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The etags program is used to create a tags table file. It knows the syntax of several languages, as described in Tag Syntax. Here is how to run etags:
The etags program reads the specified files, and writes a tags table named TAGS in the current working directory. You can optionally specify a different file name for the tags table by using the ‘--output=file’ option; specifying - as a file name prints the tags table to standard output. You can also append the newly created tags table to an existing file by using the ‘--append’ option.
If the specified files don't exist, etags looks for compressed versions of them and uncompresses them to read them. Under MS-DOS, etags also looks for file names like mycode.cgz if it is given ‘mycode.c’ on the command line and mycode.c does not exist.
If the tags table becomes outdated due to changes in the files described in it, you can update it by running the etags program again. If the tags table does not record a tag, or records it for the wrong file, then Emacs will not be able to find that definition until you update the tags table. But if the position recorded in the tags table becomes a little bit wrong (due to other editing), Emacs will still be able to find the right position, with a slight delay.
Thus, there is no need to update the tags table after each edit. You should update a tags table when you define new tags that you want to have listed, or when you move tag definitions from one file to another, or when changes become substantial.
You can make a tags table include another tags table, by passing the ‘--include=file’ option to etags. It then covers all the files covered by the included tags file, as well as its own.
If you specify the source files with relative file names when you run etags, the tags file will contain file names relative to the directory where the tags file was initially written. This way, you can move an entire directory tree containing both the tags file and the source files, and the tags file will still refer correctly to the source files. If the tags file is - or is in the /dev directory, however, the file names are made relative to the current working directory. This is useful, for example, when writing the tags to the standard output.
When using a relative file name, it should not be a symbolic link pointing to a tags file in a different directory, because this would generally render the file names invalid.
If you specify absolute file names as arguments to etags, then the tags file will contain absolute file names. This way, the tags file will still refer to the same files even if you move it, as long as the source files remain in the same place. Absolute file names start with ‘/’, or with ‘device:/’ on MS-DOS and MS-Windows.
When you want to make a tags table from a great number of files, you may have problems listing them on the command line, because some systems have a limit on its length. You can circumvent this limit by telling etags to read the file names from its standard input, by typing a dash in place of the file names, like this:
find . -name "*.[chCH]" -print | etags -
etags recognizes the language used in an input file based on its file name and contents. It first tries to match the file's name and extension to the ones commonly used with certain languages. Some languages have interpreters with known names (e.g., perl for Perl or pl for Prolog), so etags next looks for an interpreter specification of the form ‘#!interp’ on the first line of an input file, and matches that against known interpreters. If none of that works, or if you want to override the automatic detection of the language, you can specify the language explicitly with the ‘--language=name’ option. You can intermix these options with file names; each one applies to the file names that follow it. Specify ‘--language=auto’ to tell etags to resume guessing the language from the file names and file contents. Specify ‘--language=none’ to turn off language-specific processing entirely; then etags recognizes tags by regexp matching alone (see Etags Regexps). This comes in handy when an input file uses a language not yet supported by etags, and you want to avoid having etags fall back on Fortran and C as the default languages.
The option ‘--parse-stdin=file’ is mostly useful when calling etags from programs. It can be used (only once) in place of a file name on the command line. etags will read from standard input and mark the produced tags as belonging to the file file.
‘etags --help’ outputs the list of the languages etags knows, and the file name rules for guessing the language. It also prints a list of all the available etags options, together with a short explanation. If followed by one or more ‘--language=lang’ options, it outputs detailed information about how tags are generated for lang.