% This file was created automatically from files.msk.
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%W files.msk GAP documentation Frank Celler
%W & Martin Schoenert
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%H @(#)$Id: files.msk,v 1.4 2003/03/26 17:46:07 gap Exp $
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%Y Copyright 1997, Lehrstuhl D fuer Mathematik, RWTH Aachen, Germany
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%% This file contains the description of the file, filename and
%% directory functions.
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\Chapter{Files and Filenames}
Files are identified by filenames, which are represented in {\GAP} as
strings. Filenames can be created directly by the user or a program, but
of course this is operating system dependent.
Filenames 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 *always* `{'/'}' even under DOS or MacOS.
Section "Directories" describes how to construct directory objects
for the common {\GAP} and system directories.
Using the command `Filename' described in section~"Filename"
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~"File Access".
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{Portability}
For portability filenames and directory names should be restricted to at
most 8 alphanumerical characters optionally followed by a dot `{'.'}'
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 `NaMe', `Name' and `name' all refer to the same
file whereas some operating systems are case sensitive. To avoid
problems only lower case characters should be used.
Another function which is system-dependent is:
\>LastSystemError() F
`LastSystemError' returns a record describing the last system error that
has occurred.
This record contains at least the component `message' which is a
string. This message is, however, highly operating system dependent and
should only be used as an informational message for the user.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{GAP Root Directory}
When starting {\GAP} it is possible to specify various directories as
root directories. In {\GAP}'s view of the world these directories are
merged into one meta-directory. This directory is called *{\GAP} root
directory* in the following.
For example, if `<root1>;<root2>;...' is passed as argument to `-l' when
{\GAP} is started and {\GAP} wants to locate a file `lib/group.gd' in the
{\GAP} root directory it will first check if the file exists in
`<root1>', if not, it checks `<root2>', and so on.
This layout makes it possible to have one system-wide installation of
{\GAP} which is read-only but still allows users to modify individual
files. Therefore instead of constructing an absolute path name to a file
you should always use `DirectoriesLibrary' or `DirectoriesPackageLibrary'
together with `Filename' to construct a filename for a file in the {\GAP}
root directory.
*Example*
Suppose that the system-wide installation lives in `/usr/local/lib/gap4'
and you want to modify the file `lib/files.gd' without disturbing the
system installation.
In this case create a new directory `/home/myhome/gap' containing a
subdirectory `lib' which contains the modified `lib/files.gd'.
The directory/file structure now looks like
\begintt
/usr/local/lib/gap4/
/usr/local/lib/gap4/lib/
/usr/local/lib/gap4/lib/files.gd
/home/myhome/gap/
/home/myhome/gap/lib
/home/myhome/gap/lib/files.gd
\endtt
If you start {\GAP} using (under UNIX)
\begintt
you@unix> gap -l '/home/myhome/gap;/usr/local/lib/gap4'
\endtt
then the file `/home/myhome/gap/lib/files.gd' will be used whenever
{\GAP} references the file with filename `lib/files.gd' in the {\GAP}
root directory.
This setup also allows one to easily install new {\GAP} packages or
bugfixes even if no access to the system {\GAP} installation is possible.
Simply unpack the files into ``/home/myhome/gap''.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{Directories}
\>Directory( <string> ) O
returns a directory object for the string <string>.
`Directory' understands `.' for ``current directory'', that is, the
directory in which {\GAP} was started.
It also understands absolute paths.
If the variable `GAPInfo.UserHome' is defined (this may depend on the
operating system) then `Directory' understands a string with a leading
`~' character for a path relative to the user's home directory.
Paths are otherwise taken relative to the current directory.
\>DirectoryTemporary( <hint> ) F
\>DirectoryTemporary( ) F
returns a directory object in the category `IsDirectory' for a *new*
temporary directory. This is guaranteed to be newly created and empty
immediately after the call to `DirectoryTemporary'. {\GAP} will make a
reasonable effort to *remove* this directory either when a garbage
collection collects the directory object or upon termination of the
{\GAP} job that created the directory. <hint> can be used by
`DirectoryTemporary' to construct the name of the directory but
`DirectoryTemporary' is free to use only a part of <hint> or even ignore
it completely.
If `DirectoryTemporary' is unable to create a new directory, `fail' is
returned. In this case `LastSystemError' can be used to get information
about the error.
\>DirectoryCurrent( ) F
returns the directory object for the current directory.
\>DirectoriesLibrary( ) F
\>DirectoriesLibrary( <name> ) F
returns the directory objects for the {\GAP} library <name> as a list.
<name> must be one of `"lib"' (the default), `"grp"', `"prim"',
and so on.
The string `""' is also legal and with this argument `DirectoriesLibrary'
returns the list of {\GAP} root directories; the return value of
`DirectoriesLibrary("");' differs from `GAPInfo.RootPaths' in that the
former is a list of directory objects and the latter a list of strings.
The directory <name> must exist in at least one of the root directories,
otherwise `fail' is returned.
As the files in the {\GAP} root directory (see~"GAP Root Directory") can
be distributed into different directories in the filespace a list of
directories is returned. In order to find an existing file in a {\GAP}
root directory you should pass that list to `Filename' (see~"Filename")
as the first argument.
In order to create a filename for a new file inside a {\GAP} root
directory you should pass the first entry of that list.
However, creating files inside the {\GAP} root directory is not
recommended, you should use `DirectoryTemporary' instead.
\>DirectoriesSystemPrograms( ) F
`DirectoriesSystemPrograms' returns the directory objects for the list of
directories where the system programs reside as a list. Under UNIX this
would usually represent `\$PATH'.
\>DirectoryContents( <name> ) F
This function returns a list of filenames/directory names that reside in
the directory with name <name> (given as a string). It is an error, if
such a directory does not exist.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{Filename}\nolabel
\>Filename( <dir>, <name> ) O
\>Filename( <list-of-dirs>, <name> ) O
If the first argument is a directory object <dir>, `Filename' returns the
(system dependent) filename as a string for the file with name <name> in
the directory <dir>.
`Filename' returns the filename regardless of whether the directory
contains a file with name <name> or not.
If the first argument is a list <list-of-dirs> (possibly of length 1) of
directory objects, then `Filename' searches the directories in order, and
returns the filename for the file <name> in the first directory which
contains a file <name> or `fail' if no directory contains a file <name>.
*Examples*
In order to locate the system program `date' use
`DirectoriesSystemPrograms' together with the second form of `Filename'.
%notest
\beginexample
gap> path := DirectoriesSystemPrograms();;
gap> date := Filename( path, "date" );
"/bin/date"
\endexample
In order to locate the library file `files.gd' use `DirectoriesLibrary'
together with the second form of `Filename'.
%notest
\beginexample
gap> path := DirectoriesLibrary();;
gap> Filename( path, "files.gd" );
"./lib/files.gd"
\endexample
In order to construct filenames for new files in a temporary directory
use `DirectoryTemporary' together with the first form of `Filename'.
%notest
\beginexample
gap> tmpdir := DirectoryTemporary();;
gap> Filename( [ tmpdir ], "file.new" );
fail
gap> Filename( tmpdir, "file.new" );
"/var/tmp/tmp.0.021738.0001/file.new"
\endexample
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{Special Filenames}
The special filename `"*stdin*"' denotes the standard input, i.e., the
stream through which the user enters commands to {\GAP}. The exact
behaviour of reading from `"*stdin*"' is operating system dependent, but
usually the following happens. If {\GAP} 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 <ctr>-'D'. Note that
terminal streams are special, in that they may yield ordinary input
*after* an end of file. Thus when control returns to the main
read-eval-print loop the user can continue with {\GAP}. If {\GAP} 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 {\GAP} will terminate.
The special filename `"*errin*"' denotes the stream connected to the
UNIX `stderr' 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 `"*errin*"' fails.
The special filename `"*stdout*"' can be used to print to the standard
output.
The special filename `"*errout*"' 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.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{File Access}
When the following functions return `false' one can use `LastSystemError'
(see~"LastSystemError") to find out the reason (as provided by the
operating system).
\>IsExistingFile( <name-file> ) F
returns `true' if a file with the filename <name-file> exists and can be
seen by the {\GAP} process. Otherwise `false' is returned.
\>IsReadableFile( <name-file> ) F
returns `true' if a file with the filename <name-file> exists *and* the
{\GAP} process has read permissions for the file, or `false' if
this is not the case.
\>IsWritableFile( <name-file> ) F
returns `true' if a file with the filename <name-file> exists *and* the
{\GAP} process has write permissions for the file, or `false' if
this is not the case.
\>IsExecutableFile( <name-file> ) F
returns `true' if a file with the filename <name-file> exists *and* the
{\GAP} process has execute permissions for the file, or `false' 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.
\>IsDirectoryPath( <name-file> ) F
returns `true' if the file with the filename <name-file> exists *and* is
a directory and `false' otherwise.
Note that this function does not check if the {\GAP} process actually has
write or execute permissions for the directory (you can use
`IsWritableFile' (see~"IsWritableFile"), resp. `IsExecutableFile'
(see~"IsExecutableFile") to check such permissions).
*Examples*
Note, in particular, how one may use `LastSystemError'
(see~"LastSystemError") to discover the reason a file access function
returns `false'.
\beginexample
gap> IsExistingFile( "/bin/date" ); # the file `/bin/date' exists
true
gap> IsExistingFile( "/bin/date.new" ); # the file `/bin/date.new' does not exist
false
gap> IsExistingFile( "/bin/date/new" ); # `/bin/date' is not a directory
false
gap> LastSystemError().message;
"Not a directory"
gap> IsReadableFile( "/bin/date" ); # the file `/bin/date' is readable
true
gap> IsReadableFile( "/bin/date.new" ); # the file `/bin/date.new' does not exist
false
gap> LastSystemError().message;
"No such file or directory"
gap> IsWritableFile( "/bin/date" ); # the file `/bin/date' is not writable ...
false
gap> IsExecutableFile( "/bin/date" ); # ... but executable
true
\endexample
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\Section{File Operations}
\>Read( <name-file> ) O
reads the input from the file with the filename <name-file>, which must
be given as a string.
`Read' first opens the file <name-file>. If the file does not exist, or
if {\GAP} cannot open it, e.g., because of access restrictions,
an error is signalled.
Then 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 "Main Loop").
If a statement in the file causes an error a break loop is entered
(see~"Break Loops").
The input for this break loop is not taken from the file, but from the
input connected to the `stderr' output of {\GAP}.
If `stderr' is not connected to a terminal, no break loop is entered.
If this break loop is left with `quit' (or `<ctr>-D'), {\GAP} exits from
the `Read' command, and from all enclosing `Read' commands,
so that control is normally returned to an interactive prompt.
The `QUIT' statement (see~"Leaving GAP") can also be used in the break
loop to exit {\GAP} immediately.
Note that a statement must not begin in one file and end in another.
I.e., <eof> (`end-of-file') is not treated as whitespace,
but as a special symbol that must not appear inside any statement.
Note 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 operating system dependent maximum on the number of files
that may be open simultaneously. Usually it is 15.
\>ReadAsFunction( <name-file> ) O
reads the file with filename <name-file> as a function and returns this
function.
*Example*
Suppose that the file `/tmp/example.g' contains the following
\begintt
local a;
a := 10;
return a*10;
\endtt
Reading the file as a function will not affect a global variable `a'.
%notest
\beginexample
gap> a := 1;
1
gap> ReadAsFunction("/tmp/example.g")();
100
gap> a;
1
\endexample
\>PrintTo( <name-file>[, <obj1>, ...] ) F
works like `Print' (see~"Print"), except that the arguments <obj1>,
{\dots} (if present) are printed to the file with the name <name-file>
instead of the standard output. This file must of course be writable by
{\GAP}. Otherwise an error is signalled. Note that `PrintTo' will
*overwrite* the previous contents of this file if it already existed; in
particular, `PrintTo' with just the <name-file> argument empties that
file. `AppendTo' can be used to append to a file (see "AppendTo"). There
is an operating system dependent maximum on the number of output files
that may be open simultaneously, usually this is 14.
\>AppendTo( <name-file>[, <obj1>, ...] ) F
works like `PrintTo' (see~"PrintTo"), except that the output does not
overwrite the previous contents of the file, but is appended to the file.
\>LogTo( <name-file> ) O
causes the subsequent interaction to be logged to the file with the name
<name-file>, i.e., everything you see on your terminal will also appear
in this file. `LogTo' may also be used to log to a stream (see~"LogTo!for
streams").
This file must of course be writable by {\GAP}, otherwise an error is
signalled. Note that `LogTo' will overwrite the previous
contents of this file if it already existed.
\>LogTo()!{stop logging} M
In this form `LogTo' stops logging to a file or stream.
\>InputLogTo( <name-file> ) O
causes the subsequent input to be logged to the file with the name
<name-file>, i.e., everything you type on your terminal will also appear
in this file. Note that `InputLogTo' and `LogTo' cannot be used at the
same time while `InputLogTo' and `OutputLogTo' can.
Note that `InputLogTo' will overwrite the previous
contents of this file if it already existed.
\>InputLogTo()!{stop logging input} M
In this form `InputLogTo' stops logging to a file or stream.
\>OutputLogTo( <name-file> ) O
causes the subsequent output to be logged to the file with the name
<name-file>, i.e., everything {\GAP} prints on your terminal will also
appear in this file. Note that `OutputLogTo' and `LogTo' cannot be used
at the same time while `InputLogTo' and `OutputLogTo' can.
Note that `OutputLogTo' will overwrite the previous
contents of this file if it already existed.
\>OutputLogTo()!{stop logging output} M
In this form `OutputLogTo' stops logging to a file or stream.
*Note* that one should be careful not to write to a logfile with
`PrintTo' or `AppendTo'.
% The same holds of course for the redirection of output to a file.
\>CrcFile( <name-file> ) F
computes a checksum value for the file with filename <name-file> and
returns this value as an integer.
See Section~"CRC Numbers" for an example.
The function returns `fail' if a system error occurred, say, for example,
if <name-file> does not exist.
In this case the function `LastSystemError' (see~"LastSystemError")
can be used to get information about the error.
\>RemoveFile( <name-file> ) F
will remove the file with filename <name-file> and returns `true' in case
of success. The function returns `fail' if a system error occurred, for
example, if your permissions do not allow the removal of <name-file>.
In this case the function `LastSystemError' (see~"LastSystemError")
can be used to get information about the error.
\>Reread( <name-file> ) F
\>`REREADING'{REREADING}@{`REREADING'}
In general, it is not possible to read the same {\GAP} library file
twice, or to read a compiled version after reading a {\GAP} version,
because crucial global variables are made read-only (see "More about
Global Variables") and filters and methods are added to global tables.
A partial solution to this problem is provided by the function
`Reread' (and related functions `RereadLib' etc.). `Reread(
<name-file> )' sets the global variable `REREADING' to `true', reads
the file named by <name-file> and then resets `REREADING'. Various
system functions behave differently when `REREADING' is set to
true. In particular, assignment to read-only global variables is
permitted, calls to `NewRepresentation' (see "prg:NewRepresentation"
in ``Programming in {\GAP}'') and
`NewInfoClass' (see "NewInfoClass") with parameters identical to those
of an existing representation or info class will return the existing
object, and methods installed with `InstallMethod' (see
"prg:InstallMethod" in ``Programming in {\GAP}'') may sometimes displace
existing methods.
This 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 {\GAP} to read the modified file.
An additional use of `Reread' is to load the compiled version of a
file for which the {\GAP} language version had previously been read
(or perhaps was included in a saved workspace). See "The Compiler" and
"Saving and Loading a Workspace" for more information.
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%E
