Provided by: universal-ctags_5.9.20210829.0-2_amd64 
NAME
readtags - Find tag file entries matching specified names
SYNOPSIS
readtags -h | --help
readtags (-H | --help-expression) (filter|sorter)
readtags [OPTION]... ACTION
DESCRIPTION
The readtags program filters, sorts and prints tag entries in a tags file. The basic filtering is done
using actions, by which you can list all regular tags, pseudo tags or regular tags matching specific
name. Then, further filtering and sorting can be done using post processors, namely filter expressions
and sorter expressions.
ACTIONS
-l, --list
List regular tags.
[-] NAME
List regular tags matching NAME. "-" as NAME indicates arguments after this as NAME even if they
start with -.
-D, --list-pseudo-tags
Equivalent to --list-pseudo-tags.
OPTIONS
Controlling the Tags Reading Behavior
The behavior of reading tags can be controlled using these options:
-t TAGFILE, --tag-file TAGFILE
Use specified tag file (default: "tags").
-s[0|1|2], --override-sort-detection METHOD
Override sort detection of tag file. METHOD: unsorted|sorted|foldcase
The NAME action will perform binary search on sorted (including "foldcase") tags files, which is much
faster then on unsorted tags files.
Controlling the NAME Action Behavior
The behavior of the NAME action can be controlled using these options:
-i, --icase-match
Perform case-insensitive matching in the NAME action.
-p, --prefix-match
Perform prefix matching in the NAME action.
Controlling the Output
By default, the output of readtags contains only the name, input and pattern field. The Output can be
tweaked using these options:
-d, --debug
Turn on debugging output.
-E, --escape-output
Escape characters like tabs in output as described in tags(5).
-e, --extension-fields
Include extension fields in output.
-n, --line-number
Also include the line number field when -e option is give.
About the -E option: certain characters are escaped in a tags file, to make it machine-readable. e.g.,
ensuring no tabs character appear in fields other than the pattern field. By default, readtags translates
them to make it human-readable, but when utilizing readtags output in a script or a client tool, -E
option should be used. See ctags-client-tools(7) for more discussion on this.
Filtering and Sorting
Further filtering and sorting on the tags listed by actions are performed using:
-Q EXP, --filter EXP
Filter the tags listed by ACTION with EXP before printing.
-S EXP, --sorter EXP
Sort the tags listed by ACTION with EXP before printing.
These are discussed in the EXPRESSION section.
Examples
• List all tags in "/path/to/tags":
$ readtags -t /path/to/tags -l
• List all tags in "tags" that start with "mymethod":
$ readtags -p - mymethod
• List all tags matching "mymethod", case insensitively:
$ readtags -i - mymethod
• List all tags start with "myvar", and printing all fields (i.e., the whole line):
$ readtags -p -ne - myvar
EXPRESSION
Scheme-style expressions are used for the -Q and -S options. For those who doesn't know Scheme or Lisp,
just remember:
• A function call is wrapped in a pair of parenthesis. The first item in it is the function/operator
name, the others are arguments.
• Function calls can be nested.
• Missing values and boolean false are represented by #f. #t and all other values are considered to be
true.
So, (+ 1 (+ 2 3)) means add 2 and 3 first, then add the result with 1. (and "string" 1 #t) means logical
AND on "string", 1 and #t, and the result is true since there is no #f.
Filtering
The tag entries that make the filter expression produces true value are printed by readtags.
The basic operators for filtering are eq?, prefix?, suffix?, substr?, and #/PATTERN/. Language common
fields can be accessed using variables starting with $, e.g., $language represents the language field.
For example:
• List all tags start with "myfunc" in Python code files:
$ readtags -p -Q '(eq? $language "Python")' - myfunc
downcase or upcase operators can be used to perform case-insensitive matching:
• List all tags containing "my", case insensitively:
$ readtags -Q '(substr? (downcase $name) "my")' -l
We have logical operators like and, or and not. The value of a missing field is #f, so we could deal with
missing fields:
• List all tags containing "impl" in Python code files, but allow the language: field to be missing:
$ readtags -Q '(and (substr? $name "impl")\
(or (not $language)\
(eq? $language "Python")))' -l
#/PATTERN/ is for the case when string predicates (prefix?, suffix?, and substr?) are not enough. You can
use "Posix extended regular expression" as PATTERN.
• List all tags inherits from the class "A":
$ readtags -Q '(#/(^|,) ?A(,|$)/ $inherits)' -l
Here $inherits is a comma-separated class list like "A,B,C", "P, A, Q", or just "A". Notice that this
filter works on both situations where there's a space after each comma or there's not.
Case-insensitive matching can be performed by #/PATTERN/i:
• List all tags inherits from the class "A" or "a":
$ readtags -Q '(#/(^|,) ?A(,|$)/i $inherits)' -l
To include "/" in a pattern, prefix \ to the "/".
NOTE: The above regular expression pattern for inspecting inheritances is just an example to show how to
use #/PATTERN/ expression. Tags file generators have no consensus about the format of inherits:, e.g.,
whether there should be a space after a comma. Even parsers in ctags have no consensus. Noticing the
format of the inherits: field of specific languages is needed for such queries.
The expressions #/PATTERN/ and #/PATTERN/i are for interactive use. Readtags also offers an alias
string->regexp, so #/PATTERN/ is equal to (string->regexp "PATTERN"), and #/PATTERN/i is equal to
(string->regexp "PATTERN" :case-fold #t). string->regexp doesn't need to prefix \ for including "/" in a
pattern. string->regexp may simplify a client tool building an expression. See also ctags-client-tools(7)
for building expressions in your tool.
Let's now consider missing fields. The tags file may have tag entries that has no inherits: field. In
that case $inherits is #f, and the regular expression matching raises an error, since string operators
only work for strings. To avoid this problem:
• Safely list all tags inherits from the class "A":
$ readtags -Q '(and $inherits (#/(^|,) ?A(,|$)/ $inherits))' -l
This makes sure $inherits is not missing first, then match it by regexp.
Sometimes you want to keep tags where the field is missing. For example, your want to exclude reference
tags, which is marked by the extras: field, then you want to keep tags who doesn't have extras: field
since they are also not reference tags. Here's how to do it:
• List all tags but the reference tags:
$ readtags -Q '(or (not $extras) (#/(^|,) ?reference(,|$)/ $extras))' -l
Notice that (not $extras) produces #t when $extras is missing, so the whole or expression produces #t.
Run "readtags -H filter" to know about all valid functions and variables.
Sorting
When sorting, the sorter expression is evaluated on two tag entries to decide which should sort before
the other one, until the order of all tag entries is decided.
In a sorter expression, $ and & are used to access the fields in the two tag entries, and let's call them
$-entry and &-entry. The sorter expression should have a value of -1, 0 or 1. The value -1 means the
$-entry should be put above the &-entry, 1 means the contrary, and 0 makes their order in the output
uncertain.
The core operator of sorting is <>. It's used to compare two strings or two numbers (numbers are for the
line: or end: fields). In (<> a b), if a < b, the result is -1; a > b produces 1, and a = b produces 0.
Strings are compared using the strcmp function, see strcmp(3).
For example, sort by names, and make those shorter or alphabetically smaller ones appear before the
others:
$ readtags -S '(<> $name &name)' -l
This reads "If the tag name in the $-entry is smaller, it goes before the &-entry".
The <or> operator is used to chain multiple expressions until one returns -1 or 1. For example, sort by
input file names, then line numbers if in the same file:
$ readtags -S '(<or> (<> $input &input) (<> $line &line))' -l
The *- operator is used to flip the compare result. i.e., (*- (<> a b)) is the same as (<> b a).
Filter expressions can be used in sorter expressions. The technique is use if to produce integers that
can be compared based on the filter, like:
(<> (if filter-expr-on-$-entry -1 1)
(if filter-expr-on-&-entry -1 1))
So if $-entry satisfies the filter, while &-entry doesn't, it's the same as (<> -1 1), which produces -1.
For example, we want to put tags with "file" kind below other tags, then the sorter would look like:
(<> (if (eq? $kind "file") 1 -1)
(if (eq? &kind "file") 1 -1))
A quick read tells us: If $-entry has "file" kind, and &-entry doesn't, the sorter becomes (<> 1 -1),
which produces 1, so the $-entry is put below the &-entry, exactly what we want.
Inspecting the Behavior of Expressions
The print operator can be used to print the value of an expression. For example:
$ readtags -Q '(print $name)' -l
prints the name of each tag entry before it. Since the return value of print is not #f, all the tag
entries are printed. We could control this using the begin or begin0 operator. begin returns the value of
its last argument, and begin0 returns the value of its first argument. For example:
$ readtags -Q '(begin0 #f (print (prefix? "ctags" "ct")))' -l
prints a bunch of "#t" (depending on how many lines are in the tags file), and the actual tag entries are
not printed.
SEE ALSO
See tags(5) for the details of tags file format. See ctags-client-tools(7) for the tips writing a tool utilizing tags file. The official Universal Ctags web site at: <https://ctags.io/> The git repository for the library used in readtags command: <https://github.com/universal-ctags/libreadtags>
CREDITS
Universal Ctags project
<https://ctags.io/>
Darren Hiebert < <dhiebert@users.sourceforge.net> >
<http://DarrenHiebert.com/>
The readtags command and libreadtags maintained at Universal Ctags are derived from readtags.c and
readtags.h developd at
<http://ctags.sourceforge.net> .