Universal ctags can be run with --_interactive, which enters a REPL that can be used programmatically to control ctags generation. In this mode, json commands are received over stdin, and corresponding responses are emitted over stdout.

This feature needs ctags to be built with json support and this requires libjansson to be installed at build-time. If it's supported it will be listed in the output of --list-features:

$ ./ctags --list-features | grep json
json

Communication with Universal ctags over stdio uses the json lines format, where each json object appears on a single line and is terminated with a newline.

When ctags --_interactive is invoked, it will emit a single json object to stdout announcing its name and version. This signals the start of the interactive loop, and the user can begin sending commands over stdin.

$ ctags --_interactive
{"_type": "program", "name": "Universal Ctags", "version": "0.0.0"}

The following commands are currently supported in interactive mode:

• generate-tags

The generate-tags command takes two arguments:

• filename: name of the file to generate tags for (required)
• size: size in bytes of the file, if the contents will be received over stdin (optional)

The simplest way to generate tags for a file is by passing its path on filesystem(file request). The response will include one json object per line representing each tag, followed by a single json object with the completed field emitted once the file has been fully processed.

$ echo '{"command":"generate-tags", "filename":"test.rb"}' | ctags --_interactive
{"_type": "program", "name": "Universal Ctags", "version": "0.0.0"}
{"_type": "tag", "name": "foobar", "path": "test.rb", "pattern": "/^  def foobar$/", "kind": "method", "scope": "Test", "scopeKind": "class"}
{"_type":"completed", "command": "generate-tags"}

The generate-tags command can also be used to generate tags for code which is not present on filesystem(inline request). For example, an IDE might want to generate ctags for an unsaved buffer while the user is editing code. When size is specified, the corresponding number of bytes are read over stdin after the json object and newline.

$ (
  echo '{"command":"generate-tags", "filename":"test.rb", "size": 17}'
  echo 'def foobaz() end'
) | ctags --_interactive
{"_type": "program", "name": "Universal Ctags", "version": "0.0.0"}
{"_type": "tag", "name": "foobaz", "path": "test.rb", "pattern": "/^def foobaz() end$/", "kind": "method"}
{"_type": "completed", "command": "generate-tags"}

sandbox submode can be used with --_interactive=sandbox. This submode will activate a sandbox, to this limits the damage that the can be achieved when exploiting a buffer overflow in Universal-ctags.

In the sandbox submode ctags can generate tags only for inline requests because ctags has to use open system call to handle file requests. The open system call is not allowed in the sandbox.

This feature uses seccomp-bpf, and is only supported on Linux. To use the submode libseccomp is needed at build-time. If ctags was built with seccomp support, sandbox is listed in the output of --list-features option.

$ ./ctags --list-features | grep sandbox
sandbox

$ (
  echo '{"command":"generate-tags", "filename":"test.rb", "size": 17}'
  echo 'def foobaz() end'
) | ctags --_interactive=sandbox
{"_type": "program", "name": "Universal Ctags", "version": "0.0.0"}
{"_type": "tag", "name": "foobaz", "path": "test.rb", "pattern": "/^def foobaz() end$/", "kind": "method"}
{"_type": "completed", "command": "generate-tags"}