JSON In Your Shell With Heredoc

Have you ever needed JSON in a shell script? I have reached for Here Documents or Heredoc syntax whenever this comes up. It’s more readable than escaping quotes at the very least.

Suppose you have some Generic JSON™ like so:

{"id": 12345, "message": "a long message that is generic and not subject to trademarks"}

Now you’d like to CURL that payload to a POST endpoint. Here’s how you can achieve that with Heredoc syntax.

#!/bin/sh

CONTENT=$(cat << EndOfJson
{"id": 12345, "message": "a long message that is generic and not subject to trademarks"}
EndOfJson
)

curl --request POST \
    --url https://example.com/api/messages \
    --data "$CONTENT"

You might wonder: why not use Single Quotes? Single Quotes will allow you to write your payload with Double Quotes inside, however you will lose the ability for environment variable string interpolation.

#!/bin/sh

$THE_ID=$1

## feat. string interpolation
CONTENT=$(cat << EndOfJson
{"id": $THE_ID, "message": "a long message that is generic and not subject to trademarks"}
EndOfJson
)

curl --request POST \
    --url https://example.com/api/messages \
    --data "$CONTENT"

Now in this variant, you get to use $THE_ID freely in the payload. That’s why I reach for Heredoc whenever I need JSON in a shell script.

When it works

As far as I know, without exhaustive testing, this works in:

• sh
• bash
• zsh