GREP(1) manual page

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## 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 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:], [: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.

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.

Table of Contents

- Name
- Synoposis
- Description
- Regular Expressions
- Diagnostics
- Bugs