The ‘coproc’ command is a builtin command in Bash that allows users to create interactive asynchronous subshells. It is useful in situations where you want to run a command or a set of commands independently and simultaneously with the main shell. This article will illustrate various use cases of the ‘coproc’ command and provide examples for each.

Use case 1: Run a subshell asynchronously

Code:

coproc { command1; command2; ...; }

Motivation:

Running a subshell asynchronously can be useful when you want to free up the main shell for other tasks while the subshell continues to execute commands without blocking.

Explanation:

The ‘coproc’ command is used to create a subshell that will execute the given commands asynchronously. The commands are enclosed in curly braces {} and separated by semicolons (;).

Example:

coproc { echo "Hello"; sleep 5; echo "World"; }

Output:

The output will be displayed after a delay of 5 seconds, during which the main shell is free to execute other commands.

Use case 2: Create a coprocess with a specific name

Code:

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

Motivation:

Naming the coprocess allows you to easily reference it later, making it convenient for reading from and writing to the coprocess.

Explanation:

The ’name’ is the user-defined name for the coprocess. It can be any valid identifier. The commands to be executed by the coprocess are enclosed in curly braces {} and separated by semicolons (;).

Example:

coproc mycoproc { echo "Hello"; sleep 5; echo "World"; }

Output:

This will create a coprocess named ‘mycoproc’ that will execute the commands asynchronously similar to the previous use case.

Use case 3: Write to a specific coprocess stdin

Code:

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

Motivation:

Writing to a coprocess’s stdin allows you to provide input to the coprocess for processing.

Explanation:

The ’name’ enclosed in curly braces {} is the name of the coprocess whose stdin you want to write to. The >& operator is used to redirect the output of the echo command to the file descriptor associated with the coprocess stdin.

Example:

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

Output:

This will write the “input” string to the stdin of the coprocess named ‘mycoproc’.

Use case 4: Read from a specific coprocess stdout

Code:

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

Motivation:

Reading from a coprocess’s stdout allows you to retrieve the output generated by the coprocess for further processing.

Explanation:

The ’name’ enclosed in curly braces {} is the name of the coprocess whose stdout you want to read from. The <& operator is used to redirect the input of the read command to the file descriptor associated with the coprocess stdout.

Example:

read output <&"${mycoproc[0]}"
echo "$output"

Output:

This will read the output from the stdout of the coprocess named ‘mycoproc’ and store it in the ‘output’ variable. The value of ‘output’ is then printed.

Use case 5: Create a coprocess which repeatedly reads stdin and runs some commands on the input

Code:

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

Motivation:

This use case is useful when you want to create a coprocess that continuously processes input from stdin and runs specific commands on each input.

Explanation:

The ’name’ is the user-defined name for the coprocess. The while loop continuously reads input from stdin using the read command and executes a set of commands for each input line.

Example:

coproc mycoproc { while read line; do echo "$line" | grep "foo"; done }

Output:

This will create a coprocess named ‘mycoproc’ that reads input from stdin and checks if each input line contains the string “foo”. It then prints the lines containing “foo” to the stdout.

Use case 6: Create a coprocess which repeatedly reads stdin, runs a pipeline on the input, and writes the output to stdout

Code:

coproc name { while read line; do echo "$line" | command1 | command2 | ... | cat /dev/fd/0; done }

Motivation:

This use case is useful when you want to create a coprocess that processes input from stdin using a pipeline of commands and writes the output to stdout.

Explanation:

The ’name’ is the user-defined name for the coprocess. The while loop continuously reads input from stdin using the read command, processes the input with a pipeline of commands, and writes the output to stdout using the cat command.

Example:

coproc mycoproc { while read line; do echo "$line" | grep "foo" | sed 's/foo/bar/'; done }

Output:

This will create a coprocess named ‘mycoproc’ that reads input from stdin, searches for lines containing the string “foo”, and replaces “foo” with “bar” in each line. It then prints the modified lines to the stdout.

Use case 7: Create and use a coprocess running bc

Code:

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

Motivation:

This use case demonstrates how to create and interact with a coprocess running the ‘bc’ command, which is a command-line calculator.

Explanation:

The ‘BC’ is the name of the coprocess. The coprocess runs the ‘bc’ command with the ‘–mathlib’ option. The echo command writes “1/3” to the stdin of the coprocess, and the read command reads the output from the stdout of the coprocess, storing it in the ‘output’ variable.

Example:

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

Output:

This will create a coprocess named ‘BC’ running the ‘bc’ command. It writes “1/3” to the stdin of the coprocess, which performs the calculation and outputs the result “0.33333333333333333333”. The ‘output’ variable is then printed, displaying the result from the coprocess.

Conclusion:

The ‘coproc’ command in Bash allows users to create interactive asynchronous subshells. These use cases demonstrated various ways to utilize the ‘coproc’ command, from running subshells asynchronously to creating coprocesses that read from and write to stdin and stdout. By understanding how to use the ‘coproc’ command, you can enhance the efficiency and flexibility of your Bash scripts and workflows.