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.
-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 --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
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.
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.
