etags - Unix, Linux Command


etags, ctags - generate tag file for Emacs, vi


etags [-aCDGImRVh] [-i file] [-l language] [-o tagfile] [-r regexp]
[--append] [--no-defines] [--no-globals] [--include=file] [--ignore-indentation] [--language=language] [--members] [--output=tagfile] [--regex=regexp] [--no-regex] [--ignore-case-regex=regexp] [--help] [--version] file ...

ctags [-aCdgImRVh] [-BtTuvwx] [-l language] [-o tagfile] [-r regexp]
[--append] [--backward-search] [--cxref] [--defines] [--forward-search] [--globals] [--ignore-indentation] [--language=language] [--members] [--output=tagfile] [--regex=regexp] [--ignore-case-regex=regexp] [--typedefs] [--typedefs-and-c++] [--update] [--no-warn] [--help] [--version] file ...


The etags program is used to create a tag table file, in a format understood by emacs(1) ; the ctags program is used to create a similar table in a format understood by vi(1) . Both forms of the program understand the syntax of C, Objective C, C++, Java, Fortran, Ada, Cobol, Erlang, LaTeX, Emacs Lisp/Common Lisp, makefiles, Pascal, Perl, Postscript, Python, Prolog, Scheme and most assembler-like syntaxes. Both forms read the files specified on the command line, and write a tag table (defaults: TAGS for etags, tags for ctags) in the current working directory. Files specified with relative file names will be recorded in the tag table with file names relative to the directory where the tag table resides. Files specified with absolute file names will be recorded with absolute file names. The programs recognize the language used in an input file based on its file name and contents. The --language switch can be used to force parsing of the file names following the switch according to the given language, overriding guesses based on filename extensions.


Some options make sense only for the vi style tag files produced by ctags; etags does not recognize them. The programs accept unambiguous abbreviations for long option names.
-a, --append
  Append to existing tag file. (For vi-format tag files, see also --update.)
-B, --backward-search
  Tag files written in the format expected by vi contain regular expression search instructions; the -B option writes them using the delimiter ‘?’, to search backwards through files. The default is to use the delimiter ‘/’, to search forwards through files. Only ctags accepts this option.
  In C and derived languages, create tags for function declarations, and create tags for extern variables unless --no-globals is used.
-d, --defines
  Create tag entries for C preprocessor constant definitions and enum constants, too. This is the default behavior for etags.
-D, --no-defines
  Do not create tag entries for C preprocessor constant definitions and enum constants. This may make the tags file much smaller if many header files are tagged. This is the default behavior for ctags.
-g, --globals
  Create tag entries for global variables in C, C++, Objective C, Java, and Perl. This is the default behavior for etags.
-G, --no-globals
  Do not tag global variables. Typically this reduces the file size by one fourth. This is the default behavior for ctags.
-i file, --include=file
  Include a note in the tag file indicating that, when searching for a tag, one should also consult the tags file file after checking the current file. This options is only accepted by etags.
-I, --ignore-indentation
  Don’t rely on indentation as much as we normally do. Currently, this means not to assume that a closing brace in the first column is the final brace of a function or structure definition in C and C++.
-l language, --language=language
  Parse the following files according to the given language. More than one such options may be intermixed with filenames. Use --help to get a list of the available languages and their default filename extensions. The ‘auto’ language can be used to restore automatic detection of language based on the file name. The ‘none’ language may be used to disable language parsing altogether; only regexp matching is done in this case (see the --regex option).
-m, --members
  Create tag entries for variables that are members of structure-like constructs in C++, Objective C, Java.
-M, --no-members
  Do not tag member variables. This is the default behavior.
  Only tag packages in Ada files.
-o tagfile, --output=tagfile
  Explicit name of file for tag table; overrides default TAGS or tags. (But ignored with -v or -x.)
-r regexp, --regex=regexp
  Make tags based on regexp matching for each line of the files following this option, in addition to the tags made with the standard parsing based on language. When using --regex, case is significant, while it is not with --ignore-case-regex. May be freely intermixed with filenames and the -R option. The regexps are cumulative, i.e. each option will add to the previous ones. The regexps are of the form:

where tagregexp is used to match the lines that must be tagged. It should not match useless characters. If the match is such that more characters than needed are unavoidably matched by tagregexp, it may be useful to add a nameregexp, to narrow down the tag scope. ctags ignores regexps without a nameregexp. The syntax of regexps is the same as in emacs, augmented with intervals of the form \{m,n\}, as in ed or grep.
Here are some examples. All the regexps are quoted to protect them from shell interpretation.

Tag the DEFVAR macros in the emacs source files:
--regex\=’/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"\/

Tag VHDL files (this example is a single long line, broken here for formatting reasons):
--language\=none --regex=’/[ \t]*\(ARCHITECTURE\|\ CONFIGURATION\) +[^ ]* +OF/’ --regex\=’/[ \t]*\ \(ATTRIBUTE\|ENTITY\|FUNCTION\|PACKAGE\( BODY\)?\ \|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/’

Tag TCL files (this last example shows the usage of a tagregexp):
--lang\=none --regex\=’/proc[ \t]+\([^ \t]+\)/\1/’

A regexp can be preceded by {lang}, thus restricting it to match lines of files of the specified language. Use etags --help to obtain a list of the recognised languages. This feature is particularly useful inside regex files. A regex file contains one regex per line. Empty lines, and those lines beginning with space or tab are ignored. Lines beginning with @ are references to regex files whose name follows the @ sign. Other lines are considered regular expressions like those following --regex.
For example, the command
etags --regex=@regex.file *.c
reads the regexes contained in the file regex.file.

-R, --no-regex
  Don’t do any more regexp matching on the following files. May be freely intermixed with filenames and the --regex option.
-t, --typedefs
  Record typedefs in C code as tags. Since this is the default behaviour of etags, only ctags accepts this option.
-T, --typedefs-and-c++
  Generate tag entries for typedefs, struct, enum, and union tags, and C++ member functions. Since this is the default behaviour of etags, only ctags accepts this option.
-u, --update
  Update tag entries for files specified on command line, leaving tag entries for other files in place. Currently, this is implemented by deleting the existing entries for the given files and then rewriting the new entries at the end of the tags file. It is often faster to simply rebuild the entire tag file than to use this. Only ctags accepts this option.
-v, --vgrind
  Instead of generating a tag file, write index (in vgrind format) to standard output. Only ctags accepts this option.
-w, --no-warn
  Suppress warning messages about duplicate entries. The etags program does not check for duplicate entries, so this option is not allowed with it.
-x, --cxref
  Instead of generating a tag file, write a cross reference (in cxref format) to standard output. Only ctags accepts this option.
-h, -H, --help
  Print usage information.
-V, --version
  Print the current version of the program (same as the version of the emacs etags is shipped with).


Navigating the source code using vi or emacs editor could be quite a task especially if the code has multiple files. gctags utility can be quite handy in such a situation. Using gctags vi can be converted into a beautiful source code browser.

Example given below is for C program but same can be applied for other languages like C++, java, Perl, etc. Type gctags --h for to see supported languages.

Consider below source code: gctags_example.c

#include <stdio.h>
// Function Declarations
int areaRectangle(int length, int breadth);
float areaCircle(int radius);
int square(int num);
void dummy();
#define PI 3.14
typedef struct {
        int memberInt;
        float memberFloat;
        char memberChar;
} myStruct;
int rectangle;
float circle;
int areaRectangle(int length, int breadth)
        return(length * breadth);
float areaCircle(int radius)
        return(PI * square(radius));
int square(int num)
        return(num * num);
int main()
        rectangle = areaRectangle(10, 5);
        circle = areaCircle(20);
        printf("Area of rectangle: %f", rectangle);
        printf("Area of circle: %f", circle);

Step 1: Create a tags file
$ gctags --declarations --defines --global --members --output=example.tags --no-warn gctags_example.c

Above command will create the tags file example.tags. Without the --output option the file name would be tags
$ ls
example.tags  gctags_example.c

Below are the contents of example.tags:
$ cat example.tags
Mgctags_example gctags_example.c        /^int main()$/
PI      gctags_example.c        9
areaCircle      gctags_example.c        /^float areaCircle(int radius);$/
areaRectangle   gctags_example.c        /^int areaRectangle(int length, int breadth);$/
circle  gctags_example.c        18
dummy   gctags_example.c        /^void dummy();$/
memberChar      gctags_example.c        14
memberFloat     gctags_example.c        13
memberInt       gctags_example.c        12
myStruct        gctags_example.c        15
rectangle       gctags_example.c        17
square  gctags_example.c        /^int square(int num);$/

Based on the options given, tags have been created for various parts of the code. Example, option --global creates tags for global variables and due to option --defines creates tags for #define and so on. Above command has been used for single file. But with slight modification the same command can work for multiple files and also search directories recursively.

Below command creates tags for all .c and .h files. -R option is used to search directories recursively under current directory.
$ gctags -R --declarations --defines --global --members --output=example.tags --no-warn *.c *.hae

Step 2: Set the path for tags file
This is required if you have specified custom tags file and not using the default file "tags".

Open the an source file in vi editor and give below command.
:set tags=<path to tags>

Step 3: With the tags path set, you are now ready to browse source code

Below table shows the commands that can be used to navigate the source in vi editor
Ctrl+]Jump to the tag underneath the cursor
:ts Search for a particular tag
:tnGo to the next definition for the last tag
:tpGo to the previous definition for the last tag
:tsList all of the definitions of the last tag
Ctrl+tJump back up in the tag stack