GREP

grep, egrep, fgrep ? print lines matching a pattern

grep [?[ABC] NUM] [?EFGHLUVZabchilnqrsuvwxyz] [?e PATTERN | ?f FILE] [?d ACTION] [??directories=ACTION] [??extended-regexp] [??fixed-strings] [??basic-regexp] [??regexp=PATTERN] [??file=FILE] [??ignore-case] [??word-regexp] [??line-regexp] [??line-regexp] [??no-messages] [??invert-match] [??version] [??help] [??byte-offset] [??line-number] [??with-filename] [??no-filename] [??quiet] [??silent] [??text] [??files-without-match] [??files-with-matches] [??count] [??before-context=NUM] [??after-context=NUM] [??context[=NUM]] [??binary] [??unix-byte-offsets] [??mmap] [??null] [??recursive] [file...]

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, ??basic-regexp

Interpret pattern as a basic regular expression (see below). This is the default.

?E, ??extended-regexp

Interpret pattern as an extended regular expression (see below).

?F, ??fixed-strings

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 the same as grep ?E. Fgrep is the same as grep ?F.

All variants of grep understand the following options:

?A NUM, ??after-context=NUM

Print NUM lines of trailing context after matching lines.

?B NUM, ??before-context=NUM

Print NUM lines of leading context before matching lines.

?C [NUM], ??context[=NUM]

Print NUM lines (default 2) of output context.

?NUM

Same as ??context=NUM lines of leading and trailing context. However, grep will never print any given line more than once.

?V, ??version

Print the version number of grep to standard error. This version number should be included in all bug reports (see below).

?b, ??byte-offset

Print the byte offset within the input file before each line of output.

?c, ??count

Suppress normal output; instead print a count of matching lines for each input file. With the ?v, ??invert-match option (see below), count non-matching lines.

?d ACTION, ??directories=ACTION

If an input file is a directory, use ACTION to process it. By default, ACTION is read, which means that directories are read just as if they were ordinary files. If ACTION is skip, directories are silently skipped. If ACTION is recurse, grep reads all files under each directory, recursively; this is equivalent to the ?r option.

?e PATTERN, ??regexp=PATTERN

Use PATTERN as the pattern; useful to protect patterns beginning with ?.

?f FILE, ??file=FILE

Obtain patterns from FILE, one per line. The empty file contains zero patterns, and therfore matches nothing.

?H, ??with-filename

Print the filename for each match.

?h, ??no-filename

Suppress the prefixing of filenames on output when multiple files are searched.

?i, ??ignore-case

Ignore case distinctions in both the pattern and the input files.

?L, ??files-without-match

Suppress normal output; instead print the name of each input file from which no output would normally have been printed. The scanning will stop on the first match.

?l, ??files-with-matches

Suppress normal output; instead print the name of each input file from which output would normally have been printed. The scanning will stop on the first match.

?n, ??line-number

Prefix each line of output with the line number within its input file.

?q, ??quiet, ??silent

Quiet; suppress normal output. The scanning will stop on the first match. Also see the ?s or ??no-messages option below.

?r, ??recursive

Read all files under each directory, recursively; this is equivalent to the ?d recurse option.

?s, ??no-messages

Suppress error messages about nonexistent or unreadable files. Portability note: unlike GNU grep, traditional grep did not conform to POSIX.2 , because traditional grep lacked a ?q option and its ?s option behaved like GNU grep’s ?q option. Shell scripts intended to be portable to traditional grep should avoid both ?q and ?s and should redirect output to /dev/null instead.

?a, ??text

Do not suppress output lines that contain binary data. Normally, if the first few bytes of a file indicate that the file contains binary data, grep outputs only a message saying that the file matches the pattern. This option causes grep to act as if the file is a text file, even if it would otherwise be treated as binary.

?v, ??invert-match

Invert the sense of matching, to select non-matching lines.

?w, ??word-regexp

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, ??line-regexp

Select only those matches that exactly match the whole line.

?y

Obsolete synonym for ?i.

?U, ??binary

Treat the file(s) as binary. By default, under MS-DOS and MS-Windows, grep guesses the file type by looking at the contents of the first 32KB read from the file. If grep decides the file is a text file, it strips the CR characters from the original file contents (to make regular expressions with ^ and $ work correctly). Specifying ?U overrules this guesswork, causing all files to be read and passed to the matching mechanism verbatim; if the file is a text file with CR/LF pairs at the end of each line, this will cause some regular expressions to fail. This option has no effect on platforms other than MS-DOS and MS-Windows.

?u, ??unix-byte-offsets

Report Unix-style byte offsets. This switch causes grep to report byte offsets as if the file were Unix-style text file, i.e. with CR characters stripped off. This will produce results identical to running grep on a Unix machine. This option has no effect unless ?b option is also used; it has no effect on platforms other than MS-DOS and MS-Windows.

??mmap

If possible, use the mmap(2) system call to read input, instead of the default read(2) system call. In some situations, --mmap yields better performance. However, --mmap can cause undefined behavior (including core dumps) if an input file shrinks while grep is operating, or if an I/O error occurs.

?Z, ??null

Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name. For example, grep ?lZ outputs a zero byte after each file name instead of the usual newline. This option makes the output unambiguous, even in the presence of file names containing unusual characters like newlines. This option can be used with commands like find ?print0, perl ?0, sort ?z, and xargs ?0 to process arbitrary file names, even those that contain newline characters.

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:

?

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.

{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 \?, \+, \{, \|, \(, and \).

Traditional egrep did not support the { metacharacter, and some egrep implementations support \{ instead, so portable scripts should avoid { in egrep patterns and should use [{] to match a literal {.

GNU egrep attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification. For example, the shell command egrep ’{1’ searches for the two-character string {1 instead of reporting a syntax error in the regular expression. POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

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.