How to convert ctags format file to etags format file?

Question

I have a ctags generated file and would like to use it in Emacs. It was not generated using ctags (I know that ctags has a switch to create etags files).

Answer 1

Converting may be impossible because a tags file doesn't have enough information that a TAGS file needs, like byte-offsets for names. Instead, you can regenerate a TAGS file from the tags file if some conditions are met.

The latest version of readtags command that is shipped as a part of Universal ctags package (or is included in the Universal ctags source tree) may help you. The latest version of readtags has -F option. The option is needed to apply this answer to your issue.

If the format of the tags file you have follows the syntax explained in https://docs.ctags.io/en/latest/man/tags.5.html, the latest version of readtags (https://docs.ctags.io/en/latest/man/readtags.1.html) can extract all source file names:

$ readtags  -F '(list $input "\n")' -t ~/.citre/upstream-linux.tags -l | sort -u > input-file.list

Then you can pass the input-file.list to a TAGS file generator like etags or ctags:

$ cat input-file.list | xargs etags -a

or

$ ctags -L input-file.list -e 

You can simplify the steps:

$ readtags  -F '(list $input "\n")' -t ~/.citre/upstream-linux.tags -l | sort -u | xargs etags -a

or

$ readtags  -F '(list $input "\n")' -t ~/.citre/upstream-linux.tags -l | sort -u | ctags -L - -e

etags (or ctags) must support the language used in the files listed in the input-file.list. etags --help or ctags --list-languages lists the supported languages.