for loop over sequence, then make string and post via json

Question

Consider this simple bash script : (I am writing myurl as a placeholder for the correct URL; and myhost as the placeholder for the correct hostname for privacy here.)

for a in `seq 400 450`; do
curl --request POST \
  --url myurl \
  --header 'Content-Type: application/json' \
  --header 'Host: myhost' \
  --data '{
"devID": "21010010",
"count": 0,
"fileEnd": 1,
"line": "",
"file": "$a"
}'
done

This is rejected as bad request. I want, the output of seq, (e.g. the number 410) to be a quote delimated string. Notice the line "file": "$a". If I write "file": "410", this works perfectly, and also for any other file between 400 and 450 - as long as it is surrounded by "" quotes.

Question :

I want, that in each iteration of the loop, the curl request should include the number coming from the seq command surrounded by "". So the first call should be :

curl --request POST \
  --url myurl \
  --header 'Content-Type: application/json' \
  --header 'Host: myhost' \
  --data '{
"devID": "21010010",
"count": 0,
"fileEnd": 1,
"line": "",
"file": "400"
}'

The next one to be :

curl --request POST \
  --url myurl \
  --header 'Content-Type: application/json' \
  --header 'Host: myhost' \
  --data '{
"devID": "21010010",
"count": 0,
"fileEnd": 1,
"line": "",
"file": "401"
}'

and so on.

How can I do this? Thank you.

score 1 · Accepted Answer · answered Jul 05 '21 at 00:11

The --data portion is wrapped in single quotes ...

'{"devID": ... "file": "$a"}'

... which disables the expansion of variable references inside said single quotes.

Consider:

$ a=410
$ echo '"devID": "21010010", "file": "${a}"'
"devID": "21010010", "file": "${a}"              # ${a} not expanded

You could wrap the --data section in double quotes, but then you need to also escape all of the nested double quotes.

Another idea is to break the --data section into 2x sections wrapped in single quotes and place the ${a} reference between the 2x sections, eg:

$ echo '"devID": "21010010", "file": "'${a}'"'   # change "${a}" to "'${a}'"
"devID": "21010010", "file": "410"               # ${a} successfully expanded

NOTE: There are several posts regarding the difference between single and double quotes, eg, this link

for loop over sequence, then make string and post via json

1 Answers1