Intro To 'tac' Command In Linux

2023-08-02 - By Robert Elder

I use the 'tac' command to print files in reverse order:

tac numbers.txt

The 'cat' Command Versus 'tac' Command

Here, I have a file called 'numbers.txt':

one
two
three
four
five
six
seven
eight

By default, the 'tac' command works just like the 'cat' command:

cat numbers.txt

one
two
three
four
five
six
seven
eight

except the 'tac' command prints the lines in a file in reverse order:

tac numbers.txt

eight
seven
six
five
four
three
two
one

Note that the word 'tac' is the word 'cat' spelled backwards.

An Example Use Case Of The 'tac' Command

Here, I have a file called 'text-editors.txt' that contains the following text:

emacs
is better than
vim

This statement is clearly incorrect. I can use the 'tac' command to correct the statement in the file like this:

tac text-editors.txt

vim
is better than
emacs

Now it correctly says 'vim is better than emacs' (which is a universally accepted fact).

Using A Custom Separator

I can also use the '-s' flag to specify a custom separator, like a dash character, and the order of the items between the separators will be reversed:

echo -n "one-two-three-" | tac -s '-'

three-two-one-

Reversing Bytes In A File Doesn't Work

The documentation of the 'tac' command states that you can use this regular expression based separator to reverse the order of all bytes in a file:

info tac

...
   Example:

     # Reverse a file character by character.
     tac -r -s 'x\|[^x]'
...

On my machine, this example does correctly reverse the order of these bytes:

echo -en "\x00\x01" | tac -r -s 'x\|[^x]' | xxd

00000000: 0100                                     ..

but it does not reverse the order of these bytes:

echo -en "\x80\x81" | tac -r -s 'x\|[^x]' | xxd

00000000: 8081                                     ..

Further Investigation Into Byte Reversing

When I was writing this article, I expected the above 'byte reversing' example to 'just work' for all bytes, but as you can see it did not. During my tests, I wrote the following script when testing the 'tac' command which I will include here:

for i in {1..100}
do
	head -c 2 /dev/urandom > $i.test;
	cat $i.test |
	tac -r -s 'x\|[^x]' |
	tac -r -s 'x\|[^x]' > $i.test.rev;
	md5sum $i.test >> before;
	md5sum $i.test.rev >> after;
done

The above script will create 100 files with two random bytes in them. For each file, it will use the 'tac' command to reverse the file twice. The md5 hash of both the before and after files is calculated for every file. Reversing a file twice should (in theory) return its contents to the original value, so the 'before' and 'after' files should have all the same hash values. Using this 'vim' command, I compare the contents of these files:

vim -d before after

And here is the result:

Clearly, the 'tac' command is not an appropriate tool if you want to reverse all of the bytes in a file.

Why Doesn't 'tac' Correctly Reverse Bytes?

Here were the inputs for the test cases that failed:

00000000: 6dcc                                     m.
00000000: 028d                                     ..
00000000: 7da2                                     }.
00000000: 78f2                                     x.
00000000: 3ed9                                     >.
00000000: 18b5                                     ..
00000000: 7089                                     p.
00000000: 50a2                                     P.
00000000: 22c0                                     ".
00000000: 1098                                     ..
00000000: 6bbf                                     k.
00000000: 5996                                     Y.
00000000: 5dce                                     ].
00000000: 62a4                                     b.
00000000: 7983                                     y.
00000000: 34df                                     4.
00000000: 5cd0                                     \.
00000000: 4cfb                                     L.
00000000: 0a8b                                     ..
00000000: 16db                                     ..
00000000: 70ae                                     p.
00000000: 61bf                                     a.
00000000: 5282                                     R.
00000000: 41cc                                     A.

And here were the inputs for the test cases that passed:

00000000: fccb                                     ..
00000000: da2d                                     .-
00000000: 5b12                                     [.
00000000: 435f                                     C_
00000000: 3667                                     6g
00000000: a7eb                                     ..
00000000: f17b                                     .{
00000000: c4e2                                     ..
00000000: 3d2b                                     =+
00000000: c4d0                                     ..
00000000: 2d69                                     -i
00000000: 5e1c                                     ^.
00000000: ca24                                     .$
00000000: e6ff                                     ..
00000000: 8944                                     .D
00000000: 371e                                     7.
00000000: e6f6                                     ..
00000000: 96da                                     ..
00000000: 2f47                                     /G
00000000: b237                                     .7
00000000: b00a                                     ..
00000000: a2fd                                     ..
00000000: ee89                                     ..
00000000: d856                                     .V
00000000: 3327                                     3'
00000000: b52f                                     ./
00000000: 520d                                     R.
00000000: a4e2                                     ..
00000000: cedd                                     ..
00000000: b0fa                                     ..
00000000: f6c1                                     ..
00000000: f0ca                                     ..
00000000: 8921                                     .!
00000000: 8eea                                     ..
00000000: ab77                                     .w
00000000: ff99                                     ..
00000000: 3d0b                                     =.
00000000: a4d4                                     ..
00000000: bd92                                     ..
00000000: 8d37                                     .7
00000000: 284e                                     (N
00000000: b9b2                                     ..
00000000: 9c28                                     .(
00000000: e463                                     .c
00000000: 1e03                                     ..
00000000: 889b                                     ..
00000000: 6c6a                                     lj
00000000: 97bb                                     ..
00000000: 7a1f                                     z.
00000000: e5d0                                     ..
00000000: 8681                                     ..
00000000: bf82                                     ..
00000000: 1801                                     ..
00000000: 040c                                     ..
00000000: 4b09                                     K.
00000000: 8c2d                                     .-
00000000: eb9c                                     ..
00000000: 8e1c                                     ..
00000000: f7f9                                     ..
00000000: 4b57                                     KW
00000000: ba7d                                     .}
00000000: 1053                                     .S
00000000: 9772                                     .r
00000000: eb48                                     .H
00000000: f978                                     .x
00000000: c1b3                                     ..
00000000: e038                                     .8
00000000: d01c                                     ..
00000000: 9cb7                                     ..
00000000: d9de                                     ..
00000000: 193a                                     .:
00000000: 3015                                     0.
00000000: 6340                                     c@
00000000: 978a                                     ..
00000000: 84a2                                     ..
00000000: 9a0e                                     ..

The common theme above appears to be that the failing cases had an ASCII character in the first byte and a non-ASCII character in the second byte. Perhaps this is a bug? Or, perhaps this is expected behaviour (or potentially undefined behaviour) in the flavour of regular expression that the 'tac' command uses? I am not sure, and I've decided that I don't really care.

And that's why the 'tac' command is my favourite Linux command.

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