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
- -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.
- -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.
- -C NUM, --context=NUM
- Print NUM lines of output context. Places a line
containing -- between contiguous groups of matches.
- -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.
- --colour[=WHEN],
--color[=WHEN]
- Surround the matching string with the marker find in
GREP_COLOR environment variable. WHEN may be `never', `always', or
`auto'
- -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 -. May be specified more than once.
- -F, --fixed-strings
- Interpret PATTERN as a list of fixed strings,
separated by newlines, any of which is to be matched.
- -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.
- --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 --label=foo something
- --line-buffered
- Use line buffering, it can be a performance penalty.
- -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.
- -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 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.
- -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 locale's 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 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.
ENVIRONMENT VARIABLES
Grep's 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).
- 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 grep's numeric process ID.) If the
ith character of this environment variable's 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.