Combine curl and base64 into one command?

Question

I'd like to POST the Base64 encoding of a file to a server, combining curl and base64 into one command that I can execute. I've tried the following:

curl 'http://localhost:5000/files' -X POST -H 'Content-Type: application/json' -d '{"file": "$(base64 hello.txt)"}'

But it just POSTs the string literal "$(base64 hello.txt)", instead of substituting the result of executing base64 hello.txt. So how do I do it properly?

Solution

When you use simple quotes, nothing is interpreted:

~$ echo 'foo:$var bar:$(base64 foo) something'
foo:$var bar:$(base64 foo) something

You have to unquote if you want the interpretation:

~$ echo 'foo:'$var' bar:'$(base64 foo)' something'
foo:foo bar:Zm9vCg== something

And since you don't know what it can contain, it's better to double quote:

~$ echo 'foo:'"$var"' bar:'"$(base64 foo)"' something'
foo:foo bar:Zm9vCg== something

Comments

[fredtantini]

If you try curl 'http://localhost:5000/files' -X POST -H 'Content-Type: application/json' -d '{"file": "'"$(base64 hello.txt)"'"}'? (that is, "unquote" the bash command $(...) but protected by surrounding by double quotes