grep

Name

 * grep, egrep, fgrep, rgrep - print lines matching a pattern

Synopsis

 * grep PATTERN
 * grep

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,  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.

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.
 * -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 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.
 * -C NUM, --context=NUM
 * Print NUM lines of output context.  Places a line containing -- between contiguous groups of matches.
 * -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.
 * --colour[=WHEN], --color[=WHEN]
 * Surround the matching string with the marker find in GREP_COLOR environment variable. WHEN may be `never', `always', or `auto'
 * -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, which means that directories are read just as if they were ordinary files.  If ACTION is skip,  directories  are  silently skipped.  If ACTION is recurse, grep reads all files under each directory, recursively; this is  equivalent to the -r option.
 * -E, --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 newlines, any of which is to be matched.
 * -f FILE, --file=FILE
 * Obtain patterns from FILE, one per line. The empty file contains 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.
 * --label=LABEL
 * Displays input actually coming from standard input as input coming from file LABEL. This is especially useful for  tools  like zgrep, e.g.  gzip -cd foo.gz |grep --label=foo something
 * --line-buffered
 * Use line buffering, it can be a performance penalty.
 * -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.
 * -P, --perl-regexp
 * Interpret PATTERN as a Perl regular expression.
 * -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.
 * -R, -r, --recursive
 * Read all files under each directory, recursively; this is equivalent 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 conform 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-Windows.
 * -u, --unix-byte-offsets
 * Report Unix-style  byte  offsets.   This  switch causes grep to report byte offsets as if the file were  Unix-style  text  file, i.e. with CR characters stripped off.  This will produce results identical to running grep on a Unix machine.  This option has no effect  unless -b option is also used; it has no effect on platforms other than MS-DOS and MS-Windows.
 * -V, --version
 * Print the version number of grep to standard error.  This  version 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 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.
 * -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.
 * -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.

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," "extended," and "perl." In GNU grep, there is no  difference in  available functionality using either of the first two 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  add additional functionality, but the implementation used here is undocumented and is not compatible  with  other  grep implementations.

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 expresion 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 [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 operators:

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 subexpression.

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 regular expression.

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

Traditional egrep did not support the { metacharacter, and 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 specification. For example, the shell command egrep '{1' searches for the two-character  string {1 instead of reporting a syntax error in the regular expression. POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

Environment Variables
Grep's behavior is affected by the following environment variables. A locale  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 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 --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
 * 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_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  filename  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-gnu-utils@gnu.org. Be sure to include the word "grep" somewhere in the "Subject:" field.

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.

Questions

 * So, how do you grep for a string which contains spaces and quote characters? This is probably some standard feature of Linux Shell syntax which everyone knows, but I don't, and that's why this page needs examples.
 * Likewise, how do you search a group of files? Using "*.*" for FILE doesn't seem to work, and leaving the FILE argument blank tells grep to expect input from STDIN.
 * A: apparently "*" by itself; see examples below

Examples

 * grep text_to_find *.log:find text_to_find by checking all the logfiles in the current folder
 * grep -r text_to_find *:find text_to_find by checking all files in the current folder or under it

Edit Log

 * 2005-06-13 Transcribed from Ubuntu (hoary) manpages; formatting not complete.