7 releases (1 stable)
|2.0.0||Aug 13, 2022|
|0.2.3||Aug 17, 2017|
|0.1.1||Aug 15, 2017|
#126 in Command line utilities
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tac is a high-performance, simd-accelerated, cross-platform rewrite of the GNU
tac utility from Coreutils, released under a BSD-compatible (MIT) license.
tac reads input from a file (or from
stdin, but see below) and then prints it line-by-line backwards.
tac implementation uses simd-acceleration for new line detection and utilizes memory-mapped files on all supported operating systems. It is additionally written in rust for maximum integrity and safety.
Who needs a faster
Good question. Try grepping through a multi-gigabyte web access log file in reverse chronological order (
tac --line-buffered access.log | grep foo) and then get back to me.
Usage: tac [OPTIONS] [FILE1..] Write each FILE to standard output, last line first. Reads from stdin if FILE is - or not specified. Options: -h --help Print this help text and exit -v --version Print version and exit. --line-buffered Always flush output after each line.
tac reads lines from any combination of
stdin and/or zero or more files and writes the lines to the output in reverse order.
$ echo -e "hello\nworld" | tac world hello
tac may be installed via cargo, the rust package manager:
cargo install tac
Help is humbly requested in getting
tac into the package managers for various platforms. It's really a time-consuming task, especially for someone that only interacts with the various packaging tools once in a blue moon as opposed to on a daily basis.
This implementation of
tac uses the AVX2 instruction set to provide SIMD acceleration for the detection of new lines. The usage of memory-mapped files additionally boosts performance by avoiding slowdowns caused by context switches when reading from the input if speculative execution mitigations are enabled. It is significantly (2.55x if mitigations disabled, more otherwise) faster than the version of
tac that ships with GNU Coreutils, in addition to being more liberally licensed.
To obtain maximum performance:
- Try not to pipe input into
tac. e.g. instead of running
cat /usr/share/dict/words | tac, run
tac /usr/share/dict/wordsdirectly. Because
tacby definition must reach the end-of-file before it can emit its input with the lines reversed, if you use
cat foo | tac), it must buffer all
stdininput before it can begin to process the results.
tacwill try to buffer in memory, but once it exceeds a certain high-water mark (currently 4 MiB), it switches to disk-based buffering (because it can't know how large the input is or if it will end up exceeding the available free memory).
- Always try to place
tacat the start of a pipeline where possible. Even if you can guarantee that the input to
tacwill not exceed the in-memory buffering limit (see above),
tacis almost certainly faster than any other command in your pipeline, and if you are going to reverse the output, you will benefit most if you reverse it from the start. For example, instead of running
grep foo /var/log/nginx/access.log | tac, run
tac /var/log/nginx/access.log | grep foo. This will (significantly) reduce the amount of work that
grep(or any other downstream executable in the pipeline) has to do before finding a match.
- Use line-buffered output mode (
tac --line-buffered) if tac is piping into another command rather than writing to the tty directly. This gives you "live" streaming of results and lets you terminate much sooner if you're only looking for the first n matches. e.g.
tac --line-buffered access.log | grep foowill print its first match much, much sooner than
tac access.log | grep foowould.
- In the same vein, if you are chaining the output of n utilities, make sure that all commands up to n - 1 are all using line-buffered mode unless you don't care about latency and only care about throughput. For example, to print the first two matches for some grep pattern:
tac --line-buffered access.log | grep --line-buffered foo | head -n2.
License and Copyright
tac is written by Mahmoud Al-Qudsi <email@example.com> of NeoSmart Technologies, and released under the terms of the MIT public license. Copyright NeoSmart Technologies 2021. All rights not assigned by the MIT license are reserved.