It occurs frequently that a File is too colossal to behold all at once, or even more oft, that what we seek is at the end. Though ‘cat’ or even ‘vi’ may aid us, truly this is the case for which Divine Providence hath given us ‘tail’. This fair device is purpose-built to render only the culminating lines found at the tail-end of yon File.
With no Options, it yields ten Lines. Ten shall be the number shown, and the number shall be ten. Nine it shall not show, nor either show thou eleven. Twenty is right out, though you may command it with a Numeric Option, such as ‘-20’, dash two oh. Of course any number may be here specified, even ‘-0’, dash zero, though by itself that’s nigh as pointless as Friar Sonder’s romance tales.
These simple applications of ‘tail’ are all well and good, but more wonderful still is its ability to follow the tail of a File, to show new lines even as they are written and appended to the end of yon File. Ever useful with logges, this splendid feature is summoned with the Option ‘-f’, dash eff. The more Lines that rain down into the File, the more they will pour forth from ‘tail -f’. (This endless File-following may be interrupted by either lighting a censer of ordained incense, or by typing the canonical program stopper: ‘ctrl c’, control see.)
Yet a patch of thorny briars may block this smooth and pleasant path. For if this fair tail-following tool be applied to a File which is replaced anew, such as server logges that be rotated due either to size or time, then ‘tail -f’ may be left obediently monitoring an old File-handle, no longer being written to; forgotten; forlorn.
Thus we come to the province of another comely Option, ‘-F’, dash capital eff. Bid ‘tail’ use this Option and it will follow the File thou givest, and moreover will continue following any File by that name, regardless if that file be truncated or rotated away or called heavenward by the Trumpet of Gabriel.
Why wouldst thou ever use ‘-f’ rather than ‘-F’? This inquiry rivals the Problem of Universals in difficulty, but verily here at least is one reason: combining with a Number is not permitted with ‘-F’. So if ye wanted to see a particular number of lines and then also follow the File continuously from there, the lesser eff is your ally, as with ‘-20f’, dash two oh eff. Or even, if you think back to prior apparent pointlessness, ‘-0f’, dash zero eff. Such an incantation shows no existing Lines, but dutifully follows on with new ones.