Before  dpkg-source,  dpkg-gencontrol  and  dpkg-genchanges write their
       control information (to the source control file  .dsc  for  dpkg-source
       and  to  standard  output for dpkg-gencontrol and dpkg-genchanges) they
       perform some variable substitutions on the output file.

       A variable substitution has the form ${variable-name}.  Variable  names
       consist of alphanumerics, hyphens and colons and start with an alphanu-
       meric. Variable substitutions are performed repeatedly until  none  are
       left; the full text of the field after the substitution is rescanned to
       look for more substitutions.

       After all the substitutions have  been  done  each  occurrence  of  the
       string  ${}  (which  is  not a legal substitution) is replaced with a $

       While variable substitution is done on  all  control  fields,  some  of
       those fields are used and needed during the build when the substitution
       did not yet occur. That's why you can't use variables in  the  Package,
       Source and Architecture fields.

       Variable  substitution  happens on the content of the fields after they
       have been parsed, thus if you want a variable to expand  over  multiple
       lines  you  do  not  have to include a space after the newline. This is
       done implicitly when the field is output. For example, if the  variable
       ${Description}  is  set  to "foo is bar.${Newline}foo is great." and if
       you have the following field:

        Description: foo application
         More text.

       It will result in:

        Description: foo application
         foo is bar.
         foo is great.
         More text.

       Variables can be set using the -V common option. They can be also spec-
       ified in the file debian/substvars (or whatever other file is specified
       using the  -T  option).  This  file  consists  of  lines  of  the  form
       name=value.   Trailing  whitespace on each line, blank lines, and lines
       starting with a # symbol (comments) are ignored.

       Additionally, the following standard variables are available:

       Arch   The current host architecture (i.e. the architecture the package
              is being built for, the equivalent of DEB_HOST_ARCH).

              The source package version (from the changelog file). This vari-
              able is now deprecated as its  meaning  is  different  from  its
              function,  please  use  the  source:Version or binary:Version as

              The approximate total size of  the  package's  installed  files.
              This  value is copied into the corresponding control file field;
              setting it will modify the value of that field. If this variable
              is  not  set  dpkg-gencontrol  will compute the default value by
              accumulating the size of each regular file and  symlink  rounded
              to  1  KiB  used  units,  and  a baseline of 1 KiB for any other
              filesystem object type.

              Note: Take into account that this can only ever be an approxima-
              tion,  as  the  actual  size  used  on the installed system will
              depend greatly on the filesystem used and its parameters,  which
              might  end up using either more or less space than the specified
              in this field.

              Additional disk space used when the  package  is  installed.  If
              this  variable  is  set  its  value  is  added  to  that  of the
              Installed-Size variable (whether set  explicitly  or  using  the
              default  value) before it is copied into the Installed-Size con-
              trol file field.

              The value of the output field fieldname (which must be given  in
              the  canonical  capitalisation).  Setting these variables has no
              effect other than on places where they are expanded explicitly.

       Format The .changes file format version generated by  this  version  of
              the  source packaging scripts. If you set this variable the con-
              tents of the Format field in the .changes file will change too.

       Newline, Space, Tab
              These variables each hold the corresponding character.

              Variable settings with names  of  this  form  are  generated  by

              The upstream version of dpkg.

              The full version of dpkg.

       If a variable is referred to but not defined it generates a warning and
       an empty value is assumed.

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