NAME
tac —
concatenate reversed files
SYNOPSIS
tac |
[-rb] [-s
eparator] [file]… |
DESCRIPTION
Read each file (standard input stream if "-", the default), split it on the separator expression, matching from the end, and write it back out to the standard output stream, starting with the final match.
OPTIONS
-s,--separator=sep- Separate by sep instead of a single newline. An
empty separator matches nothing (and the invocation is equivalent to
cat). -r,--regex- Treat separator as basic regular expression. An empty separator is refused.
-b,--before- Write separator before its line.
EXIT STATUS
1 if a file couldn't be opened or read.
SEE ALSO
BUGS
Expects to map or read each file into main memory.
HISTORY
tail appeared in
Version 7 AT&T UNIX, allowing up to one
file, with an undocumented-in-the-manual -r mode,
largely equivalent to present-day tac, but allowing
the usual lbc specifiers for line,
512-byte
block, and byte limits (with a global 4-kilobyte buffer for
-r (and relative-to-EOF addressing it implied)).
3BSD removes
-r.
4.2BSD re-adds -r,
with broken interactions with bc specifiers.
4.3BSD bumps the buffer size to 32
kilobytes and fixes the c interaction to not affect
the result.
4.4BSD fixes bc
(-bc) interactions with r
(-r), allows multiple files (with the usual
tail headers between them),
and removes buffer limitations.
AT&T System III UNIX removes
-r.
AT&T System V Release 4 UNIX re-adds it, only allowing line limits.
As such, tac originates, fully formed,
from GNU textutils at or before version 1.11, and persists in GNU coreutils;
this implementation is compatible with the GNU system.
NetBSD 9.0 provides
tac as a link to tail,
implying -r with headings suppressed, quoting
compatibility with the GNU system.