Updates an integer value of a field at a specific path

update_register.sh

#!/bin/bash

# This is an example showing how to update a value of a specified key
# (that is potentially nested) in a JSON file using shell variables 
# in some shell script. For example, some utility for updatiing in-place
# a common set of fields in a JSON configuration file.
# In the snippet below, we assume that the input is a key ("register_name")
# in some nested JSON object whose path is "RD53B.GlobalConfig".

filename=${1}
register_name=${2}
new_value=${3}

# Notes:
#   - We use jq's "--arg" to specify the jq variables using the shell-defined variables,
#     and then use the newly-defined variables within the jq filter
#   - The first line in the jq filter (with the select(...) command) is a check that the
#     specified key ("register_name"/"register") actually exists. If it does, then its value
#     will be updated. If it does not, no updates will occur.
#   - The jq arguments are always handled as strings, so we use the "tonumber" utility
#     to convert the value to a number-type in the output JSON object.
jq --arg register "${register_name}" \
   --arg value "${new_value}" \
   '. | select(.RD53B.GlobalConfig[$register] != null)
      | .RD53B.GlobalConfig[$register] = ($value|tonumber)' \
    ${filename} > tmp.json && mv tmp.json ${filename}