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本文档提供了Grep 2.9版本的详细介绍，包括命令行选项、环境变量、退出状态等内容。涵盖基本正则表达式、扩展正则表达式及Perl正则表达式的使用方法，并附有常见应用场景实例。

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GNU Grep 2.9

grep
1 Introduction
2 Invoking grep
3 Regular Expressions
4 Usage
5 Reporting bugs
- 5.1 Known Bugs
6 Copying
- 6.1 GNU Free Documentation License
Index

Next: Introduction,Up: (dir)

grep

grep prints lines that match a pattern.

This manual is for version 2.9 of GNU Grep.

This manual is for grep, a pattern matching engine.

Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.3 orany later version published by the Free Software Foundation; with noInvariant Sections, with no Front-Cover Texts, and with no Back-CoverTexts. A copy of the license is included in the section entitled“GNU Free Documentation License”.

Next: Invoking,Previous: Top,Up: Top

1 Introduction

grep searches the input filesfor lines containing a match to a given pattern list. When it finds a match in a line,it copies the line to standard output (by default),or produces whatever other sort of output you have requested with options.

Though grep expects to do the matching on text,it has no limits on input line length other than available memory,and it can match arbitrary characters within a line. If the final byte of an input file is not a newline,grep silently supplies one. Since newline is also a separator for the list of patterns,there is no way to match newline characters in a text.

Next: Regular Expressions,Previous: Introduction,Up: Top

2 Invoking `grep`

The general synopsis of the grep command line is

     grep options pattern input_file_names

There can be zero or more options. pattern will only be seen as such(and not as aninput_file_name)if it wasn't already specified within options(by using the ‘-epattern’or ‘-f file’ options). There can be zero or moreinput_file_names.

Next: Environment Variables,Up: Invoking

2.1 Command-line Options

grep comes with a rich set of options:some fromposix.2 and some being gnu extensions. Long option names are always agnu extension,even for options that are from posix specifications. Options that are specified by posix,under their short names,are explicitly marked as suchto facilitateposix-portable programming. A few option names are providedfor compatibility with older or more exotic implementations.

Several additional options controlwhich variant of the grep matching engine is used. Seegrep Programs.

Next: Matching Control,Up: Command-line Options

2.1.1 Generic Program Information

‘ --help’

Print a usage message briefly summarizing the command-line optionsand 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.

Next: General Output Control,Previous: Generic Program Information,Up: Command-line Options

2.1.2 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 ‘ -’. (‘ -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’ ‘ -y’ ‘ --ignore-case’

Ignore case distinctions in both the pattern and the input files. ‘ -y’ is an obsolete synonym that is provided for compatibility. (‘ -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 eitherbe at the beginning of the line,or preceded by a non-word constituent character. Similarly,it must be either at the end of the lineor 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.)

Next: Output Line Prefix Control,Previous: Matching Control,Up: Command-line Options

2.1.3 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,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 andgroups of context lines) with escape sequences to display them in coloron the terminal. The colors are defined by the environment variable GREP_COLORSand default to ‘ ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36’for bold red matched text, magenta file names, green line numbers,green byte offsets, cyan separators, and default terminal colors otherwise. The deprecated environment variable GREP_COLOR is still supported,but its setting does not have priority;it defaults to `01;31' (bold red)which only covers the color for matched text. WHEN is ‘ never’, ‘ always’, or ‘ auto’.

‘ -L’ ‘ --files-without-match’

Suppress normal output;instead print the name of each input file from whichno output would normally have been printed. The scanning of every file will stop on the first match.

‘ -l’ ‘ --files-with-matches’

Suppress normal output;instead print the name of each input file from whichoutput would normally have been printed. The scanning of every file 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 positionedjust after the last matching line before exiting,regardless of the presence of trailing context lines. This enables a calling process to resume a search. For example, the following shell script makes use of it:

          while grep -m 1 PATTERN
          do
            echo xxxx
          done < FILE

But the following probably will not work because a pipe is not a regularfile:

          # This probably will not work.
          cat FILE |
          while grep -m 1 PATTERN
          do
            echo xxxx
          done

When grep stops after num matching lines,it outputs any trailing context lines. Since context does not include matching lines,grep will stop when it encounters another matching line. When the ‘-c’ or ‘--count’ option is also used,grep does not output a count greater thannum. When the ‘-v’ or ‘--invert-match’ option is also used,grep stops after outputtingnum non-matching lines.

‘ -o’ ‘ --only-matching’

Print only the matched (non-empty) parts of matching lines,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's. Portable shell scripts should avoid both‘ -q’ and ‘ -s’ and should redirectstandard and error output to /dev/null instead. (‘ -s’ is specified by posix.)

Next: Context Line Control,Previous: General Output Control,Up: Command-line Options

2.1.4 Output Line Prefix Control

When several prefix fields are to be output,the order is always file name, line number, and byte offset,regardless of the order in which these options were specified.

‘ -b’ ‘ --byte-offset’

Print the 0-based byte offset within the input filebefore each line of output. If ‘ -o’ (‘ --only-matching’) is specified,print the offset of the matching part itself. When grep runs on ms-dos or ms-Windows,the printed byte offsets depend on whetherthe ‘ -u’ (‘ --unix-byte-offsets’) option is used;see below.

‘ -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 inputas input coming from file LABEL. This isespecially useful when implementing tools like zgrep; e.g.:

          gzip -cd foo.gz | grep --label=foo -H something

‘ -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 linesfrom 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 option causes grep to report byte offsetsas if the file were a Unix-style text file,i.e., the byte offsets ignore the CR characters that were stripped. This will produce results identicalto running grep on a Unix machine. This option has no effect unless the ‘ -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 nameinstead 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.

Next: File and Directory Selection,Previous: Output Line Prefix Control,Up: Command-line Options

2.1.5 Context Line Control

Regardless of how these options are set,grep will never print any given line more than once. If the ‘-o’ or ‘--only-matching’ option is specified,these options have no effect and a warning is given upon their use.

‘ -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’ ‘ -num’ ‘ --context=num’

Print num lines of leading and trailing output context.

‘ --group-separator=string’

When -A, -B or -C are in use,print string instead of ‘ --’ around disjoint groupsof lines.

‘ --no-group-separator’

When -A, -B or -C are in use,print disjoint groups of lines adjacent to each other.

Matching lines normally use ‘:’ as a separatorbetween prefix fields and actual line content. Context (i.e., non-matching) lines use ‘-’ instead. When no context is specified,matching lines are simply output one right after another. When nonzero context is specified,lines that are adjacent in the input form a groupand are output one right after another, whilea separator appears by default between disjoint groups on a lineof its own and without any prefix. The default separatoris ‘--’, however whether to include it and its appearancecan be changed with the options above. Each group may containseveral matching lines when they are close enough to each otherthat two otherwise adjacent but divided groups connectand can just merge into a single contiguous one.

Next: Other Options,Previous: Context Line Control,Up: Command-line Options

2.1.6 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 eithera 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: ‘ --binary-files=text’ might output binary garbage,which can have nasty side effectsif the output is a terminal andif 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, FIFOs, and sockets 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(some operating systems and file systems disallow this,and will cause grepto print error messages for every directory or silently skip them). 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.

‘ --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 globsread from file (using wildcard matching as describedunder ‘ --exclude’).

‘ --exclude-dir=dir’

Exclude directories matching the pattern dir from recursivedirectory 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’ ‘ -R’ ‘ --recursive’

For each directory mentioned on the command line,read and process all files in that directory, recursively. This is the same as the ‘ --directories=recurse’ option.

Previous: File and Directory Selection,Up: Command-line Options

2.1.7 Other Options

‘ --line-buffered’

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

‘ --mmap’

This option is ignored for backwards compatibility. It used to readinput with the mmap system call, instead of the default readsystem call. On modern systems, mmap would rarely if ever yieldbetter performance.

‘ -U’ ‘ --binary’

Treat the file(s) as binary. By default, under ms-dos and ms-Windows, grep guesses the file typeby 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 effecton 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.

Next: Exit Status,Previous: Command-line Options,Up: Invoking

2.2 Environment Variables

The behavior of grep is affectedby the following environment variables.

The locale for category LC_foois specified by examining the three environment variablesLC_ALL,LC_foo, and LANG,in that order. The first of these variables that is set specifies the locale. For example, ifLC_ALL is not set,but LC_MESSAGES is set to ‘pt_BR’,then the Brazilian Portuguese locale is usedfor theLC_MESSAGES category. The ‘C’ locale is used if none of these environment variables are set,if the locale catalog is not installed,or ifgrep was not compiledwith national language support (nls).

GREP_OPTIONS

This variable specifies default options to be placed in front of anyexplicit options. For example, if GREP_OPTIONS is‘ --binary-files=without-match --directories=skip’, grepbehaves as if the two options ‘ --binary-files=without-match’ and‘ --directories=skip’ had been specified beforeany explicit options. Option specifications are separated bywhitespace. A backslash escapes the next character, so it can be used tospecify 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_COLORShave priority over it. It can only specify the color used to highlightthe 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

This variable specifies the colors and other attributesused to highlight various parts of the output. Its value is a colon-separated list of capabilitiesthat 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’ capabilityand 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’ capabilityand 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 ofthe ‘ sl=’ and ‘ cx=’ capabilitieswhen 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’) capabilityremains 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’) capabilityremains 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 insertedbetween selected line fields (‘ :’),between context line fields (‘ -’),and between groups of adjacent lineswhen 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 lineusing 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 terminalsfor 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) sectionin the documentation of your text terminalfor permitted values and their meaning as character attributes. These substring values are integers in decimal representationand can be concatenated with semicolons.grep takes care of assembling the resultinto 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 sequenceused 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.2requires that options thatfollow file names must be treated as file names;by default,such options are permuted to the front of the operand listand are treated as options. Also, POSIXLY_CORRECT disables special handling of aninvalid bracket expression. See invalid-bracket-expr.

_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 expansionand 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.

Next: grep Programs,Previous: Environment Variables,Up: Invoking

2.3 Exit Status

Normally, the 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 lineis found. Note, however, thatposix only mandates,for programs such as grep,cmp, and diff,that the exit status in case of error be greater than 1;it is therefore advisable, for the sake of portability,to use logic that tests for this general conditioninstead of strict equality with 2.

Previous: Exit Status,Up: Invoking

2.4 `grep` Programs

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 four major variants ofgrep,controlled by the following options.

‘ -G’ ‘ --basic-regexp’

Interpret the pattern as a basic regular expression (BRE). This is the default.

‘ -E’ ‘ --extended-regexp’

Interpret the pattern as an extended regular expression (ERE). (‘ -E’ is specified by posix.)

‘ -F’ ‘ --fixed-strings’

Interpret the pattern as a list of fixed strings, separatedby newlines, any of which is to be matched. (‘ -F’ is specified by posix.)

‘ -P’ ‘ --perl-regexp’

Interpret the pattern as a Perl regular expression. This is highly experimental and‘ grep -P’ may warn of unimplemented features.

In addition,two variant programs egrep andfgrep are available. egrep is the same as ‘grep -E’.fgrep is the same as ‘grep -F’. Direct invocation as eitheregrep orfgrep is deprecated,but is provided to allow historical applicationsthat rely on them to run unmodified.

Next: Usage,Previous: Invoking,Up: Top

3 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 understandsthree different versions of regular expression syntax:“basic,” (BRE) “extended” (ERE) and “perl”. Ingnu grep,there is no difference in available functionality between basic andextended 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 aredocumented in pcresyntax(3) and pcrepattern(3), but may not beavailable on every system.

Next: Character Classes and Bracket Expressions,Up: Regular Expressions

3.1 Fundamental Structure

The fundamental building blocks are the regular expressions that matcha single character. Most characters, including all letters and digits,are regular expressions that match themselves. Any meta-characterwith special meaning may be quoted by preceding it with a backslash.

A regular expression may be followed by one of severalrepetition operators:

‘ .’

The period ‘ .’ matches any single character.

‘ ?’

The preceding item is optional and will be 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.

‘ {,m}’

The preceding item is matched at most m 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 expressionmatches any string formed by concatenating two substringsthat respectively match the concatenated expressions.

Two regular expressions may be joined by the infix operator ‘|’;the resulting regular expressionmatches any string matching either alternalte expression.

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

Next: The Backslash Character and Special Expressions,Previous: Fundamental Structure,Up: Regular Expressions

3.2 Character Classes and Bracket Expressions

Abracket 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 twocharacters separated by a hyphen. It matches any single character thatsorts between the two characters, inclusive, using the locale'scollating sequence and character set. For example, in the default Clocale, ‘[a-d]’ is equivalent to ‘[abcd]’. Many locales sortcharacters in dictionary order, and in these locales ‘[a-d]’ istypically not equivalent to ‘[abcd]’;it might be equivalent to ‘[aBbCcDd]’, for example. To obtain the traditional interpretationof bracket expressions, you can use the ‘C’ locale by setting theLC_ALL environment variable to the value ‘C’.

Finally, certain named classes of characters are predefined withinbracket expressions, as follows. Their interpretation depends on theLC_CTYPE locale;the interpretation below is that of the ‘C’ locale,which is the default if noLC_CTYPE locale is specified.

‘ [:alnum:]’

Alphanumeric characters:‘ [:alpha:]’ and ‘ [:digit:]’.

‘ [:alpha:]’

Alphabetic characters:‘ [:lower:]’ and ‘ [:upper:]’.

‘ [:blank:]’

Blank characters:space and tab.

‘ [:cntrl:]’

Control characters. In ascii, these characters have octal codes 000through 037, and 177 ( DEL). In other character sets, these arethe equivalent characters, if any.

‘ [:digit:]’

Digits: 0 1 2 3 4 5 6 7 8 9.

‘ [:graph:]’

Graphical characters:‘ [:alnum:]’ and ‘ [:punct:]’.

‘ [:lower:]’

Lower-case letters: a b c d e f g h i j k l m n o p q r s t u v w x y z.

‘ [:print:]’

Printable characters:‘ [:alnum:]’, ‘ [:punct:]’, and space.

‘ [:punct:]’

Punctuation characters: ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.

‘ [:space:]’

Space characters:tab, newline, vertical tab, form feed, carriage return, and space. See Usage, for more discussion of matching newlines.

‘ [:upper:]’

Upper-case letters: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.

‘ [:xdigit:]’

Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f.

For example, ‘ [[:alnum:]]’ means ‘ [0-9A-Za-z]’, except the latterdepends upon the ‘ C’ locale and the ascii characterencoding, whereas the former is independent of locale and character set. (Note that the brackets in these class names arepart of the symbolic names, and must be included in addition tothe brackets delimiting the bracket expression.)

If you mistakenly omit the outer brackets, and search for say, ‘[:upper:]’,GNUgrep prints a diagnostic and exits with status 2, onthe assumption that you did not intend to search for the nominallyequivalent regular expression: ‘[:epru]’. Set thePOSIXLY_CORRECT environment variable to disable this feature.

Most meta-characters lose their special meaning inside bracket expressions.

‘ ]’

ends the bracket expression if it's not the first list item. So, if you want to make the ‘ ]’ character a list item,you must put it first.

‘ [.’

represents the open collating symbol.

‘ .]’

represents the close collating symbol.

‘ [=’

represents the open equivalence class.

‘ =]’

represents the close equivalence class.

‘ [:’

represents the open character class symbol, and should be followed by a valid character class name.

‘ :]’

represents the close character class symbol.

‘ -’

represents the range if it's not first or last in a list or the ending pointof a range.

‘ ^’

represents the characters not in the list. If you want to make the ‘ ^’character a list item, place it anywhere but first.

Next: Anchoring,Previous: Character Classes and Bracket Expressions,Up: Regular Expressions

3.3 The Backslash Character and Special Expressions

The ‘\’ character,when followed by certain ordinary characters,takes a special meaning:

‘ ‘\b’’

Match the empty string at the edge of a word.

‘ ‘\B’’

Match the empty string provided it's not at the edge of a word.

‘ ‘\<’’

Match the empty string at the beginning of word.

‘ ‘\>’’

Match the empty string at the end of word.

‘ ‘\w’’

Match word constituent, it is a synonym for ‘ [[:alnum:]]’.

‘ ‘\W’’

Match non-word constituent, it is a synonym for ‘ [^[:alnum:]]’.

‘ ‘\s’’

Match whitespace, it is a synonym for ‘ [[:space:]]’.

‘ ‘\S’’

Match non-whitespace, it is a synonym for ‘ [^[:space:]]’.

For example, ‘\brat\b’ matches the separate word ‘rat’,‘\Brat\B’ matches ‘crate’ but not ‘furry rat’.

Next: Back-references and Subexpressions,Previous: The Backslash Character and Special Expressions,Up: Regular Expressions

3.4 Anchoring

The caret ‘^’ and the dollar sign ‘$’ are meta-characters thatrespectively match the empty string at the beginning and end of a line.

Next: Basic vs Extended,Previous: Anchoring,Up: Regular Expressions

3.5 Back-references and Subexpressions

The back-reference ‘\n’, wheren is a single digit, matchesthe substring previously matched by the nth parenthesized subexpressionof the regular expression. For example, ‘(a)\1’ matches ‘aa’. When used with alternation, if the group does not participate in the match thenthe back-reference makes the whole match fail. For example, ‘a(.)|b\1’will not match ‘ba’. When multiple regular expressions are given with‘-e’ or from a file (‘-f file’),back-references are local to each expression.

Previous: Back-references and Subexpressions,Up: Regular Expressions

3.6 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 someegrep implementations support ‘\{’ instead, soportable scripts should avoid ‘{’ in ‘grep -E’ patterns andshould use ‘[{]’ to match a literal ‘{’.

gnu grep -E attempts to support traditional usage byassuming that ‘{’ is not special if it would be the start of aninvalid 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 scriptsshould avoid it.

Next: Reporting Bugs,Previous: Regular Expressions,Up: Top

4 Usage

Here is an example command that invokesgnu grep:

     grep -i 'hello.*world' menu.h main.c

This lists all lines in the files menu.h andmain.c thatcontain the string ‘hello’ followed by the string ‘world’;this is because ‘.*’ matches zero or more characters within a line. See Regular Expressions. The ‘-i’ option causesgrepto ignore case, causing it to match the line ‘Hello, world!’, whichit would not otherwise match. SeeInvoking, for more details abouthow to invoke grep.

Here are some common questions and answers aboutgrep usage.

How can I list just the names of matching files?
```
          grep -l 'main' *.c
```
lists the names of all C files in the current directory whose contentsmention ‘main’.
How do I search directories recursively?
```
          grep -r 'hello' /home/gigi
```
searches for ‘hello’ in all filesunder the/home/gigi directory. For more control over which files are searched,usefind, grep, andxargs. For example, the following command searches only C files:
```
          find /home/gigi -name '*.c' -print0 | xargs -0r grep -H 'hello'
```
This differs from the command:
```
          grep -rH 'hello' *.c
```
which merely looks for ‘hello’ in all files in the currentdirectory whose names end in ‘.c’. Here the-r isprobably unnecessary, as recursion occurs only in the unlikely eventthat one of ‘.c’ files is a directory. The ‘find ...’ command line above is more similar to the command:
```
          grep -rH --include='*.c' 'hello' /home/gigi
```
What if a pattern has a leading ‘-’?
```
          grep -e '--cut here--' *
```
searches for all lines matching ‘--cut here--’. Without ‘-e’,grep would attempt to parse ‘--cut here--’ as a list ofoptions.
Suppose I want to search for a whole word, not a part of a word?
```
          grep -w 'hello' *
```
searches only for instances of ‘hello’ that are entire words;it does not match ‘Othello’. For more control, use ‘\<’ and‘\>’ to match the start and end of words. For example:
```
          grep 'hello\>' *
```
searches only for words ending in ‘hello’, so it matches the word‘Othello’.
How do I output context around the matching lines?
```
          grep -C 2 'hello' *
```
prints two lines of context around each matching line.

How do I force grep to print the name of the file?

Append /dev/null:

          grep 'eli' /etc/passwd /dev/null

gets you:

          /etc/passwd:eli:x:2098:1000:Eli Smith:/home/eli:/bin/bash

Alternatively, use ‘-H’, which is a gnu extension:

          grep -H 'eli' /etc/passwd

Why do people use strange regular expressions on ps output?
```
          ps -ef | grep '[c]ron'
```
If the pattern had been written without the square brackets, it wouldhave matched not only theps output line for cron,but also theps output line for grep. Note that on some platforms,ps limits the output to the width of the screen;grep does not have any limit on the length of a lineexcept the available memory.
Why does grep report “Binary file matches”?
If grep listed all matching “lines” from a binary file, itwould probably generate output that is not useful, and it might evenmuck up your display. Sognu grep suppresses output fromfiles that appear to be binary files. To forcegnu grepto output lines even from files that appear to be binary, use the‘-a’ or ‘--binary-files=text’ option. To eliminate the“Binary file matches” messages, use the ‘-I’ or‘--binary-files=without-match’ option.
Why doesn't ‘grep -lv’ print non-matching file names?
‘grep -lv’ lists the names of all files containing one or morelines that do not match. To list the names of all files that contain nomatching lines, use the ‘-L’ or ‘--files-without-match’option.
I can do or with ‘|’, but what aboutand?
```
          grep 'paul' /etc/motd | grep 'franc,ois'
```
finds all lines that contain both ‘paul’ and ‘franc,ois’.
How can I search in both standard input and in files?
Use the special file name ‘-’:
```
          cat /etc/passwd | grep 'alain' - /etc/motd
```
How to express palindromes in a regular expression?
It can be done by using back-references;for example,a palindrome of 4 characters can be written with a BRE:
```
          grep -w -e '$.$$.$.\2\1' file
```
It matches the word "radar" or "civic".

Guglielmo Bondioni proposed a single REthat finds all palindromes up to 19 characters longusing 9 subexpressions and 9 back-references:
```
          grep -E -e '^(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?).?\9\8\7\6\5\4\3\2\1$' file
```
Note this is done by using gnu ERE extensions;it might not be portable to other implementations ofgrep.
Why is this back-reference failing?
```
          echo 'ba' | grep -E '(a)\1|b\1'
```
This gives no output, because the first alternate ‘(a)\1’ does not match,as there is no ‘aa’ in the input, so the ‘\1’ in the second alternatehas nothing to refer back to, meaning it will never match anything. (The second alternate in this example can only matchif the first alternate has matched—making the second one superfluous.)
How can I match across lines?
Standard grep cannot do this, as it is fundamentally line-based. Therefore, merely using the[:space:] character class does notmatch newlines in the way you might expect. However, if your grep iscompiled with Perl patterns enabled, the Perl ‘s’modifier (which makes. match newlines) can be used:
```
          printf 'foo\nbar\n' | grep -P '(?s)foo.*?bar'
```
With the GNU grep option -z (seeFile and Directory Selection), the input is terminated by null bytes. Thus,you can match newlines in the input, but the output will be the wholefile, so this is really only useful to determine if the pattern ispresent:
```
          printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'
```
Failing either of those options, you need to transform the inputbefore giving it togrep, or turn to awk,sed,perl, or many other utilities that aredesigned to operate across lines.
What do grep, fgrep, andegrep stand for?
The name grep comes from the way line editing was done on Unix. For example,ed uses the following syntaxto print a list of matching lines on the screen:
```
          global/regular expression/print
          g/re/p
```
fgrep stands for Fixed grep;egrep stands for Extendedgrep.

Next: Copying,Previous: Usage,Up: Top

5 Reporting bugs

Email bug reports to bug-grep@gnu.org,a mailing list whose web page ishttp://lists.gnu.org/mailman/listinfo/bug-grep. The Savannah bug tracker forgrep is located athttp://savannah.gnu.org/bugs/?group=grep.

5.1 Known Bugs

Large repetition counts in the ‘{n,m}’ construct may causegrep to use lots of memory. In addition, certain otherobscure regular expressions require exponential time andspace, and may cause grep to run out of memory.

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

Next: Index,Previous: Reporting Bugs,Up: Top

6 Copying

GNU grep is licensed under the GNU GPL, which makes itfreesoftware.

The “free” in “free software” refers to liberty, not price. Assome GNU project advocates like to point out, think of “free speech”rather than “free beer”. In short, you have the right (freedom) torun and change grep and distribute it to other people, and—if youwant—charge money for doing either. The important restriction isthat you have to grant your recipients the same rights and impose thesame restrictions.

This general method of licensing software is sometimes calledopen source. The GNU project prefers the term “free software”for reasons outlined athttp://www.gnu.org/philosophy/open-source-misses-the-point.html.

This manual is free documentation in the same sense. Thedocumentation license is included below. The license for the programis available with the source code, or athttp://www.gnu.org/licenses/gpl.html.

Up: Copying

6.1 GNU Free Documentation License

Version 1.3, 3 November 2008

     Copyright © 2000-2002, 2007-2008, 2010-2011 Free Software
     Foundation, Inc.
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