JQ Buffering Issue

Problem: Buffered Output with `jq`

When working with streaming data or commands like tail -f, jq (a tool for processing JSON) can exhibit buffering issues. By default, jq processes and outputs data only after it has been completely received. This behavior can cause delays in seeing results in real-time, which is not ideal when working with continuously updating logs or streams of data.

This issue is demonstrated in the following example:

#!/bin/bash

# Generate output every second as JSON
generate_json_output() {
    for i in {1..10}; do
        echo "{\"line\": $i, \"message\": \"This is a test\"}"
        sleep 1
    done
}

# Non-flushing jq example
non_flushing_jq() {
    generate_json_output | jq '.message' | while read -r line; do
        echo "Processed: $line"
    done
}

# Run the non-flushing jq function
non_flushing_jq

When you run this script, no output will be seen until all 10 lines have been processed. This behavior is undesirable when dealing with live data streams such as logs because it delays the output.

Solution: Using `stdbuf` to Disable Buffering

To fix the buffering issue, we can use the stdbuf command to control the buffering of jq's output. Disabling output buffering ensures that jq processes each line as it is received and outputs it immediately.

You can create a function called jq_unbuffered that wraps jq with stdbuf:

# [JQ Buffering Issue](http://www.glassthought.com/notes/wo4vgllgpeio6qcvrutsxvy)
# 
# https://stackoverflow.com/questions/3465619/how-to-make-output-stdout-stderr-of-any-shell-command-unbuffered
jq_unbuffered() {
    stdbuf -oL jq "$@"
}
export -f jq_unbuffered

Now, we can modify the previous example to use jq_unbuffered:

#!/bin/bash

# Generate output every second as JSON
generate_json_output() {
    for i in {1..10}; do
        echo "{\"line\": $i, \"message\": \"This is a test\"}"
        sleep 1
    done
}

# Unbuffered jq example
unbuffered_jq() {
    generate_json_output | jq_unbuffered '.message' | while read -r line; do
        echo "Processed: $line"
    done
}

# Run the unbuffered jq function
unbuffered_jq

With this modification, the script will now print each processed line immediately as it is generated, addressing the buffering issue.

Why This Works

The command stdbuf -oL disables output buffering by setting the output stream to "line-buffered" mode. This allows jq to output each line as soon as it is processed, rather than waiting for the entire input to complete. This is crucial for real-time applications such as log processing or streaming data.

Conclusion

If you work with continuously updating data or logs, the default behavior of jq may result in delayed output due to buffering. By creating a simple wrapper function that uses stdbuf, you can ensure that jq processes and outputs each line as it is received, allowing for real-time processing in your scripts.

Backlinks

tail & tail -f

Problem: Buffered Output with jq

Solution: Using stdbuf to Disable Buffering

Why This Works

Conclusion