Linux Commands Examples

A great documentation place for Linux commands

grep

print lines matching a pattern


see also : egrep - fgrep - rgrep - awk - cmp - diff - find - gzip - perl - sed - sort - xargs - zgrep

Synopsis

grep [OPTIONS] PATTERN [FILE...]
grep
[OPTIONS] [-e PATTERN | -f FILE] [FILE...]


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examples

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What is the difference between grep, pgrep, egrep, fgrep?

Differences between grep, pgrep, egrep, and fgrep (Linux):

grep

grep is an acronym that stands for "Global Regular Expressions Print". grep is a program which scans a specified file or files line by line, returning lines that contain a pattern. A pattern is an expression that specifies a set of strings by interpreting characters as meta-characters. For example the asterisk meta character (*) is interpreted as meaning "zero or more of the preceding element". This enables users to type a short series of characters and meta characters into a grep command to have the computer show us what lines in which files match.

The standard grep command looks like:

grep <flags> '<regular expression>' <filename>

grep prints the search results to the screen (stdout) and returns the following exit values:

0    A match was found.
1    No match was found.
>1   A syntax error was found or a file was inaccessible 
     (even if matches were found).

Some common flags are: -c for counting the number of successful matches and not printing the actual matches, -i to make the search case insensitive, -n to print the line number before each match printout, -v to take the complement of the regular expression (i.e. return the lines which don't match), and -l to print the file names of files with lines which match the expression.

egrep

egrep is an acronym that stands for "Extended Global Regular Expressions Print".

The 'E' in egrep means treat the pattern as a regular expression. "Extended Regular Expressions" abbreviated 'ERE' is enabled in egrep. egrep (which is the same as grep -E) treats +, ?, |, (, and ) as meta-characters.

In basic regular expressions (with grep), the meta-characters ?, +, {, |, (, and ) lose their special meaning. If you want grep to treat these characters as meta-characters, escape them \?, \+, \{, \|, \(, and \).

For example, here grep uses basic regular expressions where the plus is treated literally, any line with a plus in it is returned.

grep "+" myfile.txt

egrep on the other hand treats the "+" as a meta character and returns every line because plus is interpreted as "one or more times".

egrep "+" myfile.txt

Here every line is returned because the + was treated by egrep as a meta character. normal grep would have searched only for lines with a literal +.

fgrep

fgrep is an acronym that stands for "Fixed-string Global Regular Expressions Print".

fgrep (which is the same as grep -F) is fixed or fast grep and behaves as grep but does NOT recognize any regular expression meta-characters as being special. The search will complete faster because it only processes a simple string rather than a complex pattern.

For example, if I wanted to search my .bash_profile for a literal dot (.) then using grep would be difficult because I would have to escape the dot because dot is a meta character that means 'wild-card, any single character':

grep "." myfile.txt

The above command returns every line of myfile.txt. Do this instead:

fgrep "." myfile.txt

Then only the lines that have a literal '.' in them are returned. fgrep helps us not bother escaping our meta characters.

pgrep

pgrep is an acronym that stands for "Process-ID Global Regular Expressions Print".

pgrep looks through the currently running processes and lists the process IDs which matches the selection criteria to stdout. pgrep is handy when all you want to know is the process id integer of a process. For example, if I wanted to know only the process ID of my mysql process I would use the command pgrep mysql which would return a process ID like 7312.

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ps ax | grep knob | grep -v grep
1
grep -rHin -B 1 -A 1 error .



##What does it do ?

Will search in the files of the current directory ( . ) the string error ( -rHin error) and will print it, as well as the line before ( -B 1) and the line after (-A 1).



##Output:

user$ cd /var/log/apache2/

user$ grep -rHin -B 1 -A 1 error .





./error.log-29-[Tue Dec 10 22:14:49 2013] [debug] mod_deflate.c(615): [client 127.0.0.1] Zlib: Compressed 118128 to 17097 : URL /css/bootstrap.css, referer: http://lc/xmodmap

./error.log:30:[Wed Dec 11 01:36:00 2013] [error] [client ::1] File does not exist: /var/www/systopia, referer: http://www.systopia.co.uk/

./error.log-31-[Wed Dec 11 02:49:40 2013] [notice] caught SIGTERM, shutting down

--

./error.log-77-[Thu Dec 12 02:47:08 2013] [debug] mod_deflate.c(615): [client 127.0.0.1] Zlib: Compressed 202758 to 30553 : URL /index.php

./error.log:78:[Thu Dec 12 02:57:54 2013] [error] [client ::1] File does not exist: /var/www/systopia, referer: http://www.systopia.co.uk/

./error.log-79-[Thu Dec 12 03:30:02 2013] [debug] mod_deflate.c(615): [client 127.0.0.1] Zlib: Compressed 19141 to 6146 : URL /index.php

--

./access.log-71-::1 - - [10/Dec/2013:22:14:58 +0000] "OPTIONS * HTTP/1.0" 200 126 "-" "Apache/2.2.22 (Debian) (internal dummy connection)"

./access.log:72:::1 - - [11/Dec/2013:01:36:00 +0000] "GET /systopia/wp-content/plugins/jnewsticker-for-wordpress/media/jnewsticker/skins/corporate_blue.css HTTP/1.1" 404 552 "http://www.systopia.co.uk/" "Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20131030 Firefox/17.0 Iceweasel/17.0.10"

./access.log-73-127.0.0.1 - - [11/Dec/2013:12:35:04 +0000] "GET /xmodmap HTTP/1.1" 200 12164 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20131030 Firefox/17.0 Iceweasel/17.0.10"

--
example added by LeBerger
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alias grep='grep --color'
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Is there a `grep`-like utility to match strings instead of lines?

You can supply grep the -o option. Like such:

grep -o pattern file

(Hint: The mnemonic I use for this is: omit everything except match)

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/bin/grep
busybox 50
grep.grep 100
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ps aux | grep skye_server | grep -v grep
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ps ax|grep volt|grep -v grep|fold|head -2
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ps ux|grep -v grep|grep --color=auto python
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alias grep="grep --color"
alias v="$EDITOR"
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Can GNU Grep output a selected group?

You can use sed for this. On BSD sed:

echo "foo 'bar'" | sed -E "s/.*'([^']+)'.*/\\1/"

Or, without the -E option:

sed "s/.*'\([^']\+\)'.*/\1/"

This doesn't work for multiline input. For that you need:

sed -n "s/.*'\([^']\+\)'.*/\1/p"

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Combine tail -f with grep?

You almost wrote the answer, which is :

tail -f file.log | grep "foobar"

That's it.

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How can I recursively grep paticular files in a directory

find . -name '*.java' | xargs grep <your pattern here>

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Edit first line of large text file

You can use less to see what you want to edit and use sed to make the changes. This way you edit without loading the entire file.

Another way is to split the file, edit and join again:

split -b 10000k <file>

and to join:

cat xa* > <file>

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recursive grep: exclude specific directories

you can use find instead:

find . -not -path "*/.svn*" -not -type d -exec grep -ni "myfunc" {} \; -print

OK, so that's a little backwards, you get the grep results first and then the path. Maybe someoe else has a better answer?

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Counting total number of matches with grep instead of just how many lines match

try this:

grep -o -E "your expression" file |wc -l

well, -E is just an example, it could be -P, -F etc. point is -o

test:

kent$  echo "abc xxx yyy"|grep -cP "[a-z]{3}"      
1

kent$  echo "abc xxx yyy"|grep -oP "[a-z]{3}"|wc -l
3

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Advantages of cat'ing file and piping to grep

There is no advantage. Your cursor being at the end also doesn't matter much if you structure it like this instead: < inputfile grep -args foo

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How to Combine find and grep for a complex search? ( GNU/linux, find, grep )

Try

find /srv/www/*/htdocs/system/application/ -name "*.php" -exec grep "debug (" {} \; -print

This should recursively search the folders under application for files with .php extension and pass them to grep.

An optimization on this would be to execute:

find /srv/www/*/htdocs/system/application/ -name "*.php" -print0 | xargs -0 grep -H "debug ("

This uses xargs to pass all the .php files output by find as arguments to a single grep command e.g grep "debug (" file1 file2 file3. The -print0 option of find and -0 option of xargs ensure the spaces in file and directory names are correctly handled. The -H option passed to grep ensures that the filename is printed in all situations. (By default, grep prints the filename only when multiple arguments are passed in.)

From man xargs:

   -0     Input items are terminated by a null character instead of by whitespace, and the quotes and  backslash
          are  not  special  (every  character  is  taken literally).  Disables the end of file string, which is
          treated like any other argument.  Useful when input items might contain white space, quote  marks,  or
          backslashes.  The GNU find -print0 option produces input suitable for this mode.

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Ubuntu grep, find etc: "Permission denied" and "No such file or directory" output

Try redirecting stderr to /dev/null.

johndoe@johndoe-desktop:/$ grep -rnP 'YII_CORE_PATH' ./ 2> /dev/null | grep -v .svn

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grep for "term" and exclude "another term"

To and expressions with grep you need two invocations:

grep -Ei "search term" | grep -Eiv "exclude term"

If the terms you are searching for are not regular expressions use fixed string matching (-F) which is faster:

grep -F "search term" | grep -Fv "exclude term"

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Using an index to make grep faster?

grep, no. But there are several programs which use indexes and aimed at code base. ctags (there is a version provided with vim), etags (aimed for use with emacs), global (more independent of the editor) are the one I'm thinking about now but there are probably other.

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find / grep command without searching mounted shares

Use the -fstype local option to find:

find / -fstype local -name .vimrc

If you want to exclude only specific paths, you could use -prune:

find / -name /path/to/ignore -prune -o -name .vimrc


Update:

The local psuedo-fstype is available in the version of find that comes with OS X, but is not in GNU find (fstypes recognized by GNU find).

If you're using GNU find (as is used on most linux systems), you'll instead want to use -mount:

find / -mount -name .vimrc

description

grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus (-) is given as file name) for lines containing a match to the given PATTERN. By default, grep prints the matching lines.

In addition, three variant programs egrep, fgrep and rgrep are available. egrep is the same as grep -E. fgrep is the same as grep -F. rgrep is the same as grep -r. Direct invocation as either egrep or fgrep is deprecated, but is provided to allow historical applications that rely on them to run unmodified.

options

Generic Program Information

--help

Print a usage message briefly summarizing these command-line options and the bug-reporting address, then exit.

-V, --version

Print the version number of grep to the standard output stream. This version number should be included in all bug reports (see below).

Matcher Selection
-E
, --extended-regexp

Interpret PATTERN as an extended regular expression (ERE, see below). (-E is specified by POSIX .)

-F, --fixed-strings

Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched. (-F is specified by POSIX .)

-G, --basic-regexp

Interpret PATTERN as a basic regular expression (BRE, see below). This is the default.

-P, --perl-regexp

Interpret PATTERN as a Perl regular expression (PCRE, see below). This is highly experimental and grep -P may warn of unimplemented features.

Matching Control
-e
PATTERN, --regexp=PATTERN

Use PATTERN as the pattern. This can be used to specify multiple search patterns, or to protect a pattern beginning with a hyphen (-). (-e is specified by POSIX .)

-f FILE, --file=FILE

Obtain patterns from FILE, one per line. The empty file contains zero patterns, and therefore matches nothing. (-f is specified by POSIX .)

-i, --ignore-case

Ignore case distinctions in both the PATTERN and the input files. (-i is specified by POSIX .)

-v, --invert-match

Invert the sense of matching, to select non-matching lines. (-v is specified by POSIX .)

-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. (-x is specified by POSIX .)

-y

Obsolete synonym for -i.

General Output Control
-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. (-c is specified by POSIX .)

--color[=WHEN], --colour[=WHEN]

Surround the matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for fields and groups of context lines) with escape sequences to display them in color on the terminal. The colors are defined by the environment variable GREP_COLORS. The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority. WHEN is never, always, or auto.

-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. (-l is specified by POSIX .)

-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.

-o, --only-matching

Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.

-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. (-q is specified by POSIX .)

-s, --no-messages

Suppress error messages about nonexistent or unreadable files. Portability note: unlike GNU grep, 7th Edition Unix grep did not conform to POSIX , because it lacked -q and its -s option behaved like GNU grep’s -q option. USG -style grep also lacked -q but its -s option behaved like GNU grep. Portable shell scripts should avoid both -q and -s and should redirect standard and error output to /dev/null instead. (-s is specified by POSIX .)

Output Line Prefix Control
-b
, --byte-offset

Print the 0-based byte offset within the input file before each line of output. If -o (--only-matching) is specified, print the offset of the matching part itself.

-H, --with-filename

Print the file name for each match. This is the default when there is more than one file to search.

-h, --no-filename

Suppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.

--label=LABEL

Display input actually coming from standard input as input coming from file LABEL. This is especially useful when implementing tools like zgrep, e.g., gzip -cd foo.gz | grep --label=foo -H something. See also the -H option.

-n, --line-number

Prefix each line of output with the 1-based line number within its input file. (-n is specified by POSIX .)

-T, --initial-tab

Make sure that the first character of actual line content lies on a tab stop, so that the alignment of tabs looks normal. This is useful with options that prefix their output to the actual content: -H,-n, and -b. In order to improve the probability that lines from a single file will all start at the same column, this also causes the line number and byte offset (if present) to be printed in a minimum size field width.

-u, --unix-byte-offsets

Report Unix-style byte offsets. This switch causes grep to report byte offsets as if the file were a 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.

-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.

Context Line Control
-A
NUM, --after-context=NUM

Print NUM lines of trailing context after matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

-B NUM, --before-context=NUM

Print NUM lines of leading context before matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

-C NUM, -NUM, --context=NUM

Print NUM lines of output context. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

File and Directory Selection
-a
, --text

Process a binary file as if it were text; this is equivalent to the --binary-files=text option.

--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.

-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, i.e., read directories just as if they were ordinary files. If ACTION is skip, silently skip directories. If ACTION is recurse, read all files under each directory, recursively, following symbolic links only if they are on the command line. This is equivalent to the -r option.

--exclude=GLOB

Skip files whose base name matches GLOB (using wildcard matching). A file-name glob can use *, ?, and [...] as wildcards, and \ to quote a wildcard or backslash character literally.

--exclude-from=FILE

Skip files whose base name matches any of the file-name globs read from FILE (using wildcard matching as described under --exclude).

--exclude-dir=DIR

Exclude directories matching the pattern DIR from recursive searches.

-I

Process a binary file as if it did not contain matching data; this is equivalent to the --binary-files=without-match option.

--include=GLOB

Search only files whose base name matches GLOB (using wildcard matching as described under --exclude).

-r, --recursive

Read all files under each directory, recursively, following symbolic links only if they are on the command line. This is equivalent to the -d recurse option. -R, --dereference-recursive Read all files under each directory, recursively. Follow all symbolic links, unlike -r.

Other Options
--line-buffered

Use line buffering on output. This can cause a performance penalty.

--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.

-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.

-z, --null-data

Treat the input as a set of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline. Like the -Z or --null option, this option can be used with commands like sort -z to process arbitrary file names.

copyright

Copyright 1998-2000, 2002, 2005-2012 Free Software Foundation, Inc.

This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

environment variables

The behavior of grep is affected by the following environment variables.

The locale for category 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 the Brazilian Portuguese locale is used for the LC_MESSAGES category. The C locale is used if none of these environment variables are set, 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

This variable specifies the color used to highlight matched (non-empty) text. It is deprecated in favor of GREP_COLORS, but still supported. The mt, ms, and mc capabilities of GREP_COLORS have priority over it. It can only specify the color used to highlight the matching non-empty text in any matching line (a selected line when the -v command-line option is omitted, or a context line when -v is specified). The default is 01;31, which means a bold red foreground text on the terminal’s default background.

GREP_COLORS

Specifies the colors and other attributes used to highlight various parts of the output. Its value is a colon-separated list of capabilities that defaults to ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36 with the rv and ne boolean capabilities omitted (i.e., false). Supported capabilities are as follows.

sl=

SGR substring for whole selected lines (i.e., matching lines when the -v command-line option is omitted, or non-matching lines when -v is specified). If however the boolean rv capability and the -v command-line option are both specified, it applies to context matching lines instead. The default is empty (i.e., the terminal’s default color pair).

cx=

SGR substring for whole context lines (i.e., non-matching lines when the -v command-line option is omitted, or matching lines when -v is specified). If however the boolean rv capability and the -v command-line option are both specified, it applies to selected non-matching lines instead. The default is empty (i.e., the terminal’s default color pair).

rv

Boolean value that reverses (swaps) the meanings of the sl= and cx= capabilities when the -v command-line option is specified. The default is false (i.e., the capability is omitted).

mt=01;31

SGR substring for matching non-empty text in any matching line (i.e., a selected line when the -v command-line option is omitted, or a context line when -v is specified). Setting this is equivalent to setting both ms= and mc= at once to the same value. The default is a bold red text foreground over the current line background.

ms=01;31

SGR substring for matching non-empty text in a selected line. (This is only used when the -v command-line option is omitted.) The effect of the sl= (or cx= if rv) capability remains active when this kicks in. The default is a bold red text foreground over the current line background.

mc=01;31

SGR substring for matching non-empty text in a context line. (This is only used when the -v command-line option is specified.) The effect of the cx= (or sl= if rv) capability remains active when this kicks in. The default is a bold red text foreground over the current line background.

fn=35

SGR substring for file names prefixing any content line. The default is a magenta text foreground over the terminal’s default background.

ln=32

SGR substring for line numbers prefixing any content line. The default is a green text foreground over the terminal’s default background.

bn=32

SGR substring for byte offsets prefixing any content line. The default is a green text foreground over the terminal’s default background.

se=36

SGR substring for separators that are inserted between selected line fields (:), between context line fields, (-), and between groups of adjacent lines when nonzero context is specified (--). The default is a cyan text foreground over the terminal’s default background.

ne

Boolean value that prevents clearing to the end of line using Erase in Line (EL) to Right (\33[K) each time a colorized item ends. This is needed on terminals on which EL is not supported. It is otherwise useful on terminals for which the back_color_erase (bce) boolean terminfo capability does not apply, when the chosen highlight colors do not affect the background, or when EL is too slow or causes too much flicker. The default is false (i.e., the capability is omitted).

Note that boolean capabilities have no =... part. They are omitted (i.e., false) by default and become true when specified.

See the Select Graphic Rendition (SGR) section in the documentation of the text terminal that is used for permitted values and their meaning as character attributes. These substring values are integers in decimal representation and can be concatenated with semicolons. grep takes care of assembling the result into a complete SGR sequence (\33[...m). Common values to concatenate 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 16-color mode 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.

LC_ALL, LC_COLLATE, LANG

These variables specify the locale for the LC_COLLATE category, which determines the collating sequence used to interpret range expressions like [a-z].

LC_ALL, LC_CTYPE, LANG

These variables specify the locale for the LC_CTYPE category, which determines the type of characters, e.g., which characters are whitespace.

LC_ALL, LC_MESSAGES, LANG

These variables specify the locale for the LC_MESSAGES category, 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.

exit status

The exit status is 0 if selected lines are found, and 1 if not found. If an error occurred the exit status is 2. (Note: POSIX error handling code should check for ’2’ or greater.)

notes

This man page is maintained only fitfully; the full documentation is often more up-to-date.

GNU ’s not Unix, but Unix is a beast; its plural form is Unixen.

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 three different versions of regular expression syntax: “basic” (BRE), “extended” (ERE) and “perl” (PRCE). In GNU  grep, there is no difference in available functionality between basic and extended syntaxes. 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. Perl regular expressions give additional functionality, and are documented in pcresyntax(3) and pcrepattern(3), but may not be available on every system.

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 meta-character with special meaning may be quoted by preceding it with a backslash.

The period . matches any single character.

Character Classes and Bracket Expressions
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 the character class of numbers and letters in the current locale. In the C locale and ASCII character set encoding, this is the same as [0-9A-Za-z]. (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 expression.) Most meta-characters lose their special meaning inside bracket expressions. 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.

Anchoring
The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line.

The Backslash Character and Special Expressions
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. The symbol \w is a synonym for [_[:alnum:]] and \W is a synonym for [^_[:alnum:]].

Repetition
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.

Concatenation
Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated expressions.

Alternation
Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either alternate expression.

Precedence
Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole expression may be enclosed in parentheses to override these precedence rules and form a subexpression.

Back References and Subexpressions
The back-reference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression.

Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).

Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.

GNU grep -E 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 command grep -E '{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.


bugs

Reporting Bugs
Email bug reports to <bug-grep[:at:]gnu[:dot:]org>, a mailing list whose web page is <http://lists.gnu.org/mailman/listinfo/bug-grep>. grep’s Savannah bug tracker is located at <http://savannah.gnu.org/bugs/?group=grep>.

Known Bugs
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.

Back-references are very slow, and may require exponential time.


see also

Regular Manual Pages
awk , cmp , diff , find , gzip , perl , sed , sort , xargs , zgrep , mmap, read, pcre, pcresyntax, pcrepattern, terminfo, glob, regex.

POSIX Programmer’s Manual Page
grep(1p).

TeXinfo Documentation
The full documentation for grep is maintained as a TeXinfo manual, which you can read at http://www.gnu.org/software/grep/manual/. If the info and grep programs are properly installed at your site, the command

info grep

should give you access to the complete manual.

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