Bash script: Use string variable in curl JSON Post data

Update: use the simpler

request_body=$(cat <<EOF
{
  "jsonrpc": "2.0",
  "method": "Player.Open",
  "params": {
    "item": {
      "file": "$FILENAME"
    }
  }
}
EOF
)

rather than what I explain below. However, if it is an option, use jq to generate the JSON instead. This ensures that the value of $FILENAME is properly quoted.

request_body=$(jq -n --arg fname "$FILENAME" '
{
  jsonrpc: "2.0",
  method: "Player.Open",
  params: {item: {file: $fname}}
}'

---

It would be simpler to define a variable with the contents of the request body first:

#!/bin/bash
header="Content-Type: application/json"
FILENAME="/media/file.avi"
request_body=$(< <(cat <<EOF
{
  "jsonrpc": "2.0",
  "method": "Player.Open",
  "params": {
    "item": {
      "file": "$FILENAME"
    }
  }
}
EOF
))
curl -i -X POST -H "$header" -d "$request_body" http://192.167.0.13/jsonrpc

This definition might require an explanation to understand, but note two big benefits:

1. You eliminate a level of quoting
2. You can easily format the text for readability.

First, you have a simple command substitution that reads from a file:

$( < ... )   # bash improvement over $( cat ... )

Instead of a file name, though, you specify a process substitution, in which the output of a command is used as if it were the body of a file.

The command in the process substitution is simply cat, which reads from a here document. It is the here document that contains your request body.