  
  [1X9 [33X[0;0YFiles and Filenames[133X[101X
  
  [33X[0;0YFiles  are identified by filenames, which are represented in [5XGAP[105X as strings.
  Filenames  can  be  created directly by the user or a program, but of course
  this is operating system dependent.[133X
  
  [33X[0;0YFilenames  for  some  files  can  be constructed in a system independent way
  using  the  following  functions.  This is done by first getting a directory
  object for the directory the file shall reside in, and then constructing the
  filename. However, it is sometimes necessary to construct filenames of files
  in  subdirectories  relative  to  a given directory object. In this case the
  directory separator is [13Xalways[113X [10X/[110X even under DOS or MacOS.[133X
  
  [33X[0;0YSection  [14X9.3[114X describes how to construct directory objects for the common [5XGAP[105X
  and system directories. Using the command [2XFilename[102X ([14X9.4-1[114X) it is possible to
  construct a filename pointing to a file in these directories. There are also
  functions to test for accessibility of files, see [14X9.6[114X.[133X
  
  
  [1X9.1 [33X[0;0YPortability[133X[101X
  
  [33X[0;0YFor  portability  filenames  and  directory names should be restricted to at
  most  8 alphanumerical characters optionally followed by a dot [10X.[110X and between
  1  and  3  alphanumerical  characters.  Upper case letters should be avoided
  because  some operating systems do not make any distinction between case, so
  that  [10XNaMe[110X,  [10XName[110X and [10Xname[110X all refer to the same file whereas some operating
  systems  are  case  sensitive.  To avoid problems only lower case characters
  should be used.[133X
  
  [33X[0;0YAnother function which is system-dependent is [2XLastSystemError[102X ([14X9.1-1[114X).[133X
  
  [1X9.1-1 LastSystemError[101X
  
  [33X[1;0Y[29X[2XLastSystemError[102X(  ) [32X function[133X
  
  [33X[0;0Y[2XLastSystemError[102X  returns  a record describing the last system error that has
  occurred.  This  record  contains  at least the component [10Xmessage[110X which is a
  string.  This  message  is,  however,  highly operating system dependent and
  should only be used as an informational message for the user.[133X
  
  
  [1X9.2 [33X[0;0YGAP Root Directories[133X[101X
  
  [33X[0;0YWhen  [5XGAP[105X  is  started it determines a list of directories which we call the
  [13X[5XGAP[105X  root  directories[113X.  In  a running [5XGAP[105X session this list can be found in
  [10XGAPInfo.RootPaths[110X.[133X
  
  [33X[0;0YThe  core  part  of  [5XGAP[105X  knows  which  files  to  read relative to its root
  directories.   For   example  when  [5XGAP[105X  wants  to  read  its  library  file
  [11Xlib/group.gd[111X,  it  appends this path to each path in [10XGAPInfo.RootPaths[110X until
  it  finds  the  path  of  an existing file. The first file found this way is
  read.[133X
  
  [33X[0;0YFurthermore,   [5XGAP[105X   looks   for   available   packages   by  examining  the
  subdirectories [11Xpkg/[111X in each of the directories in [10XGAPInfo.RootPaths[110X.[133X
  
  [33X[0;0YThe  root  directories  are  specified  via  one  or several of the [10X-l paths[110X
  command  line  options,  see  [14X3.1[114X. Furthermore, by default [5XGAP[105X automatically
  prepends a user specific [5XGAP[105X root directory to the list; this can be avoided
  by  calling [5XGAP[105X with the [10X-r[110X option. The name of this user specific directory
  depends  on  your  operating system, it can be found in [10XGAPInfo.UserGapRoot[110X.
  This directory can be used to tell [5XGAP[105X about personal preferences, to always
  load  some  additional code, to install additional packages, or to overwrite
  some [5XGAP[105X files. See [14X3.2[114X for more information how to do this.[133X
  
  
  [1X9.3 [33X[0;0YDirectories[133X[101X
  
  [1X9.3-1 IsDirectory[101X
  
  [33X[1;0Y[29X[2XIsDirectory[102X( [3Xobj[103X ) [32X Category[133X
  
  [33X[0;0Y[2XIsDirectory[102X is a category of directories.[133X
  
  [1X9.3-2 Directory[101X
  
  [33X[1;0Y[29X[2XDirectory[102X( [3Xstring[103X ) [32X operation[133X
  
  [33X[0;0Yreturns  a directory object for the string [3Xstring[103X. [2XDirectory[102X understands [10X"."[110X
  for  [21Xcurrent  directory[121X, that is, the directory in which [5XGAP[105X was started. It
  also understands absolute paths.[133X
  
  [33X[0;0YIf  the  variable  [10XGAPInfo.UserHome[110X  is  defined  (this  may  depend  on the
  operating  system)  then  [2XDirectory[102X  understands  a  string with a leading [10X~[110X
  (tilde)  character  for  a path relative to the user's home directory (but a
  string beginning with [10X"~other_user"[110X is [13Xnot[113X interpreted as a path relative to
  [10Xother_user[110X's home directory, as in a UNIX shell).[133X
  
  [33X[0;0YPaths are otherwise taken relative to the current directory.[133X
  
  [1X9.3-3 DirectoryTemporary[101X
  
  [33X[1;0Y[29X[2XDirectoryTemporary[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns  a  directory  object  in the category [2XIsDirectory[102X ([14X9.3-1[114X) for a [13Xnew[113X
  temporary  directory.  This  is  guaranteed  to  be  newly created and empty
  immediately after the call to [2XDirectoryTemporary[102X. [5XGAP[105X will make a reasonable
  effort to remove this directory upon termination of the [5XGAP[105X job that created
  the directory.[133X
  
  [33X[0;0YIf [2XDirectoryTemporary[102X is unable to create a new directory, [9Xfail[109X is returned.
  In  this  case  [2XLastSystemError[102X ([14X9.1-1[114X) can be used to get information about
  the error.[133X
  
  [33X[0;0YA  warning  message  is  given  if  more than 1000 temporary directories are
  created in any [5XGAP[105X session.[133X
  
  [1X9.3-4 DirectoryCurrent[101X
  
  [33X[1;0Y[29X[2XDirectoryCurrent[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns the directory object for the current directory.[133X
  
  [1X9.3-5 DirectoriesLibrary[101X
  
  [33X[1;0Y[29X[2XDirectoriesLibrary[102X( [[3Xname[103X] ) [32X function[133X
  
  [33X[0;0Y[2XDirectoriesLibrary[102X returns the directory objects for the [5XGAP[105X library [3Xname[103X as
  a list. [3Xname[103X must be one of [10X"lib"[110X (the default), [10X"doc"[110X, [10X"tst"[110X, and so on.[133X
  
  [33X[0;0YThe  string  [10X""[110X  is  also  legal  and  with this argument [2XDirectoriesLibrary[102X
  returns  the  list  of  [5XGAP[105X  root directories. The return value of this call
  differs  from  [10XGAPInfo.RootPaths[110X  in  that the former is a list of directory
  objects and the latter a list of strings.[133X
  
  [33X[0;0YThe  directory  [3Xname[103X  must  exist  in  at least one of the root directories,
  otherwise [9Xfail[109X is returned.[133X
  
  [33X[0;0YAs  the  files in the [5XGAP[105X root directories (see [14X9.2[114X) can be distributed into
  different directories in the filespace a list of directories is returned. In
  order  to find an existing file in a [5XGAP[105X root directory you should pass that
  list  to  [2XFilename[102X  ([14X9.4-1[114X)  as  the  first  argument.  In order to create a
  filename  for  a  new  file  inside a [5XGAP[105X root directory you should pass the
  first  entry  of  that  list.  However,  creating  files inside the [5XGAP[105X root
  directory  is  not  recommended,  you  should use [2XDirectoryTemporary[102X ([14X9.3-3[114X)
  instead.[133X
  
  [1X9.3-6 DirectoriesSystemPrograms[101X
  
  [33X[1;0Y[29X[2XDirectoriesSystemPrograms[102X(  ) [32X function[133X
  
  [33X[0;0Y[2XDirectoriesSystemPrograms[102X  returns  the  directory  objects  for the list of
  directories  where  the  system  programs reside, as a list. Under UNIX this
  would usually represent [10X$PATH[110X.[133X
  
  [1X9.3-7 DirectoryContents[101X
  
  [33X[1;0Y[29X[2XDirectoryContents[102X( [3Xdir[103X ) [32X function[133X
  
  [33X[0;0YThis function returns a list of filenames/directory names that reside in the
  directory  [3Xdir[103X.  The argument [3Xdir[103X can either be given as a string indicating
  the  name  of  the  directory  or  as  a  directory  object (see [2XIsDirectory[102X
  ([14X9.3-1[114X)). If the specified directory does not exist, [9Xfail[109X is returned.[133X
  
  [33X[0;0YThe ordering of the list entries can depend on the operating system.[133X
  
  [33X[0;0YAn  interactive  way  to show the contents of a directory is provided by the
  function  [2XBrowseDirectory[102X  ([14XBrowse:  BrowseDirectory[114X)  from  the [5XGAP[105X package
  [5XBrowse[105X.[133X
  
  [1X9.3-8 DirectoryDesktop[101X
  
  [33X[1;0Y[29X[2XDirectoryDesktop[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns  a  directory  object  for the users desktop directory as defined on
  many  modern  operating  systems.  The  function  is  intended  to provide a
  cross-platform  interface  to  a  directory that is easily accessible by the
  user.  Under  Unix  systems  (including  Mac  OS X) this will be the [10XDesktop[110X
  directory  in  the  users  home  directory  if it exists, and the users home
  directory  otherwise. Under Windows it will the users [10XDesktop[110X folder (or the
  appropriate name under different languages).[133X
  
  [1X9.3-9 DirectoryHome[101X
  
  [33X[1;0Y[29X[2XDirectoryHome[102X(  ) [32X function[133X
  
  [33X[0;0Yreturns  a  directory  object  for  the  users  home directory, defined as a
  directory  in which the user will typically have full read and write access.
  The  function  is  intended  to  provide  a  cross-platform  interface  to a
  directory  that  is  easily  accessible  by  the  user.  Under  Unix systems
  (including  Mac  OS  X)  this  will  be the usual user home directory. Under
  Windows it will the users [10XMy Documents[110X folder (or the appropriate name under
  different languages).[133X
  
  
  [1X9.4 [33X[0;0YFile Names[133X[101X
  
  
  [1X9.4-1 [33X[0;0YFilename[133X[101X
  
  [33X[1;0Y[29X[2XFilename[102X( [3Xdir[103X, [3Xname[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XFilename[102X( [3Xlist-of-dirs[103X, [3Xname[103X ) [32X operation[133X
  
  [33X[0;0YIf  the  first  argument  is  a  directory  object [3Xdir[103X, [2XFilename[102X returns the
  (system  dependent)  filename as a string for the file with name [3Xname[103X in the
  directory  [3Xdir[103X.  [2XFilename[102X  returns  the  filename  regardless of whether the
  directory contains a file with name [3Xname[103X or not.[133X
  
  [33X[0;0YIf  the  first  argument  is  a  list [3Xlist-of-dirs[103X (possibly of length 1) of
  directory  objects,  then  [2XFilename[102X  searches  the directories in order, and
  returns the filename for the file [3Xname[103X in the first directory which contains
  a file [3Xname[103X or [9Xfail[109X if no directory contains a file [3Xname[103X.[133X
  
  [33X[0;0Y[13XFor   example[113X,   in   order   to   locate   the   system  program  [10Xdate[110X  use
  [2XDirectoriesSystemPrograms[102X ([14X9.3-6[114X) together with the second form of [2XFilename[102X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xpath := DirectoriesSystemPrograms();;[127X[104X
    [4X[25Xgap>[125X [27Xdate := Filename( path, "date" );[127X[104X
    [4X[28X"/bin/date"[128X[104X
  [4X[32X[104X
  
  [33X[0;0YIn  order to locate the library file [11Xfiles.gd[111X use [2XDirectoriesLibrary[102X ([14X9.3-5[114X)
  together with the second form of [2XFilename[102X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xpath := DirectoriesLibrary();;[127X[104X
    [4X[25Xgap>[125X [27XFilename( path, "files.gd" );[127X[104X
    [4X[28X"./lib/files.gd"[128X[104X
  [4X[32X[104X
  
  [33X[0;0YIn  order  to construct filenames for new files in a temporary directory use
  [2XDirectoryTemporary[102X ([14X9.3-3[114X) together with the first form of [2XFilename[102X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xtmpdir := DirectoryTemporary();;[127X[104X
    [4X[25Xgap>[125X [27XFilename( [ tmpdir ], "file.new" );[127X[104X
    [4X[28Xfail[128X[104X
    [4X[25Xgap>[125X [27XFilename( tmpdir, "file.new" );[127X[104X
    [4X[28X"/var/tmp/tmp.0.021738.0001/file.new"[128X[104X
  [4X[32X[104X
  
  
  [1X9.5 [33X[0;0YSpecial Filenames[133X[101X
  
  [33X[0;0YThe  special filename [10X"*stdin*"[110X denotes the standard input, i.e., the stream
  through  which  the  user  enters  commands  to  [5XGAP[105X. The exact behaviour of
  reading  from  [10X"*stdin*"[110X  is  operating  system  dependent,  but usually the
  following  happens. If [5XGAP[105X was started with no input redirection, statements
  are  read  from  the  terminal  stream until the user enters the end of file
  character,  which is usually [12XCtrl-D[112X. Note that terminal streams are special,
  in  that  they  may  yield  ordinary  input  [13Xafter[113X an end of file. Thus when
  control  returns to the main read-eval-print loop the user can continue with
  [5XGAP[105X.  If [5XGAP[105X was started with an input redirection, statements are read from
  the  current  position  in  the  input  file up to the end of the file. When
  control  returns to the main read eval view loop the input stream will still
  return end of file, and [5XGAP[105X will terminate.[133X
  
  [33X[0;0YThe  special  filename  [10X"*errin*"[110X  denotes  the stream connected to the UNIX
  [10Xstderr[110X output. This stream is usually connected to the terminal, even if the
  standard  input  was  redirected,  unless the standard error stream was also
  redirected, in which case opening of [10X"*errin*"[110X fails.[133X
  
  [33X[0;0YThe special filename [10X"*stdout*"[110X can be used to print to the standard output.[133X
  
  [33X[0;0YThe  special  filename [10X"*errout*"[110X can be used to print to the standard error
  output  file,  which  is  usually  connected  to  the  terminal, even if the
  standard output was redirected.[133X
  
  
  [1X9.6 [33X[0;0YFile Access[133X[101X
  
  [33X[0;0YWhen  the  following  functions  return  [9Xfalse[109X  one  can use [2XLastSystemError[102X
  ([14X9.1-1[114X)  to  find  out the reason (as provided by the operating system), see
  the examples.[133X
  
  [1X9.6-1 IsExistingFile[101X
  
  [33X[1;0Y[29X[2XIsExistingFile[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0Y[2XIsExistingFile[102X  returns [9Xtrue[109X if a file with the filename [3Xfilename[103X exists and
  can be seen by the [5XGAP[105X process. Otherwise [9Xfalse[109X is returned.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XIsExistingFile( "/bin/date" );     # file `/bin/date' exists[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsExistingFile( "/bin/date.new" ); # non existing `/bin/date.new'[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XIsExistingFile( "/bin/date/new" ); # `/bin/date' is not a directory[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XLastSystemError().message;[127X[104X
    [4X[28X"Not a directory"[128X[104X
  [4X[32X[104X
  
  [1X9.6-2 IsReadableFile[101X
  
  [33X[1;0Y[29X[2XIsReadableFile[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0Y[2XIsReadableFile[102X  returns [9Xtrue[109X if a file with the filename [3Xfilename[103X exists [13Xand[113X
  the  [5XGAP[105X  process has read permissions for the file, or [9Xfalse[109X if this is not
  the case.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XIsReadableFile( "/bin/date" );     # file `/bin/date' is readable[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsReadableFile( "/bin/date.new" ); # non-existing `/bin/date.new'[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XLastSystemError().message;        [127X[104X
    [4X[28X"No such file or directory"[128X[104X
  [4X[32X[104X
  
  [1X9.6-3 IsWritableFile[101X
  
  [33X[1;0Y[29X[2XIsWritableFile[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0Y[2XIsWritableFile[102X  returns [9Xtrue[109X if a file with the filename [3Xfilename[103X exists [13Xand[113X
  the  [5XGAP[105X process has write permissions for the file, or [9Xfalse[109X if this is not
  the case.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XIsWritableFile( "/bin/date" );  # file `/bin/date' is not writable[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [1X9.6-4 IsExecutableFile[101X
  
  [33X[1;0Y[29X[2XIsExecutableFile[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0Y[2XIsExecutableFile[102X  returns  [9Xtrue[109X  if a file with the filename [3Xfilename[103X exists
  [13Xand[113X  the  [5XGAP[105X process has execute permissions for the file, or [9Xfalse[109X if this
  is  not  the  case.  Note  that  execute permissions do not imply that it is
  possible to execute the file, e.g., it may only be executable on a different
  machine.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XIsExecutableFile( "/bin/date" );   # ... but executable[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [1X9.6-5 IsDirectoryPath[101X
  
  [33X[1;0Y[29X[2XIsDirectoryPath[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0Y[2XIsDirectoryPath[102X  returns  [9Xtrue[109X if the file with the filename [3Xfilename[103X exists
  [13Xand[113X  is  a  directory, and [9Xfalse[109X otherwise. Note that this function does not
  check  if  the [5XGAP[105X process actually has write or execute permissions for the
  directory.  You  can  use  [2XIsWritableFile[102X  ([14X9.6-3[114X),  resp.  [2XIsExecutableFile[102X
  ([14X9.6-4[114X) to check such permissions.[133X
  
  
  [1X9.7 [33X[0;0YFile Operations[133X[101X
  
  [1X9.7-1 Read[101X
  
  [33X[1;0Y[29X[2XRead[102X( [3Xfilename[103X ) [32X operation[133X
  
  [33X[0;0Yreads  the  input  from  the  file with the filename [3Xfilename[103X, which must be
  given as a string.[133X
  
  [33X[0;0Y[2XRead[102X  first  opens  the file [3Xfilename[103X. If the file does not exist, or if [5XGAP[105X
  cannot open it, e.g., because of access restrictions, an error is signalled.[133X
  
  [33X[0;0YThen  the  contents  of the file are read and evaluated, but the results are
  not  printed.  The  reading and evaluations happens exactly as described for
  the main loop (see [14X6.1[114X).[133X
  
  [33X[0;0YIf  a  statement  in  the  file  causes  an  error  a  break loop is entered
  (see [14X6.4[114X).  The  input  for  this break loop is not taken from the file, but
  from  the  input  connected  to  the  [10Xstderr[110X output of [5XGAP[105X. If [10Xstderr[110X is not
  connected  to  a  terminal,  no break loop is entered. If this break loop is
  left  with  [9Xquit[109X  (or [12XCtrl-D[112X), [5XGAP[105X exits from the [2XRead[102X command, and from all
  enclosing  [2XRead[102X  commands,  so  that  control  is  normally  returned  to an
  interactive  prompt.  The  [9XQUIT[109X  statement (see [14X6.7[114X) can also be used in the
  break loop to exit [5XGAP[105X immediately.[133X
  
  [33X[0;0YNote  that  a statement must not begin in one file and end in another. I.e.,
  [13Xeof[113X ([13Xe[113Xnd-[13Xo[113Xf-[13Xf[113Xile) is not treated as whitespace, but as a special symbol that
  must not appear inside any statement.[133X
  
  [33X[0;0YNote  that  one  file may very well contain a read statement causing another
  file  to  be read, before input is again taken from the first file. There is
  an upper limit of 15 on the number of files that may be open simultaneously.[133X
  
  [1X9.7-2 ReadAsFunction[101X
  
  [33X[1;0Y[29X[2XReadAsFunction[102X( [3Xfilename[103X ) [32X operation[133X
  
  [33X[0;0Yreads  the  file  with  filename  [3Xfilename[103X  as  a  function and returns this
  function.[133X
  
  [33X[0;0Y[13XExample[113X[133X
  
  [33X[0;0YSuppose that the file [11X/tmp/example.g[111X contains the following[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28Xlocal a;[128X[104X
    [4X[28X[128X[104X
    [4X[28Xa := 10;[128X[104X
    [4X[28Xreturn a*10;[128X[104X
  [4X[32X[104X
  
  [33X[0;0YReading the file as a function will not affect a global variable [10Xa[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xa := 1;[127X[104X
    [4X[28X1[128X[104X
    [4X[25Xgap>[125X [27XReadAsFunction("/tmp/example.g")();[127X[104X
    [4X[28X100[128X[104X
    [4X[25Xgap>[125X [27Xa;[127X[104X
    [4X[28X1[128X[104X
  [4X[32X[104X
  
  
  [1X9.7-3 [33X[0;0YPrintTo and AppendTo[133X[101X
  
  [33X[1;0Y[29X[2XPrintTo[102X( [3Xfilename[103X[, [3Xobj1[103X, [3X...[103X] ) [32X function[133X
  [33X[1;0Y[29X[2XAppendTo[102X( [3Xfilename[103X[, [3Xobj1[103X, [3X...[103X] ) [32X function[133X
  
  [33X[0;0Y[2XPrintTo[102X  works  like  [2XPrint[102X ([14X6.3-4[114X), except that the arguments [3Xobj1[103X, [22X...[122X (if
  present)  are  printed  to  the  file  with the name [3Xfilename[103X instead of the
  standard  output.  This file must of course be writable by [5XGAP[105X. Otherwise an
  error  is  signalled. Note that [2XPrintTo[102X will [13Xoverwrite[113X the previous contents
  of  this  file  if  it already existed; in particular, [2XPrintTo[102X with just the
  [3Xfilename[103X argument empties that file.[133X
  
  [33X[0;0Y[2XAppendTo[102X  works  like [2XPrintTo[102X, except that the output does not overwrite the
  previous contents of the file, but is appended to the file.[133X
  
  [33X[0;0YThere is an upper limit of 15 on the number of output files that may be open
  simultaneously.[133X
  
  [33X[0;0Y[13XNote[113X  that  one  should  be  careful  not  to  write to a logfile (see [2XLogTo[102X
  ([14X9.7-4[114X)) with [2XPrintTo[102X or [2XAppendTo[102X.[133X
  
  
  [1X9.7-4 [33X[0;0YLogTo[133X[101X
  
  [33X[1;0Y[29X[2XLogTo[102X( [3Xfilename[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XLogTo[102X(  ) [32X operation[133X
  
  [33X[0;0YCalling [2XLogTo[102X with a string [3Xfilename[103X causes the subsequent interaction to be
  logged  to the file with the name [3Xfilename[103X, i.e., everything you see on your
  terminal  will also appear in this file. ([2XLogTo[102X ([14X10.4-5[114X) may also be used to
  log  to a stream.) This file must of course be writable by [5XGAP[105X, otherwise an
  error  is signalled. Note that [2XLogTo[102X will overwrite the previous contents of
  this file if it already existed.[133X
  
  [33X[0;0YCalled without arguments, [2XLogTo[102X stops logging to a file or stream.[133X
  
  
  [1X9.7-5 [33X[0;0YInputLogTo[133X[101X
  
  [33X[1;0Y[29X[2XInputLogTo[102X( [3Xfilename[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XInputLogTo[102X(  ) [32X operation[133X
  
  [33X[0;0YCalling  [2XInputLogTo[102X with a string [3Xfilename[103X causes the subsequent input to be
  logged to the file with the name [3Xfilename[103X, i.e., everything you type on your
  terminal  will  also  appear  in  this  file. Note that [2XInputLogTo[102X and [2XLogTo[102X
  ([14X9.7-4[114X)  cannot  be  used  at the same time while [2XInputLogTo[102X and [2XOutputLogTo[102X
  ([14X9.7-6[114X)  can.  Note  that [2XInputLogTo[102X will overwrite the previous contents of
  this file if it already existed.[133X
  
  [33X[0;0YCalled without arguments, [2XInputLogTo[102X stops logging to a file or stream.[133X
  
  
  [1X9.7-6 [33X[0;0YOutputLogTo[133X[101X
  
  [33X[1;0Y[29X[2XOutputLogTo[102X( [3Xfilename[103X ) [32X operation[133X
  [33X[1;0Y[29X[2XOutputLogTo[102X(  ) [32X operation[133X
  
  [33X[0;0YCalling  [2XOutputLogTo[102X  with a string [3Xfilename[103X causes the subsequent output to
  be logged to the file with the name [3Xfilename[103X, i.e., everything [5XGAP[105X prints on
  your terminal will also appear in this file. Note that [2XOutputLogTo[102X and [2XLogTo[102X
  ([14X9.7-4[114X)  cannot  be  used  at  the  same  time  while [2XInputLogTo[102X ([14X9.7-5[114X) and
  [2XOutputLogTo[102X  can. Note that [2XOutputLogTo[102X will overwrite the previous contents
  of this file if it already existed.[133X
  
  [33X[0;0YCalled without arguments, [2XOutputLogTo[102X stops logging to a file or stream.[133X
  
  [1X9.7-7 CrcFile[101X
  
  [33X[1;0Y[29X[2XCrcFile[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0YCRC  (cyclic  redundancy  check)  numbers  provide a certain method of doing
  checksums. They are used by [5XGAP[105X to check whether files have changed.[133X
  
  [33X[0;0Y[2XCrcFile[102X  computes  a  checksum value for the file with filename [3Xfilename[103X and
  returns  this  value  as  an  integer. The function returns [9Xfail[109X if a system
  error  occurred,  say, for example, if [3Xfilename[103X does not exist. In this case
  the  function  [2XLastSystemError[102X  ([14X9.1-1[114X) can be used to get information about
  the error.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XCrcFile( "lib/morpheus.gi" );[127X[104X
    [4X[28X2705743645[128X[104X
  [4X[32X[104X
  
  [1X9.7-8 RemoveFile[101X
  
  [33X[1;0Y[29X[2XRemoveFile[102X( [3Xfilename[103X ) [32X function[133X
  
  [33X[0;0Ywill  remove  the  file  with  filename [3Xfilename[103X and returns [9Xtrue[109X in case of
  success.  The function returns [9Xfail[109X if a system error occurred, for example,
  if  your  permissions do not allow the removal of [3Xfilename[103X. In this case the
  function  [2XLastSystemError[102X  ([14X9.1-1[114X)  can be used to get information about the
  error.[133X
  
  [1X9.7-9 UserHomeExpand[101X
  
  [33X[1;0Y[29X[2XUserHomeExpand[102X( [3Xstr[103X ) [32X function[133X
  
  [33X[0;0YIf  the  string  [3Xstr[103X starts with a [10X'~'[110X character this function returns a new
  string  with  the  leading  [10X'~'[110X  substituted  by the users home directory as
  stored in [10XGAPInfo.UserHome[110X. Otherwise [3Xstr[103X is returned unchanged.[133X
  
  [1X9.7-10 Reread[101X
  
  [33X[1;0Y[29X[2XReread[102X( [3Xfilename[103X ) [32X function[133X
  [33X[1;0Y[29X[2XREREADING[102X [32X global variable[133X
  
  [33X[0;0YIn  general,  it is not possible to read the same [5XGAP[105X library file twice, or
  to  read  a  compiled  version  after reading a [5XGAP[105X version, because crucial
  global  variables  are  made read-only (see [14X4.9[114X) and filters and methods are
  added to global tables.[133X
  
  [33X[0;0YA  partial  solution to this problem is provided by the function [2XReread[102X (and
  related  functions  [10XRereadLib[110X  etc.).  [10XReread(  [3Xfilename[103X[10X  )[110X  sets the global
  variable [2XREREADING[102X to [9Xtrue[109X, reads the file named by [3Xfilename[103X and then resets
  [2XREREADING[102X. Various system functions behave differently when [2XREREADING[102X is set
  to  [9Xtrue[109X.  In  particular,  assignment  to  read-only  global  variables  is
  permitted, calls to [2XNewRepresentation[102X ([14X13.4-4[114X) and [2XNewInfoClass[102X ([14X7.4-1[114X) with
  parameters  identical  to  those of an existing representation or info class
  will  return  the  existing object, and methods installed with [2XInstallMethod[102X
  ([14X78.3-1[114X) may sometimes displace existing methods.[133X
  
  [33X[0;0YThis  function  may not entirely produce the intended results, especially if
  what  has  changed  is  the  super-representation of a representation or the
  requirements  of a method. In these cases, it is necessary to restart [5XGAP[105X to
  read the modified file.[133X
  
  [33X[0;0YAn  additional  use  of [2XReread[102X is to load the compiled version of a file for
  which  the  [5XGAP[105X  language  version  had previously been read (or perhaps was
  included in a saved workspace). See [14X76.3-11[114X and [14X3.3[114X for more information.[133X
  
  [33X[0;0YIt  is  not advisable to use [2XReread[102X programmatically. For example, if a file
  that  contains  calls  to  [2XReread[102X  is read with [2XReread[102X then [2XREREADING[102X may be
  reset too early.[133X
  
