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Doc needs help
What is "home directory"?
It is the directory on which swipl looks for file boot.prc, which is a zip file, and probably a Saved State File.
See also the prolog flag "home" where we read:
SWI-Prolog's notion of the home directory. SWI-Prolog uses its home directory to find its startup file as <home>/boot.prc and to find its library as <home>/library. Some installations may put architecture independent files in a shared home and also define shared_home. System files can be found using absolute_file_name/3 as swi(file). See file_search_path/2.
It is not the user's home! It should maybe be called swipl_home analogously to JAVA_HOME.
By default, it seems to be the lib/swipl subdirectory of the swipl installation. So for example, if the installation top directory is /usr/local/logic/swipl/, then by default swipl will issue this system call (easy to check using strace):
openat(AT_FDCWD, "/usr/local/logic/swipl/lib/swipl/ boot.prc", O_RDONLY) = 3
Whereas if you start swipl as
swipl --home=/home/foo
this happens:
openat(AT_FDCWD, "/home/foo/boot.prc", O_RDONLY) = -1 ENOENT (No such file or directory)
And you see:
$ swipl --home=/home/foo
[FATAL ERROR: at Fri Feb 26 10:53:59 2021
Could not find system resources]
Aborted (core dumped)
Conversely
$ swipl --home=/usr/local/logic/swipl/lib/swipl/
works nicely!
How do you pass command line arguments to swipl?
Here are two ways:
Conventional
When swipl is invoked from the command line, everything beyond the command line argument
--
is put into a list of atoms, which can be retrieved using the "current prolog flag" argv.
This is actually explained at the top of the swipl man page but it took me some time to find it.
The "double dash" is the conventional indication to a program to "stop processing options after this "double dash" and use anything that comes to the right as-is". As swipl likely calls getopt(3) for command-line processing, it inherits the conforming behaviour. As it is a way to explicitly mark command line arguments as non-options (e.g. to be able to pass strings like `--help` as argument) rather than mark a block of arguments as "arguments meant for the program", -- can be left out if it is not actually needed.
From POSIX Utility Syntax Guidelines:
The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '-' character.
Example:
Set this up in print_argv.pl where we print all results of current_prolog_flag/2 for flag argv using forall/2 (there is only one result though):
print_argv :-
format("Printing argv for great justice~n",[]),
forall(current_prolog_flag(argv, X),format("~q~n",[X])).
Then execute this on the command line:
swipl -g print_argv -t halt print_argv.pl -- foo 'baz(1)' 'hello world' --help
In detail:
$ swipl -g print_argv # goal to run immediately
-t halt # halt instead of starting toplevel REPL
print_argv.pl # file to load
-- # anything past this is definitely not an option
foo 'baz(1)' 'hello world' '--help'
And so you see this, as all the args are put into a list assigned to the argv prolog flag:
$ swipl -g print_argv -t halt print_argv.pl -- foo 'baz(1)' 'hello world' --help Printing argv for great justice [foo,'baz(1)','hello world','--help']
This also works if you leave out the -- (a bit surprisingly, even if some of the argument suspiciously look like options):
$ swipl -g print_argv -t halt print_argv.pl foo 'baz(1)' 'hello world' Printing argv for great justice [foo,'baz(1)','hello world'] $ swipl -g print_argv -t halt print_argv.pl -foo 'baz(1)' 'hello world' Printing argv for great justice ['-foo','baz(1)','hello world'] $ swipl -g print_argv -t halt print_argv.pl --foo 'baz(1)' 'hello world' Printing argv for great justice ['--foo','baz(1)','hello world']
Okay!
Unconventional: Use the Prolog database
Define file print_argv_2
print_argv_2 :-
format("Printing argv for great justice~n",[]),
forall(user:arg(X),format("~q~n",[X])). % Module "user:" qualification can be left out
Then start swipl by giving it goals which assert facts for arg/1 into the database. The facts are asserted into module user. The first use of assertz/1 implicitly declares arg/1 as "dynamic" so we don't have to do that explicitly:
$ swipl \ -g 'assertz(arg(foo))' \ -g 'assertz(arg(bar))' \ -g print_argv_2 \ -t halt \ print_argv_2.pl
And we get:
Printing argv for great justice foo bar
Comment note
The entry for -f says that the default initialization file is init.pl (searched for, according to another section of the manual, in various places); in practice, it appears to be .swiplrc (searched for in the user's home directory).
Not true, both files are looked for.
Using
$ strace swipl 2> OUT
on the command line to get the process's system calls, we find these actions (in this case, none of the files are found).
newfstatat(AT_FDCWD, "init.pl", 0xfffff6db95f0, 0) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/home/swipl/.config", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
newfstatat(AT_FDCWD, "/home/swipl/.config/swi-prolog", {st_mode=S_IFDIR|0755, st_size=4096, ...}, 0) = 0
faccessat(AT_FDCWD, "/home/swipl/.config/swi-prolog/init.pl", R_OK) = -1 ENOENT (No such file or directory)
newfstatat(AT_FDCWD, "/home/swipl/.swiplrc", 0xfffff6db95f0, 0) = -1 ENOENT (No such file or directory)
write(2, "Welcome to SWI-Prolog (threaded,"..., 256Welcome to SWI-Prolog (threaded, 64 bits, version 8.3.19-12-g9a371f586)
