sed command to replace and write

Question

I am trying to replace the following string with another one from a file which contains few lines.

"schema" : "AAAAA",

TO

"schema" : "BBBBB",

Currently, I am doing the following command to get the corresponding line with "schema"

currentSchema=cat test.json | grep schema | awk {'print $3'}

It will give me the value "AAAAA",

I would like to replace AAAAA with BBBBB

I used the following command

sed -e 's/$currentSchema/BBBBB/p' test.json

However, it is not able to replace in the same file.

Answer 1

Example JSON file:

[
  {
    "host": "myhost",
    "schema": "AAAAA",
    "lunch": "sandwich"
  },
  {
    "host": "myotherhost",
    "schema": "QQQQQ",
    "lunch": "pizza"
  }
]

We'd like to replace each schema that is AAAAA with BBBBB. We can do this with jq:

$ jq 'map(if .schema == "AAAAA" then .schema = "BBBBB" else . end)' file.json
[
  {
    "host": "myhost",
    "schema": "BBBBB",
    "lunch": "sandwhich"
  },
  {
    "host": "myotherhost",
    "schema": "QQQQQ",
    "lunch": "pizza"
  }
]

If it doesn't matter what the old schema was:

$ jq 'map(.schema = "BBBBB")' file.json
[
  {
    "host": "myhost",
    "schema": "BBBBB",
    "lunch": "sandwhich"
  },
  {
    "host": "myotherhost",
    "schema": "BBBBB",
    "lunch": "pizza"
  }
]

Since jq is a proper JSON parser, this would work even if the file was presented in a more compact form, for example as the single line

[{"host":"myhost","schema":"AAAAA","lunch":"sandwhich"},{"host":"myotherhost","schema":"QQQQQ","lunch":"pizza"}]

---

For the actual problem (in comments), which is to find the element in .rules.behavior[] that has a .name key with value mPulse and then to change that element's .options.apiKey to some other value:

jq '.rules.behaviors = [.rules.behaviors[]|select(.name == "mPulse").options.apiKey = "XXX"]' file.json

That is, rewrite the .rules.behaviour array in such a way that the element whose .name key is mPulse gets a new .options.apiKey.

Answer 2

A couple of optimizations, not least of which is eschewing a "useless use of cat". You don't need to cat file | grep pattern; you can just grep pattern file. And since you're grepping into awk, that can be simplified too:

currentSchema="$( cat test.json | grep schema | awk '{print $3}' )"

becomes

currentSchema="$( awk '/schema/ {print $3}' test.json )"

Now, for your sed script. You are using Strong Quotes (') around your sed command, which means your shell variables will not be parsed. You need to use Weak Quotes (") for this to work:

currentSchema="$( awk '/schema/ {print $3}' test.json )"
sed --in-place "s/$currentSchema/BBBBB/" test.json

Answer 3

You don't need grep, awk, and sed for this; awk can do all the task (untested):

awk '/schema/ {$3 = "BBBBB"} 1' test.json > /tmp/$$ && mv /tmp/$$ test.json

Please try and report back.