Grep from the end of a file to the beginning

tac only helps if you also use grep -m 1 (assuming GNU grep) to have grep stop after the first match:

tac accounting.log | grep -m 1 foo

From man grep:

   -m NUM, --max-count=NUM
          Stop reading a file after NUM matching lines.  

In the example in your question, both tac and grep need to process the entire file so using tac is kind of pointless.

So, unless you use grep -m, don't use tac at all, just parse the output of grep to get the last match:

grep foo accounting.log | tail -n 1 

---

Another approach would be to use Perl or any other scripting language. For example (where $pattern=foo):

perl -ne '$l=$_ if /foo/; END{print $l}' file

or

awk '/foo/{k=$0}END{print k}' file

The reason why

tac file | grep foo | head -n 1

doesn't stop at the first match is because of buffering.

Normally, head -n 1 exits after reading a line. So grep should get a SIGPIPE and exit as well as soon as it writes its second line.

But what happens is that because its output is not going to a terminal, grep buffers it. That is, it's not writing it until it has accumulated enough (4096 bytes in my test with GNU grep).

What that means is that grep will not exit before it has written 8192 bytes of data, so probably quite a few lines.

With GNU grep, you can make it exit sooner by using --line-buffered which tells it to write lines as soon as they are found regardless of whether goes to a terminal or not. So grep would then exit upon the second line it finds.

But with GNU grep anyway, you can use -m 1 instead as @terdon has shown, which is better as it exits at the first match.

If your grep is not the GNU grep, then you can use sed or awk instead. But tac being a GNU command, I doubt you'll find a system with tac where grep is not GNU grep.

tac file | sed "/$pattern/!d;q"                             # BRE
tac file | P=$pattern awk '$0 ~ ENVIRON["P"] {print; exit}' # ERE

Some systems have tail -r to do the same thing as GNU tac does.

Note that, for regular (seekable) files, tac and tail -r are efficient because they do read the files backward, they're not just reading the file fully in memory before printing it backward (as @slm's sed approach or tac on non-regular files would).

On systems where neither tac nor tail -r are available, the only options are to implement the backward-reading by hand with programming languages like perl or use:

grep -e "$pattern" file | tail -n1

Or:

sed "/$pattern/h;$!d;g" file

But those mean finding all the matches and only print the last one.

Here is a possible solution that will find the location of first occurrence of pattern from last:

tac -s "$pattern" -r accounting.log | head -n 1

This makes use of the -s and -r switches of tac which are as follows:

-s, --separator=STRING
use STRING as the separator instead of newline

-r, --regex
interpret the separator as a regular expression