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grep command help
grep, egrep, fgrep - print lines matching a pattern
SYNOPSIS
grep [-[ABC] NUM] [-EFGHLUVZabchilnqrsuvwxyz] [-e PATTERN
| -f FILE] [-d ACTION] [--directories=ACTION] [--extended-
regexp] [--fixed-strings] [--basic-regexp] [--regexp=PAT�
TERN] [--file=FILE] [--ignore-case] [--word-regexp]
[--line-regexp] [--line-regexp] [--no-messages] [--invert-
match] [--version] [--help] [--byte-offset] [--line-num�
ber] [--with-filename] [--no-filename] [--quiet]
[--silent] [--text] [--files-without-match] [--files-with-
matches] [--count] [--before-context=NUM] [--after-con�
text=NUM] [--context[=NUM]] [--binary] [--unix-byte-off�
sets] [--mmap] [--null] [--recursive] [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.
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, sepa�
rated 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).
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-match�
ing lines.
-d ACTION, --directories=ACTION
If an input file is a directory, use ACTION to pro�
cess 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 pat�
terns 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
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-match�
ing 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 charac�
ter. Similarly, it must be either at the end of
the line or followed by a non-word constituent
character. Word-constituent characters are let�
ters, 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-Win�
dows.
In some situations, --mmap yields better perfor�
mance. However, --mmap can cause undefined behav�
ior (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 new�
line. 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 expres�
sion syntax: "basic" and "extended." In GNU grep, there
is no difference in available functionality using either
syntax. In other implementations, basic regular expres�
sions 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 expres�
sions 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 sin�
gle 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
first in the list. Similarly, to include a literal ^
place it anywhere but first. Finally, to include a lit�
eral - 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 concate�
nating two substrings that respectively match the concate�
nated subexpressions.
Two regular expressions may be joined by the infix opera�
tor |; the resulting regular expression matches any string
matching either subexpression.
Repetition takes precedence over concatenation, which in
turn takes precedence over alternation. A whole subex�
pression 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
[{] to match a literal {.
GNU egrep attempts to support traditional usage by assum�
ing 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.
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 syn�
tax errors in the pattern, inaccessible input files, or
other system errors.
BUGS
Email bug reports to [email protected]. 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.
Comments - most recent first
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who say "Please send me the Linux tutorial." This website *is* your Linux Tutorial! Read everything here, learn
all you can, ask questions if you like. But don't ask me to send what you already have. :-)
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