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.
In addition, two variant programs egrep and fgrep are available. Egrep is similar (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:
A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously 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:], [: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 may be followed by one of several repetition operators:
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.
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 \{.
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.
Email bug reports to bug-gnu-utils@gnu.org. 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.