grep [options] pattern [file...]
grep [options] [-e pattern | -f patternFile] [file]


Searches the input files (or STDIN if no files are named, or if file is - )
for lines containing a match to the pattern.

-G --basic-regexp pattern is a Basic Regular Expression (default).
                                                  ? , + , { , | , ( and ) must be preceeded by \ to ENABLE special meaning

-E --extended-regexp pattern is an Extended Regular Expression.
egrep is the same as grep -E.

-F --fixed-strings pattern is a list of fixed strings, separated by newlines, any of which is to be matched.
fgrep is the same as grep -F.

-P --perl-regexp Interpret pattern as a Perl regular expression.

controling matching
Invert the sense of matching to
select non-matching lines.
-e pattern
Use pattern as the matching pattern;
useful to protect patterns beginning with - from being intrepreted as options.
-f file
Obtain patterns from file, one per line. An empty file contains zero patterns and matches nothing.
matches must form whole words.

The matching substring must either be at the beginning of the line, or preceded by a non-alphanumeric character and
must be either at the end of the line or followed by a non-alphanumeric character.

Alphanumeric characters are letters, digits, and the underscore.

exactly match the whole line.
-y Obsolete synonym for -i (--ignore-case).

controlling output
only the matching part of the line is output.
when may be never, always, or auto

The matched sections can be displayed in color as per $GREP_COLOR in the form ff[;bb]
where ff is the code used for the foreground and bb is used for the background.

magenta on yellow 35;43 blue on cyan 34;46 red 31 (default)
Foreground: black 30, green 32, yellow 33, blue 34, magenta 35, cyan 36, white 37, red 31 (default)
Background: black 40, green 42, yellow 43, blue 44, magenta 45, underscore 4, [blink] 5, inverse 7
export GREP_COLOR="33;44"

Codes are not output when inappropriate (For example: to a file or pipe) unless when=always

grep --color=always birds logfile | more -r

output num lines of context before and after match
-B num
Output num lines of context before lines containing the pattern
then the lines containing pattern.
-A num
Output num lines of context after outputting the pattern.
Places a line containing -- between sets of matches.
   grep -C1 day  grep -B2 day  grep -A2 day
 input  output
mon mon
yesterday yesterday yesterday
day day day day
later later later
------set seperator
mon2 mon2
yr2 yr2 yr2
day2 day2 day2 day2
later2 later2 later2
f f

only output count of matches (or -cv non-matches) for each input file
Prefixed by 'filename :' if multiple files, even if count is 0
grep -c images *html |grep -v 0$ #dont show lines with 0 count
Prefix each line with the line number
prefix each line with the byte offset within the input file
Use Unix-style byte offsets.
report byte offsets i.e. with CR characters stripped off. only for MS-DOS and MS-Windows.
Quiet; do not write to STDOUT.
Errors like "Permission denied" are written to STDERR and return code is set to 2.
Exit with zero status as soon as a match is found.
If no match is found $? is set to 1.
Suppress error messages about nonexistent or unaccessable files normally sent to STDERR.
filename is prepended to each match when multiple files are searched.
outputs the only the filename of files containing a match.
Once a match is found grep proceeds to the next file.
outputs the filenname with no match
terminate filename with NULL . Used when file names contain newlines!.

Used with commands find -print0, perl -0, sort -z, and xargs -0.

--label=label Displays input actually coming from standard input as input coming from file label. This is useful for tools like zgrep, e.g.
gzip -cd foo.gz |grep --label=foo something

-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, the standard input is positioned to just after the last matching line, enables calling process to resume a search.

Outputs any trailing context lines.
With -c or --count , does not output a count greater than num.

With -v or --invert-match , stops after outputting num non-matching lines.

Directory processing
-d action
If an input is a directory:
action: read: as ordinary files, default.
         recurse: reads all files under each directory, recursively.
equivalent to -d recurse .
--include=pattern Recurse only searching file matching pattern.
--exclude=pattern Recurse skip file matching pattern.

Special handling
--binary‑files=type If the first few bytes of a file indicate it contains binary data (non-ASCII text),
outputs either a one-line message saying that a binary file matches, or no message if there is no match.

binary and without-match assume binary file does not match, default.
text processe a binary file as if it were ascii text; equivalent to -a .

Note: Control characters sent to the terminal can set attributes making it unreadable. If this happens try STTY SANE

Process a binary file as if it were ASCII text
‑‑binary‑files=text equivalent.
-I Ignore a binary file (treat it as if it did not contain matching data )
‑‑binary‑files=without‑match equivalent.
Process 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 causes 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. only effective under MS-DOS and MS-Windows unless used with -b .
Display the version to standard error.
--mmap use the mmap system call to read input, instead of read which may yield better performance.
May cause undefined behavior if an input file shrinks, or if an I/O error occurs.
-D action
If an input file is a device, FIFO or socket, use action to process it.
read, which means that devices are read as ordinary files (default).
skip, devices are silently skipped.
--line‑buffered can have a performance penality.

Basic Regular Expressions: ? , + , { , | , ( and ) alone have no special meaning
   Use backslash to ENABLE special meaning (ex: \?)
   * is a meta character.

Regular Expressions

A pattern describes a set of strings, using operators to combine smaller expressions.

Grep understands two different versions of regular expression syntax: basic and extended.
GNU grep, there is no difference.

The simplest regular expression matchs a single character.
 For example: a matches a , R matches R
Regular expresions can be combined.
 For example: AR matches AR

Environment Variables

GREP_OPTIONS are placed before explicit options specified on the command.
Option specifications are separated by whitespace.
A backslash escapes the next character, used to specify an option containing whitespace or a backslash.
example: GREP_OPTIONS='--color'

GREP_COLOR Specifies the color for highlighting.

The locale LC_xxx is specified by examining: LC_ALL, LC_xxx, LANG, in 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 BrazilianPortuguese is used.
The C locale is used if none of these are set, or if the locale catalog is not installed, or if grep was not compiled with national language support (NLS).
LC_ALL, LC_COLLATE,  LANG collating sequence used to interpret range expressions like [a-z].
LC_ALL, LC_CTYPE,    LANG type of characters, e.g., which characters are whitespace.
LC_ALL, LC_MESSAGES, LANG language 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.

_N_GNU_nonoption_argv_flags_ (N is grep's process ID.)
If the ith character of this environment variable's value is 1,
do not consider the ith operand 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 expansion and therefore should not be treated as options.
Only with the GNU C library, and only when POSIXLY_CORRECT is not set.


 0: selected lines found
 1: no lines selected. if [ $? != 1 ];then …
 2: an error occurred like: no permission or file not found.

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

Backreferences are very slow, and require exponential time.

Current "official" GNU grep

See also egrep, fgrep, sed, sh, attributes, environ, largefile, regex, regexp, XPG4

GNU Project 2002/01/22 GREP(1)

From SUN /usr/bin/grep [ -bchilnsvw ] limited-regular-expression [ filename ... ]

/usr/xpg4/bin/grep [ -E | -F ] [ -c | -l | -q ] [ -bhinsvwx* ] ... pattern [ file ... ] ... -e pattern_list ... [ -f pattern_file ] ...
... [ -e pattern_list ... ] -f pattern_file ...

searches the input for a pattern and may display lines that contain that pattern.

Normally, each line found is copied to standard output.
The file name is displayed before each line found if there is more than one input file.

grep uses limited regular expressions like those described on the regexp(5) manual page to match the patterns.


    grep interprets pattern_list as a Basic Regular Expressionas described on regex(5) manual page, default.
    -E pattern_list as a full regular expression
    -F pattern_list as a fixed string.
grep uses a compact non-deterministic algorithm. which means??

Since characters $, *, [, ^, |, (, ), and \ used in the pattern_list are meaningful to the shell,
enclose the entire pattern_list in apostrophes.


    -v reVerse the sense of the operation. i.e. display all lines not containing the pattern.
    -i ignore case
    -n Precede each line by its line number in the file (first line is 1). -c only display a count of the lines that contain the pattern or
    -cv count of lines not matching. -h hide the name of the file containing the matching
    line normally appended to that line when searching multiple files. -l display unique list of the names of files with matching lines, separated by \n characters.
    -s Suppress error messages about nonexistent or unreadable files.
    -w search for the expression as a word as if surrounded by \< and \>
    -b Precede each line by the block number on which it was found, useful in locating block numbers by context (first block is 0). for /usr/xpg4/bin/grep only aka egrep :
    -e pattern_list
    Specify one or more patterns to be used during the search for input.
    patterns in pattern_list are separated by a NEWLINE character.
    A null pattern is specified by two adjacent newline characters in pattern_list.
    Unless the -E or -F is also specified, each pattern will be treated as a basic regular expression.
    Multiple -e and -f options are accepted . All of the specified patterns are used when matching lines, but the order of evaluation is unspecified.

    -E Match using Full Regular Expressions.
    If any entire full regular expression pattern matches an input line, the line will be matched.
    A null full regular expression matches every line.
    Each pattern will be interpreted as a full regular expression as and \), and including:

    followed by + that matches one or more occurrences of the full regular expression.
    followed by ? that matches 0 or 1 occurrences of the full regular expression.
    separated by | or by a new-line that match strings that are matched by any of the expressions.
    May be enclosed in parentheses ( ) for grouping.
    The order of precedence of operators is [ ], then * ? +, then concatenation, then | and new-line.

    -f pattern_file
    Read one or more patterns from the file named by the path name pattern_file. patterns in pattern_file are terminated by a NEWLINE character.
    A null pattern is specified by an empty line in pattern_file.
    Unless -E or -F is also specified, each pattern will be treated as a basic regular expression.

    -F match using fixed strings.
    Treat each pattern specified as a string instead of a regular expression. If an input line contains any of the patterns as a contiguous sequence of bytes, the line will be matched.
    A null string matches every line. See fgrep(1)

    -q Quiet. Do not write anything to the standard output, regardless of matching lines.
    Exit with zero status if a line was selected.

    -x Consider only input lines that use all characters in
    the line to match an entire fixed string or regular expression to be matching lines.


    file If no file operands are specified, the standard input will be used.

    pattern used during the search . (/usr/bin/grep )

    one or more patterns . treated as if it were specified as -epattern_list.


    The -epattern_list is useful when pattern_list begins with the hyphen and has the same effect as the pattern_list operand. Useful when it is more convenient to provide multiple patterns as separate arguments.

    Multiple -e and -f options are accepted and grep will use all of the patterns. The order of evaluation is not specified.
    If a null string exists as a pattern, it will match every line)

    The -q option provides a means of easily determining whether or not a pattern (or string) exists in a group of files. When searching several files, it provides a performance improvement (because it can quit as soon as it finds the first match) and requires less care by the user in choosing the set of files to supply as arguments (because it will exit zero if it finds a match even if grep detected an access or read error on earlier file operands).

    Exit status

    0 sucessfully found one or more matches .
    1 failed to find any matches.
    2 Syntax errors or inaccessible files (even if some matches were found).


Find all uses of the "Posix" ( -i ignoring case) in the file, and write lines with line numbers( -n ):
grep -i -n posix

Display line numbers( -n ) containg empty lines ( i.e where beginning is immediately followed by end of line)
grep -n ^$ or grep -n -v .

Display all lines containing strings abc or def or both :
grep -E 'abc def' -or- grep -F 'abc def'

Both of the following commands display all lines matching exactly abc or def:
grep -E '^abc$ ^def$' -or- grep -F -x 'abc def'

To find an A surrounded by tabs, using ANSI-C quoting for bash use:
grep $'\tA\t'

To change a line beginning with hours:minutes (where hours may be one or two digits) to be zero filled to two digits:
> echo "19:42" |sed "s/^\(.\):/0\1:/"
> echo "9:42" |sed "s/^\(.\):/0\1:/"

Green defines a subexpression, blue uses the subexpression

Environment Variables
See environ for the following environment variables : LC_COLLATE, LC_CTYPE, LC_MESSAGES, and NLSPATH.


Large File Behavior.
See largefile(5) for the description of the behavior of grep when encountering files greater than or equal to 2 Gbyte ( 2**31 bytes). Lines are limited only by the size of the available virtual memory. If there is a line with embedded nulls, grep will only match up to the first null; if it matches, it will display the entire line.

The results are unspecified if input files contain lines longer than LINE_MAX bytes or contain binary data. LINE_MAX is defined in /usr/include/limits.h. See egrep, fgrep, sed, sh, attributes(5), environ(5), largefile(5), regex(5), regexp(5), XPG4(5)