Using curl POST with variables defined in bash script functions

Solution tested with https://httpbin.org/ and inline bash script
1. For variables without spaces in it i.e. 1:
Simply add ' before and after $variable when replacing desired string

for i in {1..3}; do \
  curl -X POST -H "Content-Type: application/json" -d \
    '{"number":"'$i'"}' "https://httpbin.org/post"; \
done

2. For input with spaces:
Wrap variable with additional " i.e. "el a":

declare -a arr=("el a" "el b" "el c"); for i in "${arr[@]}"; do \
  curl -X POST -H "Content-Type: application/json" -d \
    '{"elem":"'"$i"'"}' "https://httpbin.org/post"; \
done

Curl can post binary data from a file so I have been using process substitution and taking advantage of file descriptors whenever I need to post something nasty with curl and still want access to the vars in the current shell. Something like:

curl "http://localhost:8080" \
-H "Accept: application/json" \
-H "Content-Type:application/json" \
--data @<(cat <<EOF
{
  "me": "$USER",
  "something": $(date +%s)
  }
EOF
)

This winds up looking like --data @/dev/fd/<some number> which just gets processed like a normal file. Anyway if you wanna see it work locally just run nc -l 8080 first and in a different shell fire off the above command. You will see something like:

POST / HTTP/1.1
Host: localhost:8080
User-Agent: curl/7.43.0
Accept: application/json
Content-Type:application/json
Content-Length: 43

{  "me": "username",  "something": 1465057519  }

As you can see you can call subshells and whatnot as well as reference vars in the heredoc.

We can assign a variable for curl using single quote ' and wrap some other variables in

• a single quote => ' $variable '
• a double-quote single-quote => "' $variable '"
• a single+double+single quote => '"' $variable '"'

Lets test each case, but first watch out for this catch that if we use a single quote ' for variable assignment, that variable is not evaluated.

watch out

Please notice the assignment is done by a single quote CURL_DATA='content'

cmd='ls'

CURL_DATA='{
    "cmd": "$cmd",    <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us

{
    "cmd": "$cmd",    <===== we need ls not $cmd
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

a single quote ' $variable '

• The value of a variable is evaluated
• neither single quote ' nor applies double one "

cmd='ls'

CURL_DATA='{
    "cmd": '$cmd',    <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us:

{
    "cmd": ls,    <===== neither 'ls' nor "ls", just ls
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

a double-quote single-quote "' $variable '"

• variable is evaluated
• will be surrounded by a double quote "

cmd='ls'

CURL_DATA='{
    "cmd": "'$cmd'",    <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us

{
    "cmd": "ls",    <===== we have double quote " variable "
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

a single+double+single quote => '"' $variable '"'

• variable is evaluated
• will be surrounded by a single quote '

cmd='ls'

CURL_DATA='{
    "cmd": '"'$cmd'"',
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us

{
    "cmd": 'ls',    <===== we have a single quote ' variable '
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

summary

# no quote at all
$cmd => $cmd

# a double quote (" $variable ")
"$cmd" => "$cmd"

# a single quote (' $variable ')
'$cmd' => ls

# a single quote + a double quote ("' $variable '")
"'$cmd'" => "ls"

# a single-double-single quote ('"' $variable '"')
'"'$cmd'"' => 'ls'

which one we should use?

Since JSON needs a double quote " for its key or value we can use :

• a double-quote single-quote "' $variable '"

curl

cmd='ls'

CURL_DATA='{
    "cmd": "'$cmd'",
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA" | jq '.'

curl --data "$CURL_DATA" -X POST localhost:3232/cmd | jq '.'

NOTE:
The equivalent of ' for a variable evaluation is '" which means instead of using '$cmd' we can use '"$cmd"' and it gives us ls neither with a single quote nor a double quote , but it gets more confusing if we needed to apply for curl since we need a double quoted result "ls" and would have to wrap it in another double quote => "'"

This code works well, but the above is more readable

cmd='ls'

CURL_DATA='{
    "cmd": "'"$cmd"'",     <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA" | jq '.'

curl --data "$CURL_DATA" -X POST localhost:3232/cmd | jq '.'

Will give us:

{
    "cmd": "ls",    <===== result
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

finally

We can use either of:

    "cmd": "'$cmd'",    <===== will be: "ls"

or

    "cmd": "'"$cmd"'",    <===== will be: "ls" 

and "$CURL_DATA" as a normal variable

curl --data "$CURL_DATA" -X POST localhost:3232/cmd 

A few years late but this might help someone if you are using eval or backtick substitution:

postDataJson="{\"guid\":\"$guid\",\"auth_token\":\"$token\"}"

Using sed to strip quotes from beginning and end of response

$(curl --silent -H "Content-Type: application/json" https://${target_host}/runs/get-work -d ${postDataJson} | sed -e 's/^"//' -e 's/"$//')

Here's what actually worked for me, after guidance from answers here:

export BASH_VARIABLE="[1,2,3]"
curl http://localhost:8080/path -d "$(cat <<EOF
{
  "name": $BASH_VARIABLE,
  "something": [
    "value1",
    "value2",
    "value3"
  ]
}
EOF
)" -H 'Content-Type: application/json'

Existing answers point out that curl can post data from a file, and employ heredocs to avoid excessive quote escaping and clearly break the JSON out onto new lines. However there is no need to define a function or capture output from cat, because curl can post data from standard input. I find this form very readable:

curl -X POST -H 'Content-Type:application/json' --data '@-' ${API_URL} << EOF
{
  "account": {
    "email": "$email",
    "screenName": "$screenName",
    "type": "$theType",
    "passwordSettings": {
      "password": "$password",
      "passwordConfirm": "$password"
    }
  },
  "firstName": "$firstName",
  "lastName": "$lastName",
  "middleName": "$middleName",
  "locale": "$locale",
  "registrationSiteId": "$registrationSiteId",
  "receiveEmail": "$receiveEmail",
  "dateOfBirth": "$dob",
  "mobileNumber": "$mobileNumber",
  "gender": "$gender",
  "fuelActivationDate": "$fuelActivationDate",
  "postalCode": "$postalCode",
  "country": "$country",
  "city": "$city",
  "state": "$state",
  "bio": "$bio",
  "jpFirstNameKana": "$jpFirstNameKana",
  "jpLastNameKana": "$jpLastNameKana",
  "height": "$height",
  "weight": "$weight",
  "distanceUnit": "MILES",
  "weightUnit": "POUNDS",
  "heightUnit": "FT/INCHES"
}
EOF

the info from Sir Athos worked perfectly!!

Here's how I had to use it in my curl script for couchDB. It really helped out a lot. Thanks!

bin/curl -X PUT "db_domain_name_:5984/_config/vhosts/$1.couchdb" -d '"/'"$1"'/"' --user "admin:*****"

This script will make a POST request to the specified URL with the specified JSON body, and it will output the response from the server to the console:

#!/bin/bash

# Set the URL to send the request to
url='http://example.com/endpoint'

# Set the JSON body of the request
json_data='{"key1": "value1", "key2": "value2"}'

# Make the POST request with the JSON body
response=$(curl -X POST -H "Content-Type: application/json" -d "$json_data" "$url")

echo "Response from server: $response"

Putting data into a txt file worked for me

bash --version
GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
curl --version
curl 7.29.0 (x86_64-redhat-linux-gnu)
 cat curl_data.txt 
 {  "type":"index-pattern", "excludeExportDetails": true  }

curl -X POST http://localhost:30560/api/saved_objects/_export -H 'kbn-xsrf: true' -H 'Content-Type: application/json' -d "$(cat curl_data.txt)" -o out.json

jo might help:

$ export A=this B=that C=foo D=bar
$ jo -p a=$A b=$B nested=$(jo c=$C d=$D)
{
   "a": "this",
   "b": "that",
   "nested": {
      "c": "foo",
      "d": "bar"
   }
}