grep - Unix, Linux Command
NAME
grep, egrep, fgrep - print lines matching a pattern
SYNOPSIS
grep [options] PATTERN [FILE...]
grep [options] [-e PATTERN |
-f FILE] [FILE...] 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.
In addition, two variant programs
egrep and
fgrep are available.
Egrep is the same as
grep -E. Fgrep is the same as
grep -F. OPTIONS
Tag | Description |
-A NUM, --after-context=NUM | |
Print
NUM lines of trailing context after matching lines.
Places a line containing
-- between contiguous groups of matches.
|
-a, --text | |
Process a binary file as if it were text; this is equivalent to the
--binary-files=text option.
|
-B NUM, --before-context=NUM | |
Print
NUM lines of leading context before matching lines.
Places a line containing
-- between contiguous groups of matches.
|
-C NUM, --context=NUM | |
Print
NUM lines of output context.
Places a line containing
-- between contiguous groups of matches.
|
-b, --byte-offset | |
Print the byte offset within the input file before
each line of output.
|
--binary-files=TYPE | |
If the first few bytes of a file indicate that the file contains binary
data, assume that the file is of type
TYPE. By default,
TYPE is
binary, and
grep normally outputs either
a one-line message saying that a binary file matches, or no message if
there is no match.
If
TYPE is
without-match, grep assumes that a binary file does not match; this is equivalent to the
-I option.
If
TYPE is
text, grep processes a binary file as if it were text; this is equivalent to the
-a option.
Warning: grep --binary-files=text might output binary garbage,
which can have nasty side effects if the output is a terminal and if the
terminal driver interprets some of it as commands.
|
--colour[=WHEN], --color[=WHEN] | |
Surround the matching string with the marker find in
GREP_COLOR environment variable. WHEN may be never, always, or auto
|
-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, --devices=ACTION | |
If an input file is a device, FIFO or socket, use
ACTION to process it. By default,
ACTION is
read, which means that devices are read just as if they were ordinary files.
If
ACTION is
skip, devices are silently skipped.
|
-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, --extended-regexp | |
Interpret
PATTERN as an extended regular expression (see below).
|
-e PATTERN, --regexp=PATTERN | |
Use
PATTERN as the pattern; useful to protect patterns beginning with
-. |
-F, --fixed-strings | |
Interpret
PATTERN as a list of fixed strings, separated by newlines,
any of which is to be matched.
|
-P, --perl-regexp | |
Interpret
PATTERN as a Perl regular expression.
|
-f FILE, --file=FILE | |
Obtain patterns from
FILE, one per line.
The empty file contains zero patterns, and therefore matches nothing.
|
-G, --basic-regexp | |
Interpret
PATTERN as a basic regular expression (see below). This is the default.
|
-H, --with-filename | |
Print the filename for each match.
|
-h, --no-filename | |
Suppress the prefixing of filenames on output
when multiple files are searched.
|
--help |
Output a brief help message.
|
-I |
Process a binary file as if it did not contain matching data; this is
equivalent to the
--binary-files=without-match option.
|
-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.
|
-m NUM, --max-count=NUM | |
Stop reading a file after
NUM matching lines. If the input is standard input from a regular file,
and
NUM matching lines are output,
grep ensures that the standard input is positioned to just after the last
matching line before exiting, regardless of the presence of trailing
context lines. This enables a calling process to resume a search.
When
grep stops after
NUM matching lines, it outputs any trailing context lines. When the
-c or
--count option is also used,
grep does not output a count greater than
NUM. When the
-v or
--invert-match option is also used,
grep stops after outputting
NUM non-matching lines.
|
--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.
|
-n, --line-number | |
Prefix each line of output with the line number
within its input file.
|
-o, --only-matching | |
Show only the part of a matching line that matches
PATTERN. |
--label=LABEL | |
Displays input actually coming from standard input as input coming from file
LABEL. This is especially useful for tools like zgrep, e.g.
gzip -cd foo.gz |grep -H --label=foo something |
--line-buffered | |
Use line buffering, it can be a performance penality.
|
-q, --quiet, --silent | |
Quiet; do not write anything to standard output.
Exit immediately with zero status if any match is found,
even if an error was detected.
Also see the
-s or
--no-messages option.
|
-R, -r, --recursive | |
Read all files under each directory, recursively;
this is equivalent to the
-d recurse option.
|
--include=PATTERN | |
Recurse in directories only searching file matching
PATTERN. |
--exclude=PATTERN | |
Recurse in directories skip file matching
PATTERN. |
-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
greps -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.
|
-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.
|
-V, --version | |
Print the version number of
grep to standard error. This version number should
be included in all bug reports (see below).
|
-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. |
-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.
|
REGULAR EXPRESSIONS
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
bracket expression is a list of characters enclosed by
[ and
]. It 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.
Within a bracket expression, a
range expression consists of two characters separated by a hyphen.
It matches any single character that sorts between the two characters,
inclusive, using the locales collating sequence and character set.
For example, in the default C locale,
[a-d] is equivalent to
[abcd]. Many locales sort characters in dictionary order, and in these locales
[a-d] is typically not equivalent to
[abcd]; it might be equivalent to
[aBbCcDd], for example.
To obtain the traditional interpretation of bracket expressions,
you can use the C locale by setting the
LC_ALL environment variable to the value
C.
Finally, certain named classes of characters are predefined within
bracket expressions, as follows.
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 depends upon the C locale and the
ASCII character encoding, whereas the former is independent
of locale and character set.
(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 its
not at the edge of a word.
A regular expression may be followed by one of several repetition operators:
Tag | Description |
? |
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.
ENVIRONMENT VARIABLES
Greps behavior is affected by the following environment variables.
A locale
LC_foo is specified by examining the three environment variables
LC_ALL, LC_foo, LANG, in that order.
The first of these variables that is set specifies the locale.
For example, if
LC_ALL is not set, but
LC_MESSAGES is set to
pt_BR, then Brazilian Portuguese is used for the
LC_MESSAGES locale.
The C locale is used if none of these environment variables are set,
or if the locale catalog is not installed, or if
grep was not compiled with national language support (NLS).
Tag | Description |
GREP_OPTIONS | |
This variable specifies default options to be placed in front of any
explicit options. For example, if
GREP_OPTIONS is
--binary-files=without-match --directories=skip, grep behaves as if the two options
--binary-files=without-match and
--directories=skip had been specified before any explicit options.
Option specifications are separated by whitespace.
A backslash escapes the next character,
so it can be used to specify an option containing whitespace or a backslash.
|
GREP_COLOR | |
Specifies the marker for highlighting.
|
LC_ALL, LC_COLLATE, LANG
| |
These variables specify the
LC_COLLATE locale, which determines the collating sequence used to interpret
range expressions like
[a-z]. |
LC_ALL, LC_CTYPE, LANG
| |
These variables specify the
LC_CTYPE locale, which determines the type of characters, e.g., which
characters are whitespace.
|
LC_ALL, LC_MESSAGES, LANG
| |
These variables specify the
LC_MESSAGES locale, which determines the language that
grep uses for messages.
The default C locale uses American English messages.
|
POSIXLY_CORRECT | |
If set,
grep behaves as POSIX.2 requires; otherwise,
grep behaves more like other GNU programs.
POSIX.2 requires that options that follow file names must be
treated as file names; by default, such options are permuted to the
front of the operand list and are treated as options.
Also, POSIX.2 requires that unrecognized options be diagnosed as
illegal, but since they are not really against the law the default
is to diagnose them as invalid.
POSIXLY_CORRECT also disables _N_GNU_nonoption_argv_flags_,
described below.
|
_N_GNU_nonoption_argv_flags_
| |
(Here
N is
greps numeric process ID.) If the
ith character of this environment variables value is
1, do not consider the
ith operand of
grep to be an option, even if it appears to be one.
A shell can put this variable in the environment for each command it runs,
specifying which operands are the results of file name wildcard
expansion and therefore should not be treated as options.
This behavior is available only with the GNU C library, and only
when
POSIXLY_CORRECT is not set.
|
DIAGNOSTICS
Normally, exit status is 0 if selected lines are found and 1 otherwise.
But the exit status is 2 if an error occurred, unless the
-q or
--quiet or
--silent option is used and a selected line is found.
BUGS
Email bug reports to
bug-grep@gnu.org.
Large repetition counts in the
{n,m} 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.
|