GREP(1) 10 GREP(1)
NAME
grep, egrep, fgrep - print lines matching a pattern
SYNOPOSIS
grep [ -[[AB] ]num ] [ -[CEFGVBchilnsvwx] ] [ -e ] pattern
| -ffile ] [ files... ]
DESCRIPTION
Grep searches the named input files (or standard input if
no files are named, or the file name - is given) for lines
containing a match to the given pattern. By default, grep
prints the matching lines.
There are three major variants of grep, controlled by the
following options.
-G Interpret pattern as a basic regular expression
(see below). This is the default.
-E Interpret pattern as an extended regular expression
(see below).
-F Interpret pattern as a list of fixed strings,
separated by newlines, any of which is to be
matched.
In addition, two variant programs egrep and fgrep are
available. Egrep is similiar (but not identical) to
grep -E, and is compatible with the historical Unix egrep.
Fgrep is the same as grep -F.
All variants of grep understand the following options:
-num Matches will be printed with num lines of leading
and trailing context. However, grep will never
print any given line more than once.
-A num Print num lines of trailing context after matching
lines.
-B num Print num lines of leading context before matching
lines.
-C Equivalent to -2.
-V Print the version number of grep to standard error.
This version number should be included in all bug
reports (see below).
-b Print the byte offset within the input file before
each line of output.
-c Suppress normal output; instead print a count of
matching lines for each input file. With the -v
option (see below), count non-matching lines.
-e pattern
Use pattern as the pattern; useful to protect
patterns beginning with -.
-f file
Obtain the pattern from file.
-h Suppress the prefixing of filenames on output when
multiple files are searched.
-i Ignore case distinctions in both the pattern and
the input files.
-L Suppress normal output; instead print the name of
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each input file from which no output would normally
have been printed.
-l Suppress normal output; instead print the name of
each input file from which output would normally
have been printed.
-n Prefix each line of output with the line number
within its input file.
-q Quiet; suppress normal output.
-s Suppress error messages about nonexistent or
unreadable files.
-v Invert the sense of matching, to select non-
matching lines.
-w Select only those lines containing matches that
form whole words. The test is that the matching
substring must either be at the beginning of the
line, or preceded by a non-word constituent
character. Similarly, it must be either at the end
of the line or followed by a non-word constituent
character. Word-constituent characters are
letters, digits, and the underscore.
-x Select only those matches that exactly match the
whole line.
REGULAR EXPRESSIONS
A regular expression is a pattern that describes a set of
strings. Regular expressions are constructed analagously
to arithmetic expressions, by using various operators to
combine smaller expressions.
Grep understands two different versions of regular
expression syntax: ``basic'' and ``extended.'' In
GNU grep, there is no difference in available
functionality using either syntax. In other
implementations, basic regular expressions are less
powerful. The following description applies to extended
regular expressions; differences for basic regular
expressions are summarized afterwards.
The fundamental building blocks are the regular
expressions that match a single character. Most
characters, including all letters and digits, are regular
expressions that match themselves. Any metacharacter with
special meaning may be quoted by preceding it with a
backslash.
A list of characters enclosed by [ and ] matches any
single character in that list; if the first character of
the list is the caret ^ then it matches any character not
in the list. For example, the regular expression
[0123456789] matches any single digit. A range of ASCII
characters may be specified by giving the first and last
characters, separated by a hyphen. Finally, certain named
classes of characters are predefined. Their names are
self explanatory, and they are [:alnum:], [:alpha:],
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[:cntrl:], [:digit:], [:graph:], [:lower:], [:print:],
[:punct:], [:space:], [:upper:], and [:xdigit:]. For
example, [[:alnum:]] means [0-9A-Za-z], except the latter
form is dependent upon the ASCII character encoding,
whereas the former is portable. (Note that the brackets
in these class names are part of the symbolic names, and
must be included in addition to the brackets delimiting
the bracket list.) Most metacharacters lose their special
meaning inside lists. To include a literal ] place it
first in the list. Similarly, to include a literal ^
place it anywhere but first. Finally, to include a
literal - place it last.
The period . matches any single character. The symbol \w
is a synonym for [[:alnum:]] and \W is a synonym for
[^[:alnum]].
The caret ^ and the dollar sign $ are metacharacters that
respectively match the empty string at the beginning and
end of a line. The symbols \< and \> respectively match
the empty string at the beginning and end of a word. The
symbol \b matches the empty string at the edge of a word,
and \B matches the empty string provided it's not at the
edge of a word.
A regular expression matching a single character may be
followed by one of several repetition operators:
? The preceding item is optional and matched at most
once.
* The preceding item will be matched zero or more
times.
+ The preceding item will be matched one or more
times.
{n} The preceding item is matched exactly n times.
{n,} The preceding item is matched n or more times.
{,m} The preceding item is optional and is matched at
most m times.
{n,m} The preceding item is matched at least n times, but
not more than m times.
Two regular expressions may be concatenated; the resulting
regular expression matches any string formed by
concatenating two substrings that respectively match the
concatenated subexpressions.
Two regular expressions may be joined by the infix
operator |; the resulting regular expression matches any
string matching either subexpression.
Repetition takes precedence over concatenation, which in
turn takes precedence over alternation. A whole
subexpression may be enclosed in parentheses to override
these precedence rules.
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The backreference \n, where n is a single digit, matches
the substring previously matched by the nth parenthesized
subexpression of the regular expression.
In basic regular expressions the metacharacters ?, +, {,
|, (, and ) lose their special meaning; instead use the
backslashed versions \?, \+, \{, \|, \(, and \).
In egrep the metacharacter { loses its special meaning;
instead use \{.
DIAGNOSTICS
Normally, exit status is 0 if matches were found, and 1 if
no matches were found. (The -v option inverts the sense
of the exit status.) Exit status is 2 if there were
syntax errors in the pattern, inaccessible input files, or
other system errors.
BUGS
Email bug reports to bug-gnu-utils@prep.ai.mit.edu. Be
sure to include the word ``grep'' somewhere in the
``Subject:'' field.
Large repetition counts in the {m,n} construct may cause
grep to use lots of memory. In addition, certain other
obscure regular expressions require exponential time and
space, and may cause grep to run out of memory.
Backreferences are very slow, and may require exponential
time.
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