Shell script -- how to store curl command WITH variables result into another variable?

Question

Currently I have a curl command that I want to store in a local variable. I know I could use `` or $() to get it done, but a variable inside the command stops me from doing so.

local sample_mapping="$(curl -XGET '${search_host}/${index}/_mapping')"

where search_host and index are two variables previously set. This command fails because Couldn't resolve host '$search_host'

The other posts didn't talk about what to do when dealing with variables. Anyone has any idea?

Christian Ternus · Accepted Answer · 2016-08-25 20:38:05Z

3

This is the command in your command substitution:

curl -XGET '${search_host}/${index}/_mapping'

This command will not work on its own. The problem that you are observing is due to the single quotes, not to the use of command substitution, $(...).

Inside single-quotes, variables are not expanded. Since you need variable expansion, use double-quotes:

curl -XGET "${search_host}/${index}/_mapping"

Putting the above in your original command:

local sample_mapping="$(curl -XGET "${search_host}/${index}/_mapping")"

Note that quotes and command substitutions can nest : the fact that there are double quotes inside the command substitution has no effect on the double quotes outside of the command substitution.

Example

Let's define some variables:

$ search_host=http://google.com
$ index=index.html

Now, let's try the command with single-quotes:

$ a=$(curl -XGET '$search_host/$index')
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: $search_host

The above failed with an error message similar to the one you observed: Could not resolve host: $search_host

Let's try again with double-quotes:

$ a=$(curl -XGET "$search_host/$index")
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   229  100   

229    0     0    520      0 --:--:-- --:--:-- --:--:--   521

The above succeeded.

edited Aug 25, 2016 at 20:38

Christian Ternus

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answered Aug 25, 2016 at 0:33

John1024

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4 Comments

JChao Over a year ago

I removed the single quote and used double quote instead, the problem remains. the error msg is still the same.

John1024 Over a year ago

@JChao That would appear to mean that there is a problem with the host that you are accessing. Try my example (using google) and verify that it works before trying your host.

JChao Over a year ago

Alright you're correct. I had another syntax error....... Thanks for your help! So just to make sure, in the future, I'll just have to replace single quote with double quote? That should solve the problem, right?

John1024 Over a year ago

@JChao When you want variable expansion, as you do here, use double-quotes. If you want to suppress all shell expansions, use single-quotes.

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