etags

etags [options] file1 file2 ...

-a, --append
    Append to existing tag file.

-c, --ctags
    Generate tag file in ctags format.

-d, --debug
    Enable debugging mode.

-f tagfile, --tag-file=tagfile
    Specify the name of the tag file (default is 'tags').

-i file, --include=file
    Include file in tag generation process.

-l language, --language=language
    Specify the language of the input files.

-o tagfile, --output=tagfile
    Same as -f, specifies the name of the tag file.

-r regexp, --regex=regexp
    Interpret 'regexp' as an additional regex describing a tag.

--version
    Display version information.

--help
    Display help message.

The `etags` command creates tag files, which are index files used by editors like Emacs and vi to quickly locate the definitions of functions, variables, classes, and other identifiers in source code files. It parses source code in various languages (C, C++, Java, Python, etc.) and creates a tag file (`tags` by default) that maps identifiers to their locations within the source files. This allows developers to easily jump to the definition of a symbol, improving code navigation and understanding. The `etags` command significantly speeds up development, especially in large codebases. It's a vital tool for source code analysis and code editing workflows.

The quality of the tag file depends on the complexity and structure of the source code. `etags` may not perfectly handle all language constructs or complex code layouts. Incremental updates to large tag files can be slow; regenerating the entire tag file might be faster in some cases.

Basic Tag Generation:
`etags *.c *.h`
Generates a 'tags' file in the current directory based on all .c and .h files.

Appending to an Existing Tag File:
`etags -a new_file.c`
Appends tags for 'new_file.c' to the existing 'tags' file.

Specifying a Custom Tag File Name:
`etags -f mytags *.cpp`
Generates a tag file named 'mytags' based on all .cpp files.

Generating Tags for a Specific Language:
`etags -l python *.py`
Generates a 'tags' file for Python files.

The `tags` file generated by `etags` is a plain text file. Each line represents a tag and contains the following information:
tag_name: The name of the identifier (e.g., function name, variable name).
file_name: The name of the source file where the identifier is defined.
search_pattern: A search pattern that can be used to locate the identifier within the file. This is often a regular expression.
The exact format may vary slightly depending on the options used and the language being parsed.

The `etags` command has been a part of the Unix ecosystem for a long time, primarily evolving alongside text editors like Emacs. Its purpose is to bridge the gap between the editor and source code by creating indexed tags, enabling efficient code browsing. Over the years, it has been enhanced to support more programming languages and features, reflecting the continuous development in software engineering practices.

ctags(1), vi(1), emacs(1)