grep

computing: software: operating systems: Linux: grep

Manpage Contents

Name

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

Synopsis

grep

1. REDIRECT Template:arg/opt PATTERN
2. REDIRECT Template:fmt/arg/opt

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, 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:

*?	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 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.

Examples

grep text_to_find *.log

Notes

If file is specified using wildcards (e.g. *.log or *.*), apparently the command-line parser expands this into a listing of all matching files before passing it to grep. This unfortunately can lead to the error "bash: /bin/grep: Argument list too long". Putting a backslash before the wildcard will prevent the parser from expanding it before passing to grep, but grep doesn't seem to have any wildcard-handling abilities. So as far as I can tell, there is no way to grep a really large directory (the one I'm looking at has 5014 files).

Edit Log

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