Merge branch 'lilypond/translation' into staging

[lilypond.git] / Documentation / usage / running.itely

@c -*- coding: utf-8; mode: texinfo; -*-

@ignore
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    When revising a translation, copy the HEAD committish of the
    version that you are working on.  For details, see the Contributors'
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@c \version "2.15.18"


@node Running lilypond
@chapter Running @command{lilypond}

This chapter details the technicalities of running LilyPond.

@menu
* Normal usage::
* Command-line usage::
* Error messages::
* Common errors::
@end menu


@node Normal usage
@section Normal usage

Most users run LilyPond through a GUI; if you have not done so
already, please read the @rlearning{Tutorial}.  If you use an alternate
editor to write lilypond files, see the documentation for that
program.


@node Command-line usage
@section Command-line usage

This section contains extra information about using LilyPond on the
command-line.  This may be desirable to pass extra options to the
program.  In addition, there are certain extra @q{helper} programs (such
as @code{midi2ly}) which are only available on the command-line.

By @q{command-line}, we mean the command line in the operating system.
Windows users might be more familiar with the terms @q{DOS shell} or
@q{command shell}.  MacOS@tie{}X users might be more familiar with the terms
@q{terminal} or @q{console}.  Some additional setup is required
for MacOS@tie{}X users; please see @rweb{MacOS X}.

Describing how to use this part of an operating system is outside the
scope of this manual; please consult other documentation on this topic
if you are unfamiliar with the command-line.

@menu
* Invoking lilypond::
* Command line options for lilypond::
* Environment variables::
* LilyPond in chroot jail::
@end menu

@node Invoking lilypond
@unnumberedsubsec Invoking @command{lilypond}

The @command{lilypond} executable may be called as follows from
the command line.

@example
lilypond [@var{option}]@dots{} @var{file}@dots{}
@end example


When invoked with a filename that has no extension, the @file{.ly}
extension is tried first.  To read input from stdin, use a
dash (@code{-}) for @var{file}.

When @file{filename.ly} is processed it will produce @file{filename.ps}
and @file{filename.pdf} as output.  Several files can be specified;
they will each be processed independently.  @footnote{The status of
GUILE is not reset after processing a @code{.ly} file, so be careful
not to change any system defaults from within Scheme.}

If @file{filename.ly} contains more than one @code{\book}
block, then the rest of the scores will be output in numbered files,
starting with @file{filename-1.pdf}.  In addition, the value of
@code{output-suffix} will be inserted between the basename and the
number.  An input file containing

@example
#(define output-suffix "violin")
\score @{ @dots{} @}
#(define output-suffix "cello")
\score @{ @dots{} @}
@end example

@noindent
will output @var{base}@file{-violin.pdf} and
@var{base}@file{-cello-1.pdf}.


@unnumberedsubsubsec Standard shell commands

If your shell (i.e. command window) supports normal redirects,
then you might find it useful to use the following commands to
redirect console output to a file:

@itemize

@item
@code{lilypond file.ly 1>stdout.log} to redirect normal output

@item
@code{lilypond file.ly 2>stderr.log} to redirect error messages

@item
@code{lilypond file.ly &>all.log} to redirect all output

@end itemize

Consult the documentation for your shell to see if it supports these
options, or if the syntax is different.  Note that these are shell
commands and have nothing to do with lilypond.


@node Command line options for lilypond
@unnumberedsubsec Command line options for @command{lilypond}

@cindex Invoking @command{lilypond}
@cindex command line options for @command{lilypond}
@cindex options, command line
@cindex switches

The following options are supported:

@table @code

@item -e,--evaluate=@var{expr}
Evaluate the Scheme @var{expr} before parsing any @file{.ly} files.
Multiple @option{-e} options may be given, they will be evaluated
sequentially.

The expression will be evaluated in the @code{guile-user} module, so
if you want to use definitions in @var{expr}, use

@example
lilypond -e '(define-public a 42)'
@end example

@noindent
on the command-line, and include

@example
#(use-modules (guile-user))
@end example

@warning{Windows users must use double quotes instead of single quotes.}

@noindent
at the top of the @code{.ly} file.

@item -f,--format=@var{format}
which formats should be written.  Choices for @code{format} are
@code{ps}, @code{pdf}, and @code{png}.

Example: @code{lilypond -fpng @var{filename}.ly}



@item -d,--define-default=@var{var}=@var{val}
This sets the internal program option @var{var} to the Scheme value
@var{val}.  If @var{val} is not supplied, then @var{#t} is used.  To
switch off an option, @code{no-} may be prefixed to @var{var}, e.g.

@cindex point and click, command line

@example
-dno-point-and-click
@end example

@noindent
is the same as
@example
-dpoint-and-click=#f
@end example

The following options are supported:

@cindex help, command line

@table @code

@item help
Running @code{lilypond -dhelp} will print all of the @option{-d} options
available.

@cindex paper-size, command line

@item paper-size
This option sets the default paper-size,
@example
-dpaper-size=\"letter\"
@end example

@noindent
Note that the string must be enclosed in escaped quotes ( @code{\"} ).
@c Match " in previous line to help context-sensitive editors

@cindex safe, command line

@item safe
Do not trust the @code{.ly} input.

When LilyPond formatting is available through a web server, either the
@option{--safe} or the @option{--jail} option @b{MUST} be passed.  The
@option{--safe} option will prevent inline Scheme code from wreaking
havoc, for example

@quotation
@verbatim
#(system "rm -rf /")
{
  c4^$(ly:gulp-file "/etc/passwd")
}
@end verbatim
@end quotation

The @option{-dsafe} option works by evaluating in-line Scheme
expressions in a special safe module.  This safe module is derived from
GUILE @file{safe-r5rs} module, but adds a number of functions of the
LilyPond API.  These functions are listed in @file{scm/safe-lily.scm}.

In addition, safe mode disallows @code{\include} directives and
disables the use of backslashes in @TeX{} strings.

In safe mode, it is not possible to import LilyPond variables into
Scheme.

@option{-dsafe} does @emph{not} detect resource overuse.  It is still
possible to make the program hang indefinitely, for example by feeding
cyclic data structures into the backend.  Therefore, if using LilyPond
on a publicly accessible webserver, the process should be limited in
both CPU and memory usage.

Safe mode will prevent many useful LilyPond snippets from being
compiled.  The @option{--jail} is a more secure alternative, but
requires more work to set up.

@cindex output format, setting

@item backend
the output format to use for the backend.  Choices for @code{format}
are:

@table @code
@item ps

@cindex PostScript output

for PostScript.

Postscript files include TTF, Type1 and OTF fonts.  No subsetting of
these fonts is done.  When using oriental character sets, this can lead
to huge files.

@item eps

@cindex Postscript, encapsulated
@cindex EPS (Encapsulated PostScript)

for encapsulated PostScript.  This dumps every page (system) as a
separate @file{EPS} file, without fonts, and as one collated @file{EPS}
file with all pages (systems) including fonts.

This mode is used by default by @command{lilypond-book}.

@item svg

@cindex SVG (Scalable Vector Graphics)

for SVG (Scalable Vector Graphics).

This creates a single SVG file, without embedded fonts, for every page
of output.  It is recommended to install the Century Schoolbook fonts,
included with your LilyPond installation, for optimal rendering.  Under
UNIX, simply copy these fonts from the LilyPond directory (typically
@file{/usr/share/lilypond/VERSION/fonts/otf/}) to @file{~/.fonts/}.  The
SVG output should be compatible with any SVG editor or user agent.

@item scm

@cindex Scheme dump

for a dump of the raw, internal Scheme-based drawing commands.

@item null
do not output a printed score; has the same effect as
@option{-dno-print-pages}.
@end table

Example: @code{lilypond -dbackend=svg @var{filename}.ly}

@item preview

@cindex preview, command line

This option is supported by all backends; @code{pdf}, @code{png},
@code{ps}, @code{eps} and @code{svg}, but not @code{scm} and generates
an output file containing the titles and the first system of music.  If
@code{\bookpart} blocks are used, the titles and first system of every
@code{\bookpart} will appear in the output.

An additional file in the form @code{myFile.preview.extensio} is
generated, to avoid this use the additional @option{-dprint-pages} or
@option{-dno-print-pages} options according to your requirements.

@item gui
Runs silently and redirect all output to a log file.

Note to Windows users: By default @code{lilypond.exe} outputs all
progress information to the command window, @code{lilypond-windows.exe}
does not and returns a prompt, with no progress information, immediately
at the command line.  The @option{-dgui} option can be used in this case
to redirect output to a log file.

@item print-pages
Generate the full pages, the default.  @option{-dno-print-pages} is
useful in combination with @option{-dpreview}.

@end table

@item -h,--help
Show a summary of usage.

@item -H,--header=@var{FIELD}
Dump a header field to file @file{BASENAME.@var{FIELD}}.

@cindex file searching
@cindex search path

@item --include, -I=@var{directory}
Add @var{directory} to the search path for input files.

Multiple -I options may be given.  The search will start in the first
defined directory, and if the file to be included is not found the
search will continue in subsequent directories.

@item -i,--init=@var{file}
Set init file to @var{file} (default: @file{init.ly}).

@cindex loglevel
@cindex output verbosity, setting

@item -l,--loglevel=@var{LEVEL}
Set the verbosity of the console output to @var{LEVEL}. Possible values
are:

@table @code

@item NONE
No output at all, not even error messages.

@item ERROR
Only error messages, no warnings or progress messages.

@item WARN
Warnings and error messages, no progress.

@item BASIC_PROGRESS
Basic progress messages (success), warnings and errors.

@item PROGRESS
All progress messages, warnings and errors.

@item INFO (default)
Progress messages, warnings, errors and further execution information.

@item DEBUG
All possible messages, including verbose debug output.
@end table

@cindex folder, directing output to
@cindex output filename, setting

@item -o,--output=@var{FILE} or @var{FOLDER}
Set the default output file to @var{FILE} or, if a folder with that name
exists, direct the output to @var{FOLDER}, taking the file name from the
input file.  The appropriate suffix will be added (e.g. @code{.pdf} for
pdf) in both cases.


@cindex PostScript output

@item --ps
Generate PostScript.

@cindex Portable Network Graphics (PNG) output

@item --png
Generate pictures of each page, in PNG format.  This implies
@option{--ps}.  The resolution in DPI of the image may be set with
@example
-dresolution=110
@end example

@cindex Portable Document Format (PDF) output

@item --pdf
Generate PDF.  This implies @option{--ps}.



@item -j,--jail=@var{user},@var{group},@var{jail},@var{dir}
Run @command{lilypond} in a chroot jail.

The @option{--jail} option provides a more flexible alternative to
@option{--safe} when LilyPond formatting is available through a web
server or whenever LilyPond executes externally provided
sources.

The @option{--jail} option works by changing the root of
@command{lilypond} to @var{jail} just before starting the actual
compilation process.  The user and group are then changed to match those
provided, and the current directory is changed to @var{dir}.  This setup
guarantees that it is not possible (at least in theory) to escape from
the jail.  Note that for @option{--jail} to work @command{lilypond} must
be run as root, which is usually accomplished in a safe way using
@command{sudo}.

Setting up a jail is a slightly delicate matter, as we must be sure that
LilyPond is able to find whatever it needs to compile the source
@emph{inside the jail}.  A typical setup comprises the following items:

@table @asis
@item Setting up a separate filesystem
A separate filesystem should be created for LilyPond, so that it can be
mounted with safe options such as @code{noexec}, @code{nodev}, and
@code{nosuid}.  In this way, it is impossible to run executables or to
write directly to a device from LilyPond.  If you do not want to create a
separate partition, just create a file of reasonable size and use it to
mount a loop device.  A separate filesystem also guarantees that LilyPond
cannot write more space than it is allowed.

@item Setting up a separate user
A separate user and group (say, @code{lily}/@code{lily}) with low
privileges should be used to run LilyPond inside the jail.  There should
be a single directory writable by this user, which should be passed in
@var{dir}.

@item Preparing the jail
LilyPond needs to read a number of files while running.  All these files
are to be copied into the jail, under the same path they appear in the
real root filesystem.  The entire content of the LilyPond installation
(e.g., @file{/usr/share/lilypond})
should be copied.

If problems arise, the simplest way to trace them down is to run
LilyPond using @command{strace}, which will allow you to determine which
files are missing.

@item Running LilyPond
In a jail mounted with @code{noexec} it is impossible to execute any external
program.  Therefore LilyPond must be run with a backend that does not
require any such program.  As we already mentioned, it must be also run
with superuser privileges (which, of course, it will lose immediately),
possibly using @command{sudo}.  It is a good idea to limit the number of
seconds of CPU time LilyPond can use (e.g., using @command{ulimit
-t}), and, if your operating system supports it, the amount of memory
that can be allocated.
@end table


@item -v,--version
Show version information.

@item -V,--verbose
Be verbose: show full paths of all files read, and give timing
information.

@item -w,--warranty
Show the warranty with which GNU LilyPond comes.  (It comes with
@strong{NO WARRANTY}!)
@end table


@node Environment variables
@unnumberedsubsec Environment variables


@cindex LANG
@cindex LILYPOND_DATADIR

@command{lilypond} recognizes the following environment variables:
@table @code
@item LILYPOND_DATADIR
This specifies a directory where locale messages and
data files will be looked up by default.  The directory should contain
subdirectories called @file{ly/}, @file{ps/}, @file{tex/}, etc.

@item LANG
This selects the language for the warning messages.

@item LILYPOND_LOGLEVEL
The default loglevel. If LilyPond is called without an explicit loglevel (i.e.
no @option{--loglevel} command line option), this value is used.

@item LILYPOND_GC_YIELD
A variable, as a percentage, that tunes memory management
behavior.  A higher values means the program uses more memory, a
smaller value means more CPU time is used.  The default value is
@code{70}.

@end table


@node LilyPond in chroot jail
@unnumberedsubsec LilyPond in chroot jail

Setting up the server to run LilyPond in a chroot jail is a complicated
task.  The steps are listed below.  Examples in the steps are from
Ubuntu Linux, and may require the use of @code{sudo} as appropriate.

@itemize

@item Install the necessary packages: LilyPond, GhostScript, and ImageMagick.

@item Create a new user by the name of @code{lily}:

@example
adduser lily
@end example

@noindent
This will create a new group for the @code{lily} user as well, and a home folder,
@code{/home/lily}

@item In the home folder of the @code{lily} user create a file to use as a
separate filesystem:

@example
dd if=/dev/zero of=/home/lily/loopfile bs=1k count= 200000
@end example

@noindent
This example creates a 200MB file for use as the jail filesystem.

@item Create a loop device, make a file system and mount it, then create
a folder that can be written by the @code{lily} user:

@example
mkdir /mnt/lilyloop
losetup /dev/loop0 /home/lily/loopfile
mkfs -t ext3 /dev/loop0 200000
mount -t ext3 /dev/loop0 /mnt/lilyloop
mkdir /mnt/lilyloop/lilyhome
chown lily /mnt/lilyloop/lilyhome
@end example

@item In the configuration of the servers, the JAIL will be @code{/mnt/lilyloop}
and the DIR will be @code{/lilyhome}.

@item Create a big directory tree in the jail by copying the necessary files, as
shown in the sample script below.

You can use @code{sed} to create the necessary copy commands for a given
executable:

@example
for i in "/usr/local/lilypond/usr/bin/lilypond" "/bin/sh" "/usr/bin/;  \
  do ldd $i | sed 's/.*=> \/\(.*\/\)\([^(]*\).*/mkdir -p \1 \&\&  \
    cp -L \/\1\2 \1\2/' | sed 's/\t\/\(.*\/\)\(.*\) (.*)$/mkdir -p  \
      \1 \&\& cp -L \/\1\2 \1\2/' | sed '/.*=>.*/d'; done
@end example

@end itemize

@subheading Example script for 32-bit Ubuntu 8.04

@example
#!/bin/sh
## defaults set here

username=lily
home=/home
loopdevice=/dev/loop0
jaildir=/mnt/lilyloop
# the prefix (without the leading slash!)
lilyprefix=usr/local
# the directory where lilypond is installed on the system
lilydir=/$lilyprefix/lilypond/

userhome=$home/$username
loopfile=$userhome/loopfile
adduser $username
dd if=/dev/zero of=$loopfile bs=1k count=200000
mkdir $jaildir
losetup $loopdevice $loopfile
mkfs -t ext3 $loopdevice 200000
mount -t ext3 $loopdevice $jaildir
mkdir $jaildir/lilyhome
chown $username $jaildir/lilyhome
cd $jaildir

mkdir -p bin usr/bin usr/share usr/lib usr/share/fonts $lilyprefix tmp
chmod a+w tmp

cp -r -L $lilydir $lilyprefix
cp -L /bin/sh /bin/rm bin
cp -L /usr/bin/convert /usr/bin/gs usr/bin
cp -L /usr/share/fonts/truetype usr/share/fonts

# Now the library copying magic
for i in "$lilydir/usr/bin/lilypond" "$lilydir/usr/bin/guile" "/bin/sh"  \
  "/bin/rm" "/usr/bin/gs" "/usr/bin/convert"; do ldd $i | sed 's/.*=>  \
    \/\(.*\/\)\([^(]*\).*/mkdir -p \1 \&\& cp -L \/\1\2 \1\2/' | sed  \
      's/\t\/\(.*\/\)\(.*\) (.*)$/mkdir -p \1 \&\& cp -L \/\1\2 \1\2/'  \
        | sed '/.*=>.*/d'; done | sh -s

# The shared files for ghostscript...
      cp -L -r /usr/share/ghostscript usr/share
# The shared files for ImageMagick
      cp -L -r /usr/lib/ImageMagick* usr/lib

### Now, assuming that you have test.ly in /mnt/lilyloop/lilyhome,
### you should be able to run:
### Note that /$lilyprefix/bin/lilypond is a script, which sets the
### LD_LIBRARY_PATH - this is crucial
      /$lilyprefix/bin/lilypond -jlily,lily,/mnt/lilyloop,/lilyhome test.ly
@end example

@c " keep quote signs balanced for context-sensitive editors

@node Error messages
@section Error messages

@cindex error messages
Different error messages can appear while compiling a file:

@table @emph

@item Warning
@cindex warning
Something looks suspect.  If you are requesting something out of the
ordinary then you will understand the message, and can ignore it.
However, warnings usually indicate that something is wrong with the
input file.

@item Error
@cindex error
Something is definitely wrong.  The current processing step (parsing,
interpreting, or formatting) will be finished, but the next step will
be skipped.

@item Fatal error
@cindex fatal error
Something is definitely wrong, and LilyPond cannot continue.  This
happens rarely.  The most usual cause is misinstalled fonts.

@item Scheme error
@cindex trace, Scheme
@cindex call trace
@cindex Scheme error
Errors that occur while executing Scheme code are caught by the Scheme
interpreter.  If running with the verbose option (@option{-V} or
@option{--verbose}) then a call trace of the offending
function call is printed.

@item Programming error
@cindex Programming error
There was some internal inconsistency.  These error messages are
intended to help the programmers and debuggers.  Usually, they can be
ignored.  Sometimes, they come in such big quantities that they obscure
other output.

@item Aborted (core dumped)
@cindex Aborted (core dumped)
This signals a serious programming error that caused the program to
crash.  Such errors are considered critical.  If you stumble on one,
send a bug-report.
@end table

@cindex errors, message format
If warnings and errors can
be linked to some part of the input file, then error messages have the
following form

@example
@var{filename}:@var{lineno}:@var{columnno}: @var{message}
@var{offending input line}
@end example

A line-break is inserted in the offending line to indicate the column
where the error was found.  For example,

@example
test.ly:2:19: error: not a duration: 5
  @{ c'4 e'
           5 g' @}
@end example

These locations are LilyPond's best guess about where the warning or
error occurred, but (by their very nature) warnings and errors occur
when something unexpected happens.  If you can't see an error in the
indicated line of your input file, try checking one or two lines
above the indicated position.

More information about errors is given in @ref{Common errors}.


@node Common errors
@section Common errors

The error conditions described below occur often, yet the cause
is not obvious or easily found.  Once seen and understood, they
are easily handled.


@menu
* Music runs off the page::
* An extra staff appears::
* Apparent error in ../ly/init.ly::
* Error message Unbound variable %::
* Error message FT_Get_Glyph_Name::
* Warning staff affinities should only decrease::
@end menu

@node Music runs off the page
@unnumberedsubsec Music runs off the page

Music running off the page over the right margin or appearing
unduly compressed is almost always due to entering an incorrect
duration on a note, causing the final note in a measure to extend
over the bar line.  It is not invalid if the final note in a
measure does not end on the automatically entered bar line, as the
note is simply assumed to carry over into the next measure.  But
if a long sequence of such carry-over measures occurs the music
can appear compressed or may flow off the page because automatic
line breaks can be inserted only at the end of complete measures,
i.e., where all notes end before or at the end of the measure.

@warning{An incorrect duration can cause line breaks to be
inhibited, leading to a line of highly compressed music or
music which flows off the page.}

The incorrect duration can be found easily if bar checks are used,
see @ruser{Bar and bar number checks}.

If you actually intend to have a series of such carry-over measures
you will need to insert an invisible bar line where you want the
line to break.  For details, see @ruser{Bar lines}.


@node An extra staff appears
@unnumberedsubsec An extra staff appears

If contexts are not created explicitly with @code{\new} or
@code{\context}, they will be silently created as soon as a
command is encountered which cannot be applied to an existing
context.  In simple scores the automatic creation of contexts is
useful, and most of the examples in the LilyPond manuals take
advantage of this simplification.  But occasionally the silent
creation of contexts can give rise to unexpected new staves or
scores.  For example, it might be expected that the following code
would cause all note heads within the following staff to be
colored red, but in fact it results in two staves with the note
heads remaining the default black in the lower staff.

@lilypond[quote,verbatim,relative=2]
\override Staff.NoteHead #'color = #red
\new Staff { a }
@end lilypond

This is because a @code{Staff} context does not exist when the
override is processed, so one is implicitly created and the override
is applied to it, but then the @code{\new Staff} command creates
another, separate, staff into which the notes are placed.  The
correct code to color all note heads red is

@lilypond[quote,verbatim,relative=2]
\new Staff {
  \override Staff.NoteHead #'color = #red
  a
}
@end lilypond

As a second example, if a @code{\relative} command is placed inside
a @code{\repeat} command, two staves result, the second offset from
the first, because the @code{\repeat} command generates two
@code{\relative} blocks, which each implicitly create @code{Staff}
and @code{Voice} blocks.

@lilypond[quote,verbatim]
\repeat unfold 2 {
  \relative c' { c4 d e f }
}
@end lilypond

Explicitly instantiating the @code{Voice} context fixes the
problem:

@lilypond[quote,verbatim]
\new Voice {
  \repeat unfold 2 {
    \relative c' { c4 d e f }
  }
}
@end lilypond


@node Apparent error in ../ly/init.ly
@unnumberedsubsec Apparent error in @code{../ly/init.ly}

Various obscure error messages may appear about syntax errors in
@file{../ly/init.ly} if the input file is not correctly formed,
for example, if it does not contain correctly
matched braces or quote signs.

The most common error is a missing brace, (@code{@}}), at the end of
a @code{score} block.  Here the solution is obvious: check the
@code{score} block is correctly terminated.  The correct structure
of an input file is described in @rlearning{How LilyPond input files work}.
Using an editor which automatically highlights matching brackets and
braces is helpful to avoid such errors.

A second common cause is no white space between the last syllable
of a lyrics block and the terminating brace, (@code{@}}).  Without
this separation the brace is taken to be part of the syllable.  It
is always advisable to ensure there is white space before and after
@emph{every} brace.  For the importance of this when using lyrics,
see @ruser{Entering lyrics}.

This error message can also appear if a terminating quote sign,
(@code{"}), is omitted.  In this case an accompanying error message
@c keep "-matching straight in fancy editors
should give a line number close to the line in error.  The
mismatched quote will usually be on the line one or two above.

@node Error message Unbound variable %
@unnumberedsubsec Error message Unbound variable %

This error message will appear at the bottom of the console
output or log file together with a @qq{GUILE signalled an error @dots{}}
message every time a Scheme routine is called which (invalidly)
contains a @emph{LilyPond} rather than a @emph{Scheme} comment.

LilyPond comments begin with a percent sign, (@code{%}), and must
not be used within Scheme routines.  Scheme comments begin with a
semi-colon, (@code{;}).

@node Error message FT_Get_Glyph_Name
@unnumberedsubsec Error message FT_Get_Glyph_Name

This error messages appears in the console output or log file if
an input file contains a non-ASCII character and was not saved in
UTF-8 encoding.  For details, see @ruser{Text encoding}.


@node Warning staff affinities should only decrease
@unnumberedsubsec Warning staff affinities should only decrease
This warning can appear if there are no staves in the printed
output, for example if there are just a @code{ChordName} context
and a @code{Lyrics} context as in a lead sheet.  The warning
messages can be avoided by making one of the contexts behave as a
staff by inserting

@example
\override VerticalAxisGroup #'staff-affinity = ##f
@end example

@noindent
at its start.  For details, see @qq{Spacing of non-staff lines} in
@ruser{Flexible vertical spacing within systems}.