9 Files and Filenames

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.

9.2 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

/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

If you start GAP using (under UNIX)

you@unix> gap -l '/home/myhome/gap;/usr/local/lib/gap4'

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''.

9.3 Directories

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 USER_HOME 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.

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.

returns the directory object for the current directory.

returns the directory objects for the GAP library lib as a list. lib 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 GAP_ROOT_PATHS in that the former is a list of directory objects and the latter a list of strings.

The directory lib 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 the 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 the 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 returns the directory objects for the list of directories where the system programs reside as a list. Under UNIX this would usually represent $PATH.

9.4 Filename

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.

gap> path := DirectoriesSystemPrograms();;
gap> date := Filename( path, "date" );
"/bin/date"

In order to locate the library file files.gd use DirectoriesLibrary together with the second form of Filename.

gap> path := DirectoriesLibrary();;
gap> Filename( path, "files.gd" );
"./lib/files.gd"

In order to construct filenames for new files in a temporary directory use DirectoryTemporary together with the first form of Filename.

gap> tmpdir := DirectoryTemporary();;
gap> Filename( [ tmpdir ], "file.new" );
fail
gap> Filename( tmpdir, "file.new" );    
"/var/tmp/tmp.0.021738.0001/file.new"

9.5 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.

9.6 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).

returns true if a file with the filename name-file exists and can be seen by the GAP process. Otherwise false is returned.

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.

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.

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.

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.

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

9.7 File Operations

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.

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

local a;

a := 10;
return a*10;

Reading the file as a function will not affect a global variable a.

gap> a := 1;
1
gap> ReadAsFunction("/tmp/example.g")();
100
gap> a;
1

works like Print (see Print), except that the arguments obj1, ... (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.

works like PrintTo (see PrintTo), except that the output does not overwrite the previous contents of the file, but is appended to the file.

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.

In this form LogTo stops logging to a file or stream.

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.

In this form InputLogTo stops logging to a file or stream.

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.

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.

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.

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.

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 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 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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GAP 4 manual
May 2002