9c620cab64ca86a0044de2c4a7c610b13332558e
max
  Mon Sep 7 19:43:50 2026 -0700
trackDb/README: document the braced ${name} form only

The bare $name form still works and old pages use it, but braces are the form
worth writing. Inside braces any character is allowed up to the closing brace,
so ${name} can carry a structured name like ${hgTrackUi/caddSuper} if we ever
want one, while $name stops at the first character outside [0-9A-Za-Z_] and
cannot express it. Braces also settle the adjacency case: ${db}Something is
unambiguous, $dbSomething reads as one long name.

Every variable in the list is now written braced, and the closing paragraph says
not to write or document the bare form in anything new.

Also spells out why ${hgsid} is a hub page variable in practice. Native trackDb
html is substituted once by hgTrackDb when it loads trackDb, and that happens
without a cart, so there is no session id to put there. Only hgc and hgTrackUi
substitute with a cart, at render time.

One non-variable change: 'an $otherDb field' becomes 'an otherDb field'. That is
the name of a .ra setting rather than a substitution, and leaving a dollar on it
next to a braces-only rule would only confuse.

refs #38283

diff --git src/hg/makeDb/trackDb/README src/hg/makeDb/trackDb/README
index 8bbd24333bc..c256d9bf542 100644
--- src/hg/makeDb/trackDb/README
+++ src/hg/makeDb/trackDb/README
@@ -1,155 +1,162 @@
 This directory contains the track database for
 the UCSC human genome browser.  The program
 hgTrackDb converts the information here into
 a 'trackDb' table in the assembly's MySQL database.
 
 SEE ALSO: source tree file: src/product/README.trackDb
 	  for a general discussion of developing tracks.
 
           trackDb.ra documentation:
           http://genome.ucsc.edu/goldenPath/help/trackDb/trackDbDoc.html
 
 	  http://genome.ucsc.edu/admin/hgFindSpecHowTo.html
 	  for instructions on hgFindSpec.
 
 The bulk of the information about a track is
 contained in the trackDb.ra file.  Tracks are
 separated from each other in this file by
 a blank line.  Each line begins with an
 attribute name and is followed by attribute
 value separated by white space.
 
 The html text about a track is in separate
 files named xxx.html where xxx is a
 track name.  The contents of the HTML is subject
 to variable substitution as described below.
 Note that the html file in lower-level directories
 can override the html in higher-level directories
 without requiring an entry in trackDb.ra.
 
 There may be subdirectories for each organism such
 as 'mouse', and underneath these directories for
 each database, such as 'mm10'.  There are a trackDb.ra
 file and xxx.html files in these subdirectories.
 Track descriptions in the subdirectory will
 over-ride track descriptions in the parent directory.
 This allows you to customize a track for
 each version of the database.
 
 xxx.html files may include the contents of other files
 using this syntax:
 
 <!--#insert file="sharedText.html"-->
 
 where "sharedText.html" is a relative path, relative to
 the directory that contains the xxx.html file.
 This can also be made conditional on a database:
 
 <!-- #if (db==hg19) #insert file="human/hg19/specialNote.html" -->
 
 Variable substitution:
 
 The shortLabel and longLabel fields and the associated HTML files
 may have the following variables, which will be substituted:
-     $ORGANISM - all upper case organism, like 'MOUSE'
-     $Organism - initial capped organism, like 'Mouse'
-     $organism - all lower case organism, like 'mouse'
-     $db - database (like mm3, hg15, etc.)
-     $date - freeze date of underlying assembly
-     $blurb - If there is a blurb field in the .ra file this echos it.
-     $matrix - content of the matrix and optional matrixHeader trackDb setting
+     ${ORGANISM} - all upper case organism, like 'MOUSE'
+     ${Organism} - initial capped organism, like 'Mouse'
+     ${organism} - all lower case organism, like 'mouse'
+     ${db} - database (like mm3, hg15, etc.)
+     ${date} - freeze date of underlying assembly
+     ${blurb} - If there is a blurb field in the .ra file this echos it.
+     ${matrix} - content of the matrix and optional matrixHeader trackDb setting
                which will be converted to an HTML table. If there is no matrix
                setting, an empty string is substituted.
-     $chainMinScore -  value that gets substituted into this statement on the
+     ${chainMinScore} -  value that gets substituted into this statement on the
                chain or chainNet html page: 'Chains scoring below a minimum
-               score of "$chainMinScore" were discarded'.
-     $chainLinearGap - value for the -linearGap matrix used with axtChain
+               score of "${chainMinScore}" were discarded'.
+     ${chainLinearGap} - value for the -linearGap matrix used with axtChain
                (e.g. loose, medium). Gets substituted into the chain or
                chainNet html page.
-     $downloadsServer - the value of the hg.conf downloads.server variable, or
+     ${downloadsServer} - the value of the hg.conf downloads.server variable, or
                          hgdownload.soe.ucsc.edu if not set.
-     $track - the track's own name, as hgTrackUi's g= parameter needs it. For a
-               hub track that includes the hub_<id>_ prefix.
-     $parentTrack - the name of the container the track sits in, a superTrack or
-               a composite, in the same form as $track. Views are skipped, since
-               a view has no description page of its own. For a track that is
-               not in a container this is the same as $track. Useful for linking
-               a subtrack's description page back to its container, e.g.
-               <a href="hgTrackUi?db=$db&amp;g=$parentTrack">.
-     $hgsid - the current session id. Substituted only where there is a cart,
-               that is by the CGIs; hgTrackDb leaves it empty, so it is of use
-               in hub description pages rather than in native trackDb.
-In addition, if there is an $otherDb field set in the .ra file, these
+     ${track} - the track's own name, as hgTrackUi's g= parameter needs it. For
+               a hub track that includes the hub_<id>_ prefix.
+     ${parentTrack} - the name of the container the track sits in, a superTrack
+               or a composite, in the same form as ${track}. Views are skipped,
+               since a view has no description page of its own. For a track that
+               is not in a container this is the same as ${track}. Useful for
+               linking a subtrack's description page back to its container, e.g.
+               <a href="hgTrackUi?db=${db}&amp;g=${parentTrack}">.
+     ${hgsid} - the current session id. Only a CGI has a cart to take this
+               from. Native trackDb html is substituted once by hgTrackDb when
+               it loads trackDb, and that happens without a cart, so ${hgsid}
+               is empty there. It is meant for hub description pages, which are
+               substituted at render time by hgc and hgTrackUi instead.
+In addition, if there is an otherDb field set in the .ra file, these
 variables are available:
-     $o_ORGANISM - all upper case other organism, like 'MOUSE'
-     $o_Organism - initial capped other organism, like 'Mouse'
-     $o_organism - all lower case other organism, like 'mouse'
-     $o_db - other database (like mm3, hg15, etc.)
-     $o_date - freeze date of underlying other assembly
+     ${o_ORGANISM} - all upper case other organism, like 'MOUSE'
+     ${o_Organism} - initial capped other organism, like 'Mouse'
+     ${o_organism} - all lower case other organism, like 'mouse'
+     ${o_db} - other database (like mm3, hg15, etc.)
+     ${o_date} - freeze date of underlying other assembly
 
 Any other ra fields may be referenced as a variable.
-The reference can be in the form $name or ${name}.  Without
-the braces, name is terminated by a character other than
-[0-9A-Za-Z_].  A literal $ is represented as $$.
+
+Always write the braces.  The bare form $name also works, terminated by a
+character other than [0-9A-Za-Z_], and older pages use it, but do not write it
+in anything new and do not document it.  Inside braces any character is allowed
+up to the closing brace, so ${name} leaves room for structured variable names
+later, something $name cannot express.  It also settles what happens when a
+reference is followed by a letter: ${db}Something is unambiguous, $dbSomething
+is read as one long name.  A literal $ is represented as $$.
 
 Here are some details on how to load various track types into the database
 tables:
 
 - type bedGraph [column]
 In this case, the data table is a bed type of table loaded
 with hgLoadBed.  The [column] specified is a numeric column of data
 in the table to be used for the graphing value.  The default column
 to graph would be column five, the 'score'.  All graphing options as
 described in 'type wig' apply to this type of track.
 
 - type bigBed <fieldCount> <extraInfo>
 This uses a binary indexed file rather than a database table.  It is, other than
 substituting "bigBed" for "bed" in the type line, the same as the "type bed".
 
 The database loading procedure for the table is:
  hgBbiDbLink hg18 itemRgbBbTest /gbdb/hg18/bbi/itemRgb.hg18.bb
 The file name given should be a full path name that can be accessed
 by the Apache WEB server.  Or, the file name can also be a
 valid URL to the bb file.
 This loader is equivalent to:
  hgsql hg18 -e 'drop table if exists itemRgbBbTest;
   create table itemRgbBbTest (fileName varchar(255) not null);
   insert into itemRgbBbTest values
   ("/gbdb/hg18/bbi/itemRgb.hg18.bb");'
 
 - type bigWig [lower] [upper]
 This uses a binary indexed file rather than a database table.  It is, other
 than substituting "bigWig" for "wig" in the type line, the same as the
 "type wig".  
 
 The database loading procedure is:
  hgBbiDbLink hg19 myLocalBigWig /gbdb/hg19/bbi/myLocalBigWig.bw
 The file name given should be a full path name that can be accessed
 by the Apache WEB server.  Or, the file name can also be a
 valid URL to the bw file.
 This loader is equivalent to:
  hgsql hg19 -e 'drop table if exists myLocalBigWig; \
   create table myLocalBigWig (fileName varchar(255) not null); \
   insert into myLocalBigWig values
   ("/gbdb/hg19/bbi/myLocalBigWig.bw");'
 
 - type pgSnp
 This type is used for the display of variants (for example, personal genome
 variants).  It will display stacked boxes showing the proportion of reads
 supporting each allele when frequency is available (in case of 2 alleles
 only).  Also the details page will use the UCSC genes to compute any amino
 acid changes.  
 
 The track can be loaded using hgLoadBed and the -sqlTable 
 option (see pgSnp.sql for the format).
 
 
 After editing a track definition here, run ./loadTracks
 which invokes hgTrackDb to bring it into the MySQL database.
 To update your personal copy (on hgwdev-userName) do
     make update
 to do it on genome-test do
     make alpha
 You can also restrict a track to alpha, beta, or public
 by using the release field.