A command-line tool for reading and writing JSON configuration files. This tool is written in pure C with no external dependencies, making it suitable for cross-compilation and embedded systems.

• Read values from JSON configuration files using dot notation
• Write values to JSON configuration files using dot notation
• Create new JSON configuration files
• Print entire JSON configuration files
• Restore configuration files to original state (OverlayFS support)
• Support for nested objects and arrays
• Support for various data types (strings, numbers, booleans, null)
• Pretty-printing of JSON output

To build the project for your local machine, run:

make

This will compile the source files and create the jct executable.

The Makefile supports multiple toolchains using the standard CROSS_COMPILE prefix:

# Use prefix for a toolchain found in PATH
make CROSS_COMPILE=mipsel-linux-gnu-

# Use any custom toolchain with a full path
make CROSS_COMPILE=/path/to/toolchain/bin/prefix-

You can also use these toolchain options with the release target:

make CROSS_COMPILE=/path/to/toolchain/bin/mipsel-linux-musl- release
make CROSS_COMPILE=mipsel-linux-gnu- release

For optimized builds with smaller binary size:

make release         # Optimized build (stripped of debug info)

The optimized builds use the following techniques to reduce binary size:

• -Os optimization flag for size
• Removal of unused code with -ffunction-sections and -fdata-sections
• Stripping of debug information

To clean up the build artifacts:

make clean           # Remove object files and executables
make distclean       # Remove all generated files (complete cleanup)

Usage: jct [--trace-resolve] <config_file> <command> [options]

Commands:
  <config_file> get <key>              Get a value from the config file
  <config_file> set <key> <value>      Set a value in the config file
  <config_file> create                 Create a new empty config file
  <config_file> print                  Print the entire config file
  <config_file> restore                Restore config file to original state (OverlayFS)

Options:
  --trace-resolve                      Trace short-name resolution steps

Short-name resolution (when <config_file> has no '/' and does not end with .json):
  Tries, in order: ./<name>, ./<name>.json, /etc/<name>.json (POSIX only)
  - If none found: exit 2 with an error listing tried paths
  - If a candidate exists but is not readable: exit 13 (do not try later candidates)
  - Symlinks are followed; final target must be a regular file (directories are skipped)

Creation rules:
  - create requires an explicit path
  - set may create a new file only with an explicit path
  - Using a short name that does not resolve will not create; the command exits 2 with guidance

Examples:
  jct prudynt get server.host           Resolve short name 'prudynt' and read
  jct prudynt set app.name "My App"     Resolve and update existing; to create, use explicit path
  jct ./prudynt set app.name "My App"   Explicit path; allowed to create
  jct config.json print                 Print the entire config file

• 0: Success
• 2: Not found
  • Short name did not resolve to any candidate (get/print/restore/set)
  • Short name used with create (and with set when it would create) — use an explicit path instead
• 13: Permission denied
  • A candidate file was found during short-name resolution but is not readable; later candidates are not tried

Note: The restore command defines additional exit codes specific to OverlayFS operations; see the Restore section below for details.

When <config_file> is a short name (no path separators and no .json extension), jct searches for a JSON file deterministically:

1. ./
2. ./.json
3. /etc/.json (POSIX systems only)

Rules:

• If a candidate does not exist, jct continues to the next.
• If a candidate exists and is a directory, it is skipped.
• If a candidate exists, symlinks are followed and the final target must be a regular file.
• If a candidate is a regular file but not readable, jct exits immediately with code 13 (permission denied) and does not try later candidates.
• If no candidate is selected, jct exits with code 2 (not found) and prints: jct: no JSON file found for ''; tried: ./, ./.json[, /etc/.json]

Creation behavior:

• create requires an explicit path; short names are not accepted for creation and will return code 2 with guidance.
• set may create a new file only when invoked with an explicit path; using a short name that does not resolve will return code 2 and advise supplying an explicit path (e.g., ./.json).

Tip:

• Use --trace-resolve to print the resolution steps and the final chosen path to stderr.

./jct new_config.json create

./jct config.json set server.host localhost
./jct config.json set server.port 8080
./jct config.json set server.ssl true
./jct config.json set app.name "My Application"
./jct config.json set app.version 1.0

Query JSON data using Goessner JSONPath.

Examples:

• jct books.json path "$..author" --mode values
• jct books.json path "$.store.book[?(@.price < 10)].title"
• jct books.json path "$.store.book[0:3]" --mode pairs

Options:

• --mode values|paths|pairs (default: values)
• --limit N to stop after N matches
• --strict causes parse/eval errors to exit nonzero (2 parse, 3 eval); default lenient emits [] and warns to stderr
• --pretty pretty-prints JSON output
• --unwrap-single when mode=values, emit the lone value instead of [value]

./jct config.json get server.host
# Output: localhost

./jct config.json get server.port
# Output: 8080

./jct config.json get app.name
# Output: My Application

./jct config.json print

./jct /etc/config.json restore

This command is designed for embedded systems using OverlayFS. It:

1. Requires an absolute path (must start with '/') for the config file
2. Validates that the original file exists in /rom/<config_file>
3. Checks that a modified version exists in /overlay/<config_file>
4. Removes the overlay file to expose the original ROM version
5. Remounts the overlay filesystem to apply changes

Important: The config file path must be absolute. Relative paths are not accepted.

Examples:

./jct /etc/prudynt.json restore          # ✓ Valid - absolute path
./jct /opt/app/config.json restore       # ✓ Valid - absolute path
./jct prudynt.json restore               # ✗ Invalid - relative path
./jct ./config.json restore              # ✗ Invalid - relative path

Exit codes:

• 0: Success - file restored to original state
• 1: Original ROM file not found
• 2: File is already original (no overlay to remove)
• 3: Failed to remove overlay file
• 4: Failed to remount overlay filesystem
• 5: Invalid arguments or non-absolute path

• src/json_config.h - Header file with type definitions and function declarations
• src/json_value.c - Implementation of JSON value handling functions
• src/json_parse.c - Implementation of JSON parsing functions
• src/json_serialize.c - Implementation of JSON serialization functions
• src/json_config.c - Implementation of configuration manipulation functions
• src/json_config_cli.c - Main file with CLI interface
• Makefile - Build configuration

This project is open source and available under the MIT License.