replace a value in file with jq (one liner)

Question

I want to replace the value of "CTE_LG_LL_OE" from "warn" to "ready"

{ "abcd": { "aav": "on", "vvg": "iio7890_APPID", "ct": "b-tte", "eSL": true, "it": "https://%ght%.mjk.com/", "tpo": "i-1", "pd": false, "pm": false, "pr": "" }, "en": { "CTE_LG_LL_OE": "warn", "S_G_EL_OD": "INFO", "G_LG_EL_OVE": "info" }, "EN_HTTPS": false }

I am able to find the same in file using the below command , but how do I replace it with one liner command.

jq '.' /abc/temp/config.json | grep CTE_LG_LL_OE

Answer 1

To get the key's value, you would use a JSON-aware tool, like jq rather than grep. You would do this for a few reasons:

1. The value that you are extracting may be encoded. The jq tool would decode this for you if you use it with the -r (--raw-output) option.
2. Using grep makes no distinction between values and keys, so you may accidentally extract data that you did not plan to extract.

Extracting the value of the CTE_LG_LL_OE key in the top-level en entry:

jq -r '.en.CTE_LG_LL_OE' file

Setting the value to the string ready and writing the resulting document to the file new-file:

jq '.en.CTE_LG_LL_OE |= "ready"' file >new-file

The |= operator is the "update operator" and it takes a "path" to a key to the left and a new value for the key on the right.

To set the value from a shell variable:

jq --arg newval "$newvalue" '.en.CTE_LG_LL_OE |= $newval' file >new-file

This creates a jq variable called $newval from the shell variable newvalue, which we then use in the jq expression. The value in the variable will automatically be JSON-encoded by jq.

For readability, space things out a bit (assuming this is part of a shell script, since the question is tagged with shell-script):

jq --arg newval "$newvalue" \
    '.en.CTE_LG_LL_OE |= $newval' file >new-file

To do in-place editing of the file (jq does not support in-place editing by itself):

tmpfile=$(mktemp)

cp file "$tmpfile" &&
jq --arg newval "$newvalue" \
    '.en.CTE_LG_LL_OE |= $newval' "$tmpfile" >file &&
rm -f "$tmpfile"

As a once-liner:

tmpfile=$(mktemp); cp file "$tmpfile" && jq --arg newval "$newvalue" '.en.CTE_LG_LL_OE |= $newval' "$tmpfile" >file && rm -f "$tmpfile"

---

If you're not sure of where in the document structure the CTE_LG_LL_OE key is located and just want to update the values of all CTE_LG_LL_OE keys that have the value warn:

jq '
    (
        .. |
        select(type == "object" and .CTE_LG_LL_OE? == "warn").CTE_LG_LL_OE
    ) |= "ready"' file

This examines all keys and values recursively in the whole document. It finds all objects that has a CTE_LG_LL_OE key with th evalue warn and updates these to instead be ready. The newlines are only for readability.

The value could be taken from a shell variable as before:

jq --arg newval "$somevariable" '
    (
        .. |
        select(type == "object" and .CTE_LG_LL_OE? == "warn").CTE_LG_LL_OE
    ) |= $newval' file

This could obviously be combined with doing in-place editing too, as show in the first half of this answer.

Answer 2

With your example content in file.json, the following one-liner works:

cat file.json | jq '.en.CTE_LG_LL_OE = "ready"' | tee file.json

To break this down:

1. cat … copies the contents of file.json to standard output (stdout).
2. | sends this output to the standard input (stdin) of the next command.
3. jq '.en.CTE_LG_LL_OE = "ready"' has what jq calls a ‘filter’ ('.en.CTE_LG_LL_OE = "ready"', assigning that specific value to that specific key) but does not specify any input file(s). This causes jq to read its input from stdin.
4. | tee sends the output from jq to the tee command, which writes it to both stdout and the named file.

I found this question after being puzzled by the behaviour of jq myself. For some reason (compare answers at https://stackoverflow.com/questions/48964305/write-output-to-a-file-after-piped-to-jq) output redirection does not work, nor does having jq do the file read itself. In short, none of the following work; any of them will truncate file.json, leaving it totally empty:

jq '.en.CTE_LG_LL_OE = "ready"' file.json >file.json

jq '.en.CTE_LG_LL_OE = "ready"' file.json | tee file.json

cat file.json | jq '.en.CTE_LG_LL_OE = "ready"' >file.json

It's possible some of the standard command-line options would alter this behaviour.