Sgrep (sorted grep) searches sorted input files for lines that
match a search key and outputs the matching lines.
When searching large files sgrep is much faster
than traditional Unix grep, but with significant restrictions.
All input files must be sorted regular files.
The sort key must start at the beginning of the line.
The search key matches only at the beginning of the line.
No regular expression support.
Sgrep uses a binary search algorithm, which is very fast, but
requires sorted input. Each iteration of the search eliminates
half of the remaining input. In other words, doubling the size
of the file adds just one iteration.
sgrep [ -i | -n ] [ -c ] [ -b ] [ -r ] [ -- ] key
[ sorted_file ... ]
- -i (or -f)
By default sgrep uses exact byte comparison. The -i flag (or -f)
causes sgrep to use case insensitive byte comparison.
For example aa matches AA or aA, etc.
The input file must be sorted with sort -f.
The -n flag causes sgrep to use numeric comparison. The input file
must be sorted with sort -n. Each line of the file should
begin with optional white space and a numeric text string such as
055 or -27 or 73.57. Sgrep compares the floating point
value of the numeric string with the floating point value of the
search key. For example, the search key 10 matches 10.0,
010, 010.00, etc. Non numeric text strings have the value
zero. The numeric search key may optionally be followed by a
colon and a second number, which specifies a range for the key,
such as 10.0:10.999.
The -b flag causes sgrep to ignore leading white space at the
beginning of a line and at the beginning of the search key.
The input file must be sorted with sort -b.
By default sgrep outputs all lines where the search key matches
the beginning of the line. The -c flag causes sgrep to output
the number of matching lines.
The -r flag specifies that the input file is sorted in reverse
(descending) order using sort -r.
The -- flag terminates the option flags and the next argument
is used as the search key. You need to use -- if the search
key begins with -, such as
sgrep -n -- -25 file
If no input files are specified, then sgrep uses stdin which
must be a sorted regular file. You cannot pipe into sgrep.
Sgrep compares bytes using the native hardware byte order, so the file
must be sorted accordingly. You should set the environment
variable LC_ALL to C before running sort or sgrep.
sort -o sorted unsorted
sgrep key sorted
Sgrep offers negligible speed improvement over traditional grep
on files smaller than 100 KBytes, is measurably faster on
files larger than a few MBytes, and is dramatically faster
on files larger than 1 GByte.
Sgrep searches large sorted files very efficiently. But the
overhead of sorting a very large file is considerable, so sgrep
is most useful in situations where many searches are performed
on the same file.
Some files, such as system logs, are naturally output in
chronological order. If the date is output at the beginning
of the line in a format such as YYYY/MM/DD:hh:mm:ss,
then the file is already sorted and suitable for quick
search by date using sgrep.
Stephen C. Losen, University of Virginia