Escape strings sent in curl data when using application/json

Question

I'm using CURL to send JSON data. Here is an example:

mycomputer$ curl -H "Content-Type: application/json" 
     -d  "{ "some_string": "Hello mom it's me!" }"
     "http://localhost:3001/api_v2/blocks/42af6ab04d9d9635a97f8abec14ed023?api_key=fe5cf0d86af27c086ab5cd4d0eab6641"

How can I escape the contents of any value for some_string?

For example if someone wants to put in the string Abe Lincoln's favorite character is the backslash \. He said "I love the \ and single quotes like ''". how can I escape this while using curl?

I think I need to do the following:

If the string contains a " escape it with a triple backslash \\\".
If the string contains a ' escape it doesn't need escaping '.
If the string contains a \ escape it with a triple backslash \\\\.

Are there any characters I am forgetting?

phemmer · Accepted Answer · 2014-10-16 01:15:39Z

I'm going to assume your objective is just getting the string past the shell's parsing. If so, Use read:

For example

$ IFS='' read -r var

Then manually paste the line.

If it's multiple lines you can use:

$ IFS='' read -r -d '' var

And paste again, but this time use CTRL+d to end the input.

Or use a heredoc:

$ IFS='' read -r -d '' var <<'EOF'
{ "some_string": "Hello mom it's me!" }
EOF

Whichever method you use, you can then just use the variable $var for accessing it:

$ curl -H "Content-Type: application/json" \
 -d  "$var" \
 "http://localhost:3001/api_v2/blocks/42af6ab04d9d9635a97f8abec14ed023?api_key=fe5cf0d86af27c086ab5cd4d0eab6641"

mikeserv · Accepted Answer · 2014-10-16 02:35:06Z

You probably wouldn't need 3 of those backslashes if you can remove the shell from the equation. Luckily, you can:

curl --config - <<\DATA
    url    = "http://some.url"
    header = "Content-Type: application/json"
    data   = "{ \"some_string\": "Abe Lincoln's favorite character is the backslash \\. He said \"I love the \\ and single quotes like '.\""
#END
DATA

Look at man curl. I suggest paying close attention to the differences between --data-ascii (which is what -d means), --data-binary, and --data-urlencoded.

You might also want to compare the different behaviors you get with --header, --data, and --form (which, by default, all do a POST in slightly different ways), and what you might get with --get which can instead encode the data specified with any of the previously mentioned into the request url following a ?.

Kusalananda · Accepted Answer · 2022-02-05 15:24:25Z

Consider using a JSON-aware tool to create the JSON document.

Using jq:

json=$(
    jq -n --arg 'Some key "string"' "My cat's useless" '$ARGS.named'
)

Using jo:

json=$( jo 'Some key "string"'="My cat's useless" )

Note that jo would try to infer that the value is an array entry if the key's value ends in [].

With both variations of the code above, the json variable would get the value equivalent of

{
  "Some key \"string\"": "My cat's useless"
}

You may then use the json variable in your call to curl:

curl -H 'Content-Type: application/json' \
    -d "$json" \
    'http://localhost:3001/api_v2/some/endpoint'

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