blobl.sh

A simple, flexible Bash script to run Bloblang mappings using Benthos.

It supports:

• Reading input from stdin or a file (.json or .jsonl)
• Writing output to stdout or a file
• Inline mapping strings
• Mapping from a .blobl file
• Auto-compacting pretty-printed JSON files
• Clean handling of partial JSON line inputs

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Installation

1. Install Benthos

On macOS:

brew install benthos

Or manually download a binary from Benthos GitHub releases.

Check installation:

benthos -v

Make sure you have version 4.x or later.

2. Install blobl.sh

Save the script blobl.sh locally and make it executable:

chmod +x blobl.sh

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Usage

./blobl.sh [-m mapping_string | -f mapping_file] [-i input_file] [-o output_file]

Options:

Flag	Description
-m, --mapping	(Required) Inline Bloblang mapping string
-f, --mapping-file	(Required) Path to a .blobl file containing a mapping
-i, --input	Input file (.json or .jsonl). Defaults to stdin
-o, --output	Output file. Defaults to stdout
-h, --help	Show help and usage information

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Help Output

Run:

./blobl.sh -h

Outputs:

Usage: blobl.sh [-m mapping_string | -f mapping_file] [-i input_file] [-o output_file]

Options:
  -m, --mapping       Inline Bloblang mapping string.
  -f, --mapping-file  Path to a .blobl file containing a mapping.
  -i, --input         Input file (.json or .jsonl). Defaults to stdin.
  -o, --output        Output file. Defaults to stdout.
  -h, --help          Show this help message.

Notes:
  - Either -m or -f must be specified, but not both.
  - If input is a .json file, it will be automatically converted to .jsonl internally.
  - The script automatically cleans up temporary files created during conversion.

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Why .jsonl Files?

.jsonl (JSON Lines) format means each line contains exactly one complete JSON object.

Example .jsonl file:

{"foo": "bar"}
{"foo": "baz"}
{"foo": "qux"}

Each line is treated as a separate message by benthos blobl.

• ✅ Correct: One JSON object per line
• ❌ Incorrect: A single large array [ {...}, {...} ]

If you provide a .json file instead of .jsonl, blobl.sh will automatically compact it line-by-line using jq.

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Examples

1. Read from stdin, output to stdout

printf '{"foo":"bar"}\n' | ./blobl.sh -m 'root = this.foo.uppercase()'

Output:

"BAR"

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2. Read from .jsonl file, output to stdout

./blobl.sh -m 'root = this.foo.uppercase()' -i events.jsonl

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3. Read from .json (pretty-printed) file, output to stdout

./blobl.sh -m 'root = this.foo.uppercase()' -i event.json

The script will auto-compact the input behind the scenes.

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4. Read from stdin, write to a file

printf '{"foo":"bar"}\n' | ./blobl.sh -m 'root = this.foo.uppercase()' -o output.jsonl

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5. Read from a file, write to another file

./blobl.sh -m 'root = this.foo.uppercase()' -i events.jsonl -o output.jsonl

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Complex Example: Modifying a Field

In this example, we use the provided sample files:

• event.json — Pretty-printed JSON input
• events.jsonl — Compact one-line JSON input
• event.blobl — Mapping file to modify CreatedById

The goal is to update the CreatedById field while preserving the rest of the data.

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Input JSON (event.json or events.jsonl):

{
  "Account__c": { "string": "Dickenson plc" },
  "Amount__c": { "double": 15000 },
  "CreatedById": "005bm00000AWUWfAAP",
  "CreatedDate": 1736968069554,
  "Subsidiary__c": { "string": "ACME UK Ltd" }
}

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(A) Using the event.json file

Run:

./blobl.sh -i event.json -f event.blobl

✅ The .json file will be auto-compacted internally.

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(B) Using the events.jsonl file

Run:

./blobl.sh -i events.jsonl -f event.blobl

✅ The .jsonl file is already compact, so no transformation is needed.

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Mapping (event.blobl):

root = this
root.CreatedById = "005bm00000ZZZZZ"

✅ This mapping copies the original event and replaces CreatedById with the new ID.

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About the Sleep Hack

A tiny sleep 0.1 is inserted after the input command to force a proper EOF signal into Benthos.

Without it:

• Benthos might wait indefinitely for more input.
• Particularly when piping very short inputs.

This ensures:

• Benthos flushes and processes the message immediately.
• Outputs appear without needing manual Enter presses.

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Enjoy piping JSON through Bloblang!