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Capture `stderr` of a command *inside* a process substitution (`diff <(cmd 2>&1) `)

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Understanding Bash Process Substitution and Stderr Handling

Q: What is process substitution in Bash?

A: Process substitution is a feature of the Bash shell that allows a process's input or output to be redirected to a file-like construct, typically formatted as <(command)> or >(command). It lets you treat the output of a process as if it were a filename. This can be extremely useful in cases where a command expects a file as an argument rather than standard input or output.

Q: How does capturing stderr work typically in Bash scripting?

A: In Bash scripting, standard output (stdout) and standard error (stderr) are two separate streams of data. By default, they both print to the terminal, but they can be redirected separately. You capture stderr by redirecting it, which is commonly done by specifying 2> followed by a filename or another destination. To merge stderr with stdout, you use 2>&1, redirecting stderr to wherever stdout is currently going.

Q: Can you explain the specific challenge with stderr in the context of process substitution?

A: The challenge arises because process substitution outputs a filename that points to a file descriptor. This is fine for stdout but becomes tricky for stderr because the error messages are not automatically included in the output of the substituted process but are directed to the terminal by default. Capturing stderr inside process substitution like diff <(cmd 2>&1) ... can be used to compare the combined stdout and stderr of a command with something else, affecting how errors are handled or logged.

Background and Examples

Simple Example of Process Substitution: Consider you want to compare the output of two commands directly:

diff <(ls) <(ls ~/Documents)

This command will show the differences between the directory listing of the current directory and the Documents directory.

Capturing stderr inside process substitution: Let's take a scenario where you wish to capture both stdout and stderr of a command while comparing its output with another command:

diff <(some_command 2>&1) <(other_command)

Here, some_command 2>&1 redirects stderr to stdout, and its entire output is provided to diff as if it were a file.

Executable Script: Capturing Standard Error with Process Substitution

#!/bin/bash

# Command that generates both stdout and stderr
generate_output() {
  echo "Normal Output"
  echo "Error Output" >&2
}

# Command that generates only stdout
generate_normal_output() {
  echo "Only Normal Output"
}

# Use process substitution to compare their outputs, including stderr
diff <(generate_output 2>&1) <(generate_normal_output)

Run this script and observe how the output from stderr is captured and included in the comparison.

Conclusion

Capturing stderr inside process substitution represents a valuable technique in Bash scripting for managing and controlling both the standard output and error streams of commands seamlessly. It allows for sophisticated manipulations and comparisons, making it a powerful tool for scripting and debugging. This approach can streamline workflows and enhance the automation of tasks, especially in complex scripts where output validity and error detection are crucial.

Further Reading

For further reading on Bash process substitution and handling of stderr, consider exploring these resources:

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