GAP (Utils) - Chapter 8: Web Downloads

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8 Web Downloads
8.1 Functions for downloading files from the web
8.1-1 Download
8.1-2 User preference `DownloadVerifyCertificate`

### 8 Web Downloads

The `Download` operation has been written by Thomas Breuer, incorporating a number of suggestions from Max Horn, for version 0.77 of Utils. It implements downloading a file from within GAP. It can use the IO or curlInterface packages, or wget or curl, if installed, and it can be extended with other download methods quite easily. It is envisaged that, once other packages have started to use it, and any problems have been addressed, that the functions will be transferred to the main GAP library.

#### 8.1 Functions for downloading files from the web

##### 8.1-1 Download

`‣ Download`( url[, opt] ) | ( function )

This function downloads the file with the web address url, which must be a string.

The result is a record which has at least the component `success`, with value `true` if the download was successful and `false` otherwise. In the former case, the component `result` is bound, whose value is a string that contains the contents of the downloaded file. In the latter case, the component `error` is bound, whose value is a string that describes the problem.

The function calls the methods stored in the global list `Download_Methods` until one of them is successful. Currently there are methods based on the GAP functions `DownloadURL` (curl: DownloadURL) and `SingleHTTPRequest` (IO: SingleHTTPRequest), and methods based on the external programs `wget` and `curl`.

An optional record opt can be given. The following components are supported.

`target`

  If this component is bound then its value must be a string that is a local filename, and the function writes the downloaded contents to this file; the returned record does not have a `result` component in this case.

`verifyCert`

  If this component is bound and has the value `false` then those download methods that are based on `curl` or `wget` will omit the check of the server's certificate. The same effect is achieved for all `Download` calls by setting the user preference `DownloadVerifyCertificate` (see 8.1-2) to `false` and omitting the `verifyCert` component from opt.

    gap> url:= "https://www.gap-system.org/Packages/utils.html";;
    gap> res1:= Download( url );;
    gap> res1.success;
    true
    gap> IsBound( res1.result ) and IsString( res1.result );
    true
    gap> res2:= Download( Concatenation( url, "xxx" ) );;
    gap> res2.success;
    false
    gap> IsBound( res2.error ) and IsString( res2.error );
    true

##### 8.1-2 User preference `DownloadVerifyCertificate`

The value `true` (the default) means that the server's certificate is checked in calls of `Download` (8.1-1), such that nothing gets downloaded if the certificate is invalid.

If the value is `false` then download methods are supposed to omit the check of the server's certificate (this may not be supported by all download methods).

One can set the value of the preference to be `val` via `SetUserPreference` (Reference: SetUserPreference), by calling `SetUserPreference( "utils", "DownloadVerifyCertificate", val )`, and access the current value via `UserPreference` (Reference: UserPreference), by calling `UserPreference( "utils", "DownloadVerifyCertificate" )`.

We recommend leaving this preference at its default value `true`. Sometimes it can be necessary to change it, e.g. to work around issues with old operating systems which may not be able to correctly verify new certificates. In general it is better to update such a system, but if that is not an option, then disabling certificate checks may be a good last resort.

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