Bash script: Use string variable in curl JSON Post data

Question

I want to send a json request and embedd a variable in the post data. I did a little research and I came up with the single quotes around the variable.

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

Unfortunately I get some errors:

curl: (6) Couldn't resolve host '—d'
curl: (3) [globbing] nested braces not supported at pos 54
HTTP/1.1 200 OK
Content-Length: 76
Content-Type: application/json
Date: Wed, 29 Jan 2014 19:16:56 GMT

{"error":{"code":-32700,"message":"Parse error."},"id":null,"jsonrpc":"2.0"}

Appearently there are some problems with the braces and the http answer states, that the command could not be executed. What's wrong with my code here? Thanks!

This is my curl version:

curl 7.30.0 (mips-unknown-linux-gnu) libcurl/7.30.0 OpenSSL/0.9.8y
Protocols: file ftp ftps http https imap imaps pop3 pop3s rtsp smtp smtps tftp 
Features: IPv6 Largefile NTLM NTLM_WB SSL 

Answer 1

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.

Answer 2

My suggestion:

#!/bin/bash
FILENAME="/media/file 2.avi"
curl -i -X POST -H "Content-Type: application/json" -d '{"jsonrpc": "2.0", "method": "Player.Open", "params":{"item":{"file":"'"$FILENAME"'"}}}' http://192.167.0.13/jsonrpc

The differences are hyphen in -d (instead of a dash) and double quotes around $FILENAME.

Answer 3

Here is another way to insert data from a file into a JSON property. This solution is based on a really cool command called jq.

Below is an example which prepares request JSON data, used to create a CoreOS droplet on Digital Ocean:

# Load the cloud config to variable
user_data=$(cat config/cloud-config)

# Prepare the request data 
request_data='{
  "name": "server name",
  "region": "fra1",
  "size": "512mb",
  "image": "coreos-stable",
  "backups": false,
  "ipv6": true,
  "user_data": "---this content will be replaced---",
  "ssh_keys": [1234, 2345]
}'

# Insert data from file into the user_data property
request_data=$(echo $request_data | jq ". + {user_data: \"$user_data\"}")