: Read from a specific coprocess stdout

Code:

read {{variable}} <&"${{{name}}[0]}"

Motivation: Reading from the stdout of a specific coprocess allows you to capture its output and use it within your script. This can be helpful when you need to process the output or make decisions based on the information received.

Explanation: In this example, “name” refers to the name of the coprocess created earlier. The command “read {{variable}} <&"${{{name}}[0]}” reads the input from the stdout of the coprocess and assigns it to the variable specified. The [0] indicates the stdout of the coprocess, which allows you to capture its output.

Example output: The output of the coprocess is stored in the variable specified and can be used further in your script.

Use case 5: Create a coprocess which repeatedly reads stdin and runs commands

Code:

coproc {{name}} { while read line; do {{command1; command2; ...}}; done }

Motivation: Creating a coprocess that continuously reads from stdin and performs specific commands allows for dynamic and interactive processing of input. This can be useful when you want to process input on-the-fly or perform certain actions based on the received input.

Explanation: In this example, “name” refers to the name you want to assign to the coprocess. The coprocess is created using the “coproc” command, followed by the name, and then a while loop that repeatedly reads from the stdin. Within the loop, specific commands can be executed based on the received input.

Example output: No output is typically produced in this use case, as the output depends on the commands executed within the coprocess based on the input received.

Use case 6: Create and use a coprocess running “bc”

Code:

coproc BC { bc --mathlib; }; echo "1/3" >&"${BC[1]}"; read output <&"${BC[0]}"; echo "$output"

Motivation: Running a coprocess with “bc” (a command-line calculator) allows you to perform mathematical calculations in your script. It can be useful when you need to automate complex calculations or perform computations within your script.

Explanation: In this example, “BC” is the name of the coprocess. The coprocess is created using the “coproc” command, followed by the name and the command “bc –mathlib”, which runs the “bc” calculator with the extended mathematical library. The output “1/3” is then sent to the stdin of the coprocess, and the resulting output is read from the stdout and stored in the “output” variable. Finally, the value of “output” is echoed.

Example output: 0.33333333333333333333

Conclusion:

The “coproc” command in bash provides powerful functionality for managing interactive asynchronous subshells. Understanding its various use cases allows you to effectively utilize coprocesses and enhance the capabilities of your Bash scripts.