grep

globally search for a regular expression and print matching lines

grep [options] pattern [file...]
grep [options] [-e pattern | -f patternFile] [file…]
bzgrep, zgrep

Search files (or STDIN if no files are named, or if file is - )
for lines containing a match to the pattern.

-G --basic-regexp pattern is a Basic Regular Expression (default).
                                                  ? , + , { , | , ( and ) must be preceeded by \ to ENABLE special meaning

-E --extended-regexp pattern is an Extended Regular Expression.
egrep is the same as grep -E.

-F --fixed-strings pattern is a list of fixed strings, separated by newlines, any of which is to be matched.
fgrep is the same as grep -F.

-P --perl-regexp Interpret pattern as a Perl regular expression. PCRE for Perl Compatible Regular Expressions.

controling matching
-v
--invert-match 		Invert the sense of matching to
select non-matching lines.
-i
--ignore-case		 
-e pattern
--regexp=pattern 		Use pattern as the matching pattern;
useful to protect patterns beginning with - from being intrepreted as options.
-f file
--file=file		 Obtain patterns from file, one per line. An empty file contains zero patterns and matches nothing.
-w
--word-regexp 		matches must form whole words.

The matching substring must either be at the beginning of the line, or preceded by a non-alphanumeric character and
must be either at the end of the line or followed by a non-alphanumeric character.

Alphanumeric characters are letters, digits, and the underscore.

-x
--line-regexp 		exactly match the whole line.
-y Obsolete synonym for -i (--ignore-case).

controlling output
-o
--only-matching		 only the matching part of the line is output.
--color[=when]
-colour[=when] 		when may be never, always, or auto

The matched sections can be displayed in color as per $GREP_COLOR in the form ff[;bb]
where ff is the code used for the foreground and bb is used for the background.

Examples:
magenta on yellow 35;43 blue on cyan 34;46 red 31 (default)
Foreground: black 30, green 32, yellow 33, blue 34, magenta 35, cyan 36, white 37, red 31 (default)
Background: black 40, green 42, yellow 43, blue 44, magenta 45, underscore 4, [blink] 5, inverse 7
export GREP_COLOR="33;44"

Codes are not output when inappropriate (For example: to a file or pipe) unless when=always

grep --color=always birds logfile | more -r

-C† num
--context=num 		output num lines of Context before and after match
-B num
--before-context=num		 Output num lines of context before lines containing the pattern
then the lines containing pattern.
-A num
--after-context=num 		Output num lines of context after outputting the pattern.
Places a line containing -- between sets of matches.
Examples:
   grep -C1 day  grep -B2 day  grep -A2 day
inputoutput
week  
mon mon
yesterday yesterday yesterday
day day day day
later later later
tuesday
sunday
------set seperator
week2  
mon2 mon2
yr2 yr2 yr2
day2 day2 day2 day2
later2 later2 later2
f f

-c
--count 		output count of matches (or -cv non-matches) for each file
Prefixed by 'filename :' if multiple files, even if count is 0!
 grep -c images *html |grep -v 0$ #dont show lines with 0 count
BackupStrageties.html:1
Characters.html:2
clonezilla.1.html:6
colors.html:3
css-box.html:4
-m num
--max-count= num 		stop reading after num matching lines
With 0 no lines are read.
If the input is standard input from a regular file, and num matching lines are output, the standard input is positioned to just after the last matching line, enables calling process to resume a search.

Example:Show 2 lines before body with ++ between each occurance.

> while grep -B 2 -m 1 body ; do echo ++; done<index.html
Outputs any trailing context lines.
With -c or --count , does not output a count greater than num.

With -v or --invert-match, stops after outputting num non-matching lines.

-n
--line-number
not with --count

Prefix each line with the line number

-b
--byte-offset 		prefix each line with the byte offset within the input file
-u
--unix-byte-offsets 		Use Unix-style byte offsets.
report byte offsets i.e. with CR characters stripped off. only for MS-DOS and MS-Windows.
-T
--initial-tab		 Force line content to a tab stop, causing tabs to be aligned, useful with : -H,-n, and -b.
Causes the line number and byte offset in a minimum width.
-q
--quiet
--silent		 Quiet; do not write to STDOUT.
Errors like "Permission denied" are written to STDERR and return code is set to 2.
Exit with zero status as soon as a match is found.
If no match is found $? is set to 1.
Useful when embedded in conditional statments
-s
--no-messages 		Suppress error messages about nonexistent, unaccessable files or Is a directory normally sent to STDERR.
-H
--with-filename		 filename is prepended to each match when multiple files are searched.
-h
--no-filename 		don't
-l
--files-with-matches 		outputs the only the filename of files containing a match.
Once a match is found proceeds to the next file.
-L
--files-without-match 		outputs the filename with no match
-Z
--null 		terminate filename with NULL . Used when file names contain newlines!.

Used with commands find -print0, perl -0, sort -z, and xargs -0.

--label=label Displays input actually coming from standard input as input coming from file label. Useful for tools like zgrep, e.g.
gzip -cd foo.gz |grep --label=foo something

Directory processing
-d action
--directories=action 		If an input is a directory:
action: read: as ordinary files, default.
         skip.
         recurse: reads all files under each directory, recursively.
-R
-r
--recursive 		equivalent to -d recurse .
--include=pattern Recurse only searching file matching pattern.
--exclude=pattern Recurse skip file matching pattern.

binary file handling
--binary‑files=type If the first few bytes of a file indicate it contains binary data (non-ASCII text†),
On a match, outputs Binary file name matches .

type
binary and
without-match assume binary file does not match, default.
text processe all files as text; equivalent to -a .

Control characters sent to the terminal can set attributes making it unreadable. If this happens try STTY SANE

-a
--text 		Process all file as text
‑‑binary‑files=text equivalent.
-I binary files do not match.
‑‑binary‑files=without‑match equivalent.
-U
--binary 		Process file(s) as binary.

If the file appears to be a text file, ␍ s are ignored to make regular expressions with ^ and $ work correctly.
Causes all files to be read and passed to the matching mechanism verbatim;
if the file is a text file with ␍␊ pairs at the end of each line, this will cause some regular expressions to fail. only under MS-Windows unless used with -b .

By default, under MS-Windows, uses the first 32KB of the file to determine if a file is text.

--help
-V
--version 		Display the version to standard error.
--mmap use the mmap system call to read input, instead of read which may yield better performance.
May cause undefined behavior if an input file shrinks, or if an I/O error occurs.
-D action
--devices=action		 If an input file is a device, FIFO or socket, use action to process it.
read, which means that devices are read as ordinary files (default).
skip, devices are silently skipped.
--line-buffered flush after each match. This permits piping to process each match as it occurs rather than waiting for full buffer.
has a performance penality.

Basic Regular Expressions: ? , + , { , | , ( and ) alone have no special meaning
   Use backslash to ENABLE special meaning (ex: \?)
   * is a meta character.

Regular Expressions

A pattern describes a set of strings, using operators to combine smaller expressions.

grep will process both basic regular expressions and extended regular expressions ( with GNU grep, there is no difference. )

The simplest regular expression matchs a single character.  For example: a matches an a , R matches an R.
Regular expresions can be combined.  For example: AR matches AR.

Regular expressions joined by | match any string matching either expression.

Metacharacters, can be treated as normal character by preceding it with a \  (backslash) (the opposite of BRE)!
.  (period) matches any single character except null.
^  (caret) matches the beginning of a line
$  (dollar sign) matches the end of a line.

repetition operators may follow a regular expression.

preceeding item is matched
? at most once (more) †

* zero or more times †
Match is greedy attempting to match as much as possible.
For example /.*xx/ matches everything in the line:asdfbxx asdfxx asdfxx

For stingy (aka laxy ) matching:
a question mark after any of the greedy quantifiers, chooses the smallest quantity for the first try.
/.*foo/ matches BOTH words in barfoo stoolfoo .
/.*?foo/ matches the FIRST word in barfoo stoolfoo

perlpcre

vim does NOT use ? rather .\{-} ( } is optional)
see :help non-greedy and :ver and plugin eregex.vim vimdoc/pattern.

+ one or more times †
{n} exactly n times
{n,} n or more times
{n,m} at least n times, but not more than m times.

The braces may need to be escaped as in z\{1,3\} means: z zz or zzz

/9\{2,3\} bytes/ matches 99 bytes and 999 bytes
\> matches the empty string at the beginning of a word,    \< … at the end of a word.
\b matches the empty string at the edge of a word,
\B matches the empty string provided it's not at the edge of a word.

†

subexpression is defined using \(…\)

backreference \n, where n is a single digit, matches the substring previously matched by the nth subexpression
Frequently on the right hand side of a substitute command

Precedence: Repetition, concatenation, alternation.
A subexpression overrides precedence rules.

Environment Variables

$GREP_OPTIONS are placed before explicit options specified on the command.
Option specifications are separated by whitespace.
A backslash escapes the next character to specify an option containing whitespace or a backslash.
example: $GREP_OPTIONS='--color'

$GREP_COLORS Specifies the color for highlighting. GREP_COLOR†
Colors and attributes used for highlighting.

cx=fbcontext lines 7 inverted
sl=fb selected lines or unselected with -v
rv reverses meanings of sl and cx with -v
mt=fbmatching text 01;31. equivalent to ms and mc to the same value. Default :bold red text foreground / current background.
ms=fb matched string text in a selected line. (used when -v omitted.) The effect of the sl= (or cx= if rv) capability remains active when this kicks in.
Default bold red text
mc=fb matching text in a context line. (only with -v )
The effect of the cx (or sl if rv) capability remains active when this kicks in.
Default bold red text
fn=fb file names prefixing any content line. default magenta text 35
ln=fb line numbers prefix. default green text 32
bn=byte number offsets prefixing any content line. Default green text 32
se=fbseparators inserted between selected line fields (:), between context line fields, (-), and between groups of adjacent lines when nonzero context is specified (--). default cyan text

fb: Select Graphic Rendition (SGR) forgroundBackground integers concatenated with semicolons. ANSI).
Common values include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.

The locale $LC_xxx is specified by examining: $LC_ALL, $LC_xxx, $LANG, in 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 BrazilianPortuguese is used.
The C locale is used if none of these are set, or if the locale catalog is not installed, or if grep was not compiled with national language support (NLS).
$LC_ALL, $LC_COLLATE,  LANG collating sequence used to interpret range expressions like [a-z].
$LC_ALL, $LC_CTYPE,    LANG type of characters, e.g., which characters are whitespace.
$LC_ALL, $LC_MESSAGES, LANG language 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.

_N_GNU_nonoption_argv_flags_ (N is grep's process ID.)
If the ith character of this environment variable's value is 1,
do not consider the ith operand 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.
Only with the GNU C library, and only when POSIXLY_CORRECT is not set.

Returns

 0: selected lines WERE found
 1: no lines selected. if [ $? != 1 ];then …
 2: an error occurred like: no permission or file not found.

Large repetition counts in the {n,m} construct may cause grep to use lots of memory.
Certain obscure regular expressions require exponential time and space.

Backreferences are very slow, and require exponential time.

Current "official" GNU grep

See also egrep, fgrep, sed, sh, attributes, environ, largefile, regex, regexp, XPG4

Examples

Find all uses of the "Posix" ( -i ignoring case) in the file text.mm, and write lines with line numbers( -n ):
grep -i -n posix text.mm

Display line numbers( -n ) containg empty lines ( i.e where beginning is immediately followed by end of line)
grep -n ^$ or grep -n -v .

Display all lines containing strings abc or def or both :
grep -E 'abc def' -or- grep -F 'abc def'

Both of the following commands display all lines matching exactly abc or def:
grep -E '^abc$ ^def$' -or- grep -F -x 'abc def'

To find an A surrounded by tabs, using ANSI-C quoting for bash use:
grep $'\tA\t'

Change a line beginning with hours:minutes (where hours may be one or two digits) to be zero filled to two digits if it was only one digit:
Green defines a subexpression, brown uses the subexpression
> echo "8:27 stuff stuff" |sed "s/^\(.\):/0\1:/" # Take first any character, folllowed by a colon, substitute a 0 before it and a colon
08:27 stuff stuff

> echo "19:42 stuff stuff" |sed "s/^\(.\):/0\1:/"
19:42 stuff stuff # no change as there are 2 characters between the start of the line and the colon

Environment Variables
See environ for the following environment variables : $LC_COLLATE, $LC_CTYPE, $LC_MESSAGES, and $NLSPATH.

Notes

A line with embedded nulls will only be compared up to the first null; if it matches entire line is output.

The results are unspecified if input files contain binary data or
lines longer than LINE_MAX (2048) bytes (defined in /usr/include/sys/syslimits.h or /usr/include/limits.h

Large File Behavior.
See largefile(5) for the description of the behavior of grep when encountering files greater than or equal to 2 Gbyte ( 2**31 bytes). Lines are limited only by the size of the available virtual memory.

International Components for Unicode

See egrep, fgrep, sed, sh, attributes(5), environ(5), largefile(5), regex(5), regexp(5), XPG4(5)