How to use the grep command

grep

grep, egrep, fgrep - print lines matching a pattern

SYNOPSIS grep [options] PATTERN [FILE...] grep [options] [-e PATTERN | -f FILE] [FILE...]

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

In addition, two variant programs egrep and fgrep are available. Egrep is the same as grep -E. Fgrep is the same as grep -F.

OPTIONS -A NUM, --after-context=NUM Print NUM lines  of  trailing  context  after  matching  lines. Places a  line  containing—between  contiguous  groups  of              matches.

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

-B NUM, --before-context=NUM Print NUM  lines  of  leading  context  before  matching lines. Places a  line  containing—between  contiguous  groups  of              matches.

-C NUM, --context=NUM Print NUM lines of output context. Places a line containing—between contiguous groups of matches.

-b, --byte-offset Print the byte offset within the input file before each line of              output.

--binary-files=TYPE If the first few bytes of a file indicate that the file contains binary data, assume that the file is of type TYPE. By default, TYPE is binary, and grep normally outputs either a one-line mes- sage 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 termi- nal driver interprets some of it as commands.

--colour[=WHEN], --color[=WHEN] Surround the matching string with the marker find in GREP_COLOR environment variable. WHEN may be `never', `always', or `auto'

-c, --count Suppress normal output; instead print a count of matching lines for each  input  file. With the -v, --invert-match option (see             below), count non-matching lines.

-D ACTION, --devices=ACTION If an input file is a device, FIFO or socket, use ACTION to pro- cess it. By default, ACTION is read, which means that devices are read just as if they were ordinary  files. If ACTION  is              skip, devices are silently skipped.

-d ACTION, --directories=ACTION If an  input file is a directory, use ACTION to process it. By             default, ACTION is read, which means that directories  are  read just as if they were ordinary files. If ACTION is skip, direc- tories are silently skipped. If ACTION is recurse, grep  reads all files under each directory, recursively; this is equivalent to the -r option.

-E, --extended-regexp Interpret PATTERN as an extended regular expression (see below).

-e PATTERN, --regexp=PATTERN Use PATTERN as the pattern; useful to protect patterns beginning with -.

-F, --fixed-strings Interpret PATTERN as a list of fixed strings, separated by new- lines, any of which is to be matched. -P, --perl-regexp Inter- pret PATTERN as a Perl regular expression.

-f FILE, --file=FILE Obtain patterns from FILE, one per line. The empty  file  con- tains zero patterns, and therefore matches nothing.

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

-H, --with-filename Print the filename for each match.

-h, --no-filename Suppress the prefixing of filenames  on  output  when  multiple files are searched.

--help Output a brief help message.

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

-i, --ignore-case Ignore case distinctions in both  the  PATTERN  and  the  input files.

-L, --files-without-match Suppress normal  output;  instead  print the name of each input file from which no output would normally have been printed. The scanning will stop on the first match.

-l, --files-with-matches Suppress normal  output;  instead  print the name of each input file from which output would normally have been  printed. The scanning will stop on the first match.

-m NUM, --max-count=NUM Stop reading  a file after NUM matching lines. If the input is             standard input from a regular file, and NUM matching  lines  are output, grep  ensures  that the standard input is positioned to              just after the last matching line before exiting, regardless  of              the  presence of trailing context lines. This enables a calling process to resume a search. When grep stops after NUM matching lines, it  outputs  any trailing context lines. When the -c or—count option is also used,  grep  does  not  output  a  count greater than NUM. When the -v or—invert-match option is also used, grep stops after outputting NUM non-matching lines.

--mmap If possible, use the mmap(2) system call to read input, instead of the default read(2) system call. In some situations, --mmap yields better performance. However, --mmap can cause undefined behavior (including  core dumps) if an input file shrinks while grep is operating, or if an I/O error occurs.

-n, --line-number Prefix each line of output with the line number within its input file.

-o, --only-matching Show only the part of a matching line that matches PATTERN.

--label=LABEL Displays input actually coming from standard input as input com- ing from file LABEL. This is especially useful for tools  like zgrep, e.g. gzip -cd foo.gz |grep—label=foo something—line-buffering Use line buffering, it can be a performance penalty.

-q, --quiet, --silent Quiet; do  not write anything to standard output. Exit immedi- ately with zero status if any match is found, even if an  error was detected. Also see the -s or—no-messages option.

-R, -r, --recursive Read all files under each directory, recursively; this is equiv- alent to the -d recurse option.

--include=PATTERN Recurse in directories only searching file matching PATTERN.

--exclude=PATTERN Recurse in directories skip file matching PATTERN.

-s, --no-messages Suppress error messages about nonexistent or unreadable  files. Portability note: unlike GNU grep, traditional grep did not con- form to POSIX.2, because traditional grep lacked a -q option and its -s option behaved like GNU grep's -q option. Shell scripts intended to be portable to traditional grep should avoid both -q and -s and should redirect output to /dev/null instead.

-U, --binary Treat the  file(s) as binary. By default, under MS-DOS and MS-Windows, grep guesses the file type by looking at the  contents of the first 32KB read from the file. If grep decides the file is a text file, it strips the CR characters from  the  original file contents  (to  make  regular expressions with ^ and $ work              correctly). Specifying -U overrules this guesswork, causing all files to be read and passed to the matching mechanism verbatim; if the file is a text file with CR/LF pairs at the end of  each line, this  will  cause some regular expressions to fail. This option has no effect on platforms other than MS-DOS and MS-Win- dows.

-u, --unix-byte-offsets Report Unix-style  byte  offsets. This switch causes grep to              report byte offsets as if the file were  Unix-style  text  file, i.e. with CR characters stripped off. This will produce results identical to running grep on a Unix machine. This option has no             effect  unless -b option is also used; it has no effect on plat- forms other than MS-DOS and MS-Windows.

-V, --version Print the version number of grep to standard error. This ver- sion number should be included in all bug reports (see below).

-v, --invert-match Invert the sense of matching, to select non-matching lines.

-w, --word-regexp Select only  those  lines  containing  matches  that form whole words. The test is that the matching substring must either  be              at  the  beginning  of  the line, or preceded by a non-word con- stituent character. Similarly, it must be either at the end of              the line or followed by a non-word constituent character. Word- constituent characters are letters, digits, and the underscore.

-x, --line-regexp Select only those matches that exactly match the whole line.

-y    Obsolete synonym for -i.

-Z, --null Output a  zero  byte  (the  ASCII NUL character) instead of the character that normally follows a file name. For example, grep -lZ outputs  a  zero  byte  after each file name instead of the usual newline. This option makes the output unambiguous,  even in the presence of file names containing unusual characters like newlines. This option can be  used  with  commands  like  find -print0, perl  -0,  sort  -z, and xargs -0 to process arbitrary file names, even those that contain newline characters.

REGULAR EXPRESSIONS A regular expression is a pattern that describes  a  set  of  strings. Regular expressions  are constructed analogously to arithmetic express- sions, by using various operators to combine smaller expressions.

Grep understands two different versions of regular expression  syntax: "basic" and "extended." In GNU grep, there is no difference in avail- able functionality using either  syntax. In other  implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards.

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

A bracket expression is a list of characters enclosed by [ and ]. It      matches  any  single  character in that list; if the first character of       the list is the caret ^ then it matches any character not in the  list. For example,  the  regular  expression [0123456789] matches any single digit.

Within a bracket expression, a range expression consists of two charac- ters 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 dictio- nary order,  and in these locales [a-d] is typically not equivalent to       [abcd]; it might be equivalent to [aBbCcDd], for  example. To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value C.

Finally, certain named classes of  characters  are  predefined  within bracket expressions, as follows. Their names are self-explanatory, and they are  [:alnum:],  [:alpha:],  [:cntrl:],   [:digit:],   [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. For example, alnum: means [0-9A-Za-z],  except  the  latter  form depends upon the C locale and the ASCII character encoding, whereas the former is independent of locale and character  set. (Note that  the       brackets  in these class names are part of the symbolic names, and must       be included in addition to the brackets delimiting the  bracket  list.) Most metacharacters  lose  their  special  meaning  inside  lists. To      include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal - place it last.

The period. matches any single character. The symbol \w is a synonym for alnum: and \W is a synonym for [^[:alnum]].

The caret ^ and the dollar sign $ are metacharacters that respectively match the empty string at the beginning and end of a line. The symbols \< and \> respectively match the empty string at the beginning and end of a word. The symbol \b matches the empty string at the  edge  of  a       word,  and \B matches the empty string provided it's not at the edge of       a word.

A regular expression may be followed by one of several repetition oper- ators: ?     The preceding item is optional and matched at most once. *     The preceding item will be matched zero or more times. +     The preceding item will be matched one or more times. {n}   The preceding item is matched exactly n times. {n,}  The preceding item is matched n or more times. {n,m} The  preceding  item  is  matched at least n times, but not more than m times.

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

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

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

The backreference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regu- lar expression.

In basic regular expressions the metacharacters ?, +, {, |, (,  and  ) lose their  special  meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).

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

GNU egrep attempts to support traditional usage by assuming that {  is       not  special if it would be the start of an invalid interval specifica- tion. For example, the shell command egrep '{1' searches for the two- character string {1 instead of reporting a syntax error in the regular expression. POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

ENVIRONMENT VARIABLES Grep's behavior is affected by the following environment variables.

A locale  LC_foo is specified by examining the three environment vari- ables LC_ALL, LC_foo, LANG, in that order. The first of  these  vari- ables that is set specifies the locale. For example, if LC_ALL is not set, but LC_MESSAGES is set to pt_BR, then Brazilian Portuguese is used for the  LC_MESSAGES  locale. The C locale is used if none of these environment variables are  set,  or  if  the  locale  catalog  is  not installed, or  if grep was not compiled with national language support (NLS).

GREP_OPTIONS This variable specifies default options to be placed in front of             any   explicit   options. For example,  if  GREP_OPTIONS  is              '--binary-files=without-match—directories=skip', grep  behaves as if the two options—binary-files=without-match and—direc- tories=skip had been specified  before  any  explicit  options. Option specifications are separated by whitespace. A backslash escapes the next character, so it can be  used  to  specify  an              option containing whitespace or a backslash.

GREP_COLOR Specifies the marker for highlighting.

LC_ALL, LC_COLLATE, LANG These variables specify the LC_COLLATE locale, which determines tories=skip had been specified  before  any  explicit  options. Option specifications are separated by whitespace. A backslash escapes the next character, so it can be  used  to  specify  an              option containing whitespace or a backslash.

GREP_COLOR Specifies the marker for highlighting.

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

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

LC_ALL, LC_MESSAGES, LANG These variables specify the LC_MESSAGES locale, which determines the language that grep uses for messages. The default C locale uses American English messages.

POSIXLY_CORRECT If set, grep  behaves  as  POSIX.2  requires;  otherwise,  grep behaves more  like  other  GNU programs. POSIX.2 requires that options that follow file names must be treated as file names; by             default,  such  options are permuted to the front of the operand list and are treated as options. Also, POSIX.2  requires  that unrecognized options  be diagnosed as "illegal", but since they are not really against the law the default is to diagnose  them as  "invalid". POSIXLY_CORRECT also  disables  _N_GNU_nonop- tion_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 com- mand it runs, specifying which operands are the results of file name wildcard  expansion and therefore should not be treated as              options. This behavior  is  available  only  with  the  GNU  C              library, and only when POSIXLY_CORRECT is not set.

DIAGNOSTICS Normally, exit status is 0 if selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred, unless the -q or—quiet or—silent option is used and a selected line is found.

BUGS Email bug reports to bug-grep@gnu.org.

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

Backreferences are very slow, and may require exponential time.
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