Tail Command – Fixing ‘tail -s N’ Not Sleeping for N Seconds

(take this with a pinch of salt) As far as I remember, the problem lies in the way libiconv works. Multi-byte encodings need a state machine to decode them, and libiconv prefers to receive entire characters, so you can't just give it half a character in one function call and the other half in the next.

I can think of another two solutions, one is a good out-of-band method, the other is an in-band hack.

Change Terminal Emulator encoding (out-of-band): one is to change the character encoding in your terminal emulator, so its native encoding is Shift JIS. I just checked konsole, and is supports this. From the menu, View→Character encoding→Japenese→sjis. You can then just tail -f the file, and konsole will take care of decoding the multibyte characters and matching them up to font glyphs.

Transcode terminal encoding on the fly (in-band; best): courtesy of Gilles, who reminded me of luit after a very long time. Use luit, which should have come with your XOrg distribution (on Debian, it's package x11-utils). Use it like this:

$ luit -encoding SJIS -- tail -f x

This will make the terminal transcode SJIS to/from your terminal encoding, and run tail -f x. The downside of luit is that it doesn't support the wealth of encodings supported by libiconv. The upside is it's available almost everywhere.

Transcode terminal encoding on the fly (in-band; hack): ttyconv is a hack I wrote many years ago (initially in C, later redone in Python) which uses libiconv to transcode terminal I/O. It spawns a new pseudoterminal and (a) transcodes the characters you type from your local encoding into the remote encoding, and (b) transcodes the characters you receive from the remote encoding to your local encoding. I used it to talk to servers that used encodings not supported by the standard Linux terminals. Please note that all of the remote encodings I tested it with were single-byte encodings, so I can't guarantee it'd work for Shift JIS. I don't often find call to use it these days, with most systems switching to Unicode.

This is how you would use it:

$ ttyconv -rsjis -- tail -f x

The downside of ttyconv is that I wrote it, no-one uses it but me, it's probably full of bugs. I excel at this. The upside is that it uses libiconv, so if your encoding is unusual, it's your best bet. At last count, ttyconv --list supports 100 encodings.

No, tail doesn't read the whole file, it seeks to the end then read blocks backwards until the expected number of lines have been reached, then it displays the lines in the proper direction until the end of the file, and possibly stays monitoring the file if the -f option is used.

Note however that tail has no choice but to read the whole data if provided a non seekable input, for example when reading from a pipe.

Similarily, when asked to look for lines starting from the beginning of the file, with using the tail -n +linenumber syntax or tail +linenumber non standard option when supported, tail obviously reads the whole file (unless interrupted).