Daat HaRav — The Shulchan Aruch HaRav on Siman שמ"ד

דִּין הַהוֹלֵךְ בַּמִּדְבָּר בְּשַׁבָּת וּבוֹ ב' סְעִיפִים:

הַהוֹלֵךְ בַּמִּדְבָּר וְאֵינוֹ יוֹדֵעַ מָתַי שַׁבָּת — מוֹנֶה ז' יָמִים מִיּוֹם שֶׁנָּתַן אֶל לִבּוֹ שִׁכְחָתוֹ, וּמְקַדֵּשׁ שְׁבִיעִי בְּקִדּוּשׁ וְהַבְדָּלָה לְזִכָּרוֹן בְּעָלְמָא, שֶׁיְּהֵא לוֹ שָׁם יוֹם חָלוּק מִשְּׁאָר יָמִים, וְלֹא תִשְׁתַּכַּח תּוֹרַת שַׁבָּת מִמֶּנּוּ. אֲבָל מֻתָּר לֵילֵךְ בּוֹ אֲפִלּוּ כַּמָּה פַּרְסָאוֹת, כְּדֵי לְמַהֵר לָצֵאת מִן הַמִּדְבָּר.

וְאִם יֵשׁ לוֹ מִמַּה לְהִתְפַּרְנֵס — אָסוּר לוֹ לַעֲשׂוֹת שׁוּם מְלָאכָה גְמוּרָה אֲפִלּוּ בְּשֵׁשֶׁת יְמֵי הַחֹל שֶׁלּוֹ עַד שֶׁיִּכְלֶה מַה שֶּׁיֵּשׁ לוֹ, מִפְּנֵי שֶׁכָּל יוֹם סְפֵק שַׁבָּת הוּא. וּדְבָרִים הָאֲסוּרִים בְּשַׁבָּת מִדִּבְרֵי סוֹפְרִים — מֻתָּר לַעֲשׂוֹתָם בְּכָל יְמֵי הַחֹל שֶׁלּוֹ, מִפְּנֵי שֶׁכָּל סְפֵק דִּבְרֵי סוֹפְרִים לְהָקֵל. אֲבָל בְּז' שֶׁלּוֹ, כֵּיוָן שֶׁחָל עָלָיו שַׁבָּת מִדִּבְרֵי סוֹפְרִים — אָסוּר בְּכָל שְׁבוּת, כְּמוֹ יוֹם טוֹב שֵׁנִי שֶׁהוּא גַם כֵּן סָפֵק, וְאָסוּר בְּכָל שְׁבוּת מִטַּעַם זֶה.

וּלְאַחַר שֶׁיִּכְלֶה כָּל מַה שֶּׁיֵּשׁ לוֹ — מֻתָּר לוֹ לַעֲשׂוֹת מְלָאכָה אֵצֶל בְּנֵי הַשַּׁיָּרָא לְהִשְׂתַּכֵּר כְּדֵי פַרְנָסָתוֹ בְּכָל יוֹם וָיוֹם אֲפִלּוּ בַּשְּׁבִיעִי שֶׁלּוֹ, מִפְּנֵי פִּקּוּחַ נֶפֶשׁ. וְלֹא יַעֲשֶׂה אֶלָּא כְּדֵי פַּרְנָסָה מְצֻמְצֶמֶת שֶׁיּוּכַל לִחְיוֹת בָּהּ אוֹתוֹ הַיּוֹם בִּלְבָד, אֲבָל לֹא מִיּוֹם לַחֲבֵרוֹ, אֲפִלּוּ מִשִּׁשִּׁי לַשְּׁבִיעִי שֶׁלּוֹ, כִּי שֶׁמָּא הוּא עוֹשֶׂה מִשַּׁבָּת לְחֹל, וְאֵין בְּזֶה צֹרֶךְ לְפִקּוּחַ נֶפֶשׁ, שֶׁהֲרֵי יוּכַל לַעֲשׂוֹת גַּם לְמָחָר בְּחֹל.

וְאִם יֵשׁ לוֹ לֶחֶם לֶאֱכוֹל — אֵינוֹ רַשַּׁאי לְבַשֵּׁל לוֹ, מִפְּנֵי שֶׁאֵין בְּזֶה פִּקּוּחַ נֶפֶשׁ:

The law concerning one who walks in the wilderness on Shabbos — and it contains 2 seifim:

One who walks in the wilderness and does not know when Shabbos falls — counts seven days from the day he took note in his heart of his forgetting, and sanctifies the seventh with Kiddush and Havdalah as a mere remembrance, so that he should have there a day distinct from the rest of his days, and that the substance of Shabbos (toras Shabbos) not be forgotten from him. But it is permitted for him to walk on it even several parsa'os, in order to hasten his exit from the wilderness.

And if he has from what to sustain himself — it is forbidden for him to do any full melachah even on the six weekdays he counts, until what he has runs out — for every day is for him a safek Shabbos. And things which are forbidden on Shabbos by the words of the Sofrim — it is permitted for him to do them on all his weekdays, since every safek of rabbinic origin is leniently treated (safek divrei sofrim le-hakel). But on his seventh day, since Shabbos has come upon him by the words of the Sofrim — he is forbidden in every shevus, like the second day of Yom Tov which is likewise a doubt, and which is forbidden in every shevus for this same reason.

And after all that he has runs out — it is permitted for him to do melachah with the members of the caravan (bnei ha-shayara) to earn his sustenance each and every day, even on his seventh, on account of pikuach nefesh. And he shall not do more than a strictly limited measure of sustenance — enough to live by for that day only — but not from one day to another, even from the sixth to his seventh, since perhaps he is making [provisions] from Shabbos to a weekday — and there is no pikuach nefesh need in this, since he will be able to do [the melachah] tomorrow as a weekday.

And if he has bread to eat — he is not permitted to cook for himself, since there is no pikuach nefesh in this:

הָיָה יוֹדֵעַ מִנְיַן יוֹם שֶׁיָּצָא בוֹ, כְּגוֹן שֶׁיּוֹדֵעַ שֶׁהַיּוֹם יוֹם רְבִיעִי אוֹ יוֹם חֲמִישִׁי לִיצִיאָתוֹ, אֲבָל אֵינוֹ יוֹדֵעַ בְּאֵיזֶה יוֹם מִימֵי הַשַּׁבָּת יָצָא — מֻתָּר לוֹ לַעֲשׂוֹת מְלָאכָה כָּל כַּמָּה שֶׁיִּרְצֶה בְּיוֹם שְׁמִינִי לִיצִיאָתוֹ, שֶׁבְּיוֹם כַּזֶה לֹא יָצָא מִבֵּיתוֹ, כִּי בְּוַדַּאי לֹא יָצָא בְּשַׁבָּת, וְכֵן בְּיוֹם ט"ו, וּכְמוֹ כֵן בְּיוֹם כ"ב, וְכֵן לְעוֹלָם:

If he knew the number of the day he set out on — for instance, if he knows that today is the fourth or fifth day from his departure, but he does not know on which day of the week he departed — it is permitted for him to do melachah however much he wishes on the eighth day from his departure, since on such a day he did not leave his house (for he certainly did not leave on Shabbos); and likewise on the fifteenth day, and likewise on the twenty-second, and so forever: