grep : Introduction

Finds text within a file.

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

-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	ations, --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.

Patterns for searching.

.	Matches single character.
*	Wild character Example C* if found would pull up CC or CAT...
{}	Matches any character contained within the bracket.
^	Represents the beginning of the line, so if you did ^T it would search for any sentence starting with a T.
$	Represents the end of the line, so if you did $. then it would pull up any lines that ended with .
\	Means to take the next character serious so you could search for C\ C.

Note: Be careful using the characters $, *, [, ^, |, (, ), and \ in the pattern list because they are also meaningful to the shell. It is safest to enclose the entire pattern list in single quotes '... '.

The Wildcard Character

So the first question that probably comes to mind is something like "does this grep thing support wildcards ? And the answer is better than yes. In fact saying that grep supports wildcards is a big understatement. grep uses regular expressions which go a few steps beyond wildcards. But we will start with wildcards. The canonical wildcard character is the dot "." Here is an example :

>cat file

big
bad bug 
bag
bigger
boogy

>grep b.g file

big
bad bug 
bag
bigger

notice that boogy didn't match, since the "." matches exactly one character.

The repetition character

To match repetitions of a character, we use the star, which works in the following way:

the expression consisting of a character followed by a star matches any number (possibly zero) of repetitions of that character. In particular, the expression ".*" matches any string, and hence acts as a "wildcard".

To illustrate, we show some examples:

Examples: Wildcards

The File for These Examples >cat file big bad bug bag bigger boogy

Wildcards #1 >grep "b.*g" file big bad bug bag bigger boogy

Wildcards #2 >grep "b.*g." file bigger boogy

repetition >grep "ggg*" file bigger

Read the repetion example carefully, and pay careful attention to the fact that the "*" in grep patterns denotes repetition. It does not behave as a wildcard in regular expression syntax (as it is in UNIX or DOS glob patterns). Recall that the pattern ".*" behaves as a wildcard (because .* means "repeat any character any number of times). The pattern "g*" matches the string "", "g", "gg", etc. Likewise, "gg*" matches "g", "gg", "ggg", so "ggg*" matches "gg", "ggg", "gggg", etc.
The $ character matches the end of the line. The ^ character matches the beginning of the line.