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search input files for lines that match a given pattern.
grep programs
`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. There are three major variants of `grep', controlled by the
following options.
`-G'
`--basic-regexp'
Interpret pattern as a basic regular expression. This is the
default.
`-E'
`--extended-regexp'
Interpret pattern as an extended regular expression.
`-F'
`--fixed-strings'
Interpret pattern as a list of fixed strings, separated by
newlines, any of which is to be matched.
In addition, two variant programs EGREP and FGREP are available.
EGREP is the same as `grep -E'. FGREP is the same as `grep -F'.
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 two different versions of
regular expression syntax: "basic" and "extended". In GNU `grep', there is no
difference in available 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 list of characters enclosed by `[' and `]' 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. A range of ASCII
characters may be specified by giving the first and last characters, separated
by a hyphen.
Finally, certain named classes of characters are predefined,
as follows. Their interpretation depends on the `LC_CTYPE' locale;
the
interpretation below is that of the POSIX locale, which is the default if
no `LC_CTYPE' locale is specified.
`[:alnum:]'
Any of `[:digit:]' or `[:alpha:]'
`[:alpha:]'
Any letter:
`a b c d e f g h i j k l m n o p q r s t u v w x y z',
`A B C D E F G H I J K L M N O P Q R S T U V W X Y Z'.
`[:blank:]'
Space or tab.
`[:cntrl:]'
Any character with octal codes 000 through 037, or `DEL' (octal
code 177).
`[:digit:]'
Any one of `0 1 2 3 4 5 6 7 8 9'.
`[:graph:]'
Anything that is not a `[:alnum:]' or `[:punct:]'.
`[:lower:]'
Any one of `a b c d e f g h i j k l m n o p q r s t u v w x y z'.
`[:print:]'
Any character from the `[:space:]' class, and any character that is
*not* in the `[:graph:]' class.
`[:punct:]'
Any one of `! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _
` { | } ~'.
`[:space:]'
Any one of `CR FF HT NL VT SPACE'.
`[:upper:]'
Any one of `A B C D E F G H I J K L M N O P Q R S T U V W X Y Z'.
`[:xdigit:]'
Any one of `a b c d e f A B C D E F 0 1 2 3 4 5 6 7 8 9'.
For example, `[[:alnum:]]' means `[0-9A-Za-z]', except the latter
form is dependent upon the ASCII character encoding, whereas the former
is portable. (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 will be 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.
Usage
*****
Here is an example shell command that invokes GNU `grep':
grep -i 'hello.*world' menu.h main.c
This lists all lines in the files `menu.h' and `main.c' that contain
the string `hello' followed by the string `world'; this is because `.*'
matches zero or more characters within a line. *Note Regular
Expressions::. The `-i' option causes `grep' to ignore case, causing
it to match the line `Hello, world!', which it would not otherwise
match. *Note Invoking::, for more details about how to invoke `grep'.
Here are some common questions and answers about `grep' usage.
1. How can I list just the names of matching files?
grep -l 'main' *.c
lists the names of all C files in the current directory whose
contents mention `main'.
2. How do I search directories recursively?
grep -r 'hello' /home/gigi
searches for `hello' in all files under the directory
`/home/gigi'. For more control of which files are searched, use
`find', `grep' and `xargs'. For example, the following command
searches only C files:
find /home/gigi -name '*.c' -print | xargs grep 'hello' /dev/null
3. What if a pattern has a leading `-'?
grep -e '--cut here--' *
searches for all lines matching `--cut here--'. Without `-e',
`grep' would attempt to parse `--cut here--' as a list of options.
4. Suppose I want to search for a whole word, not a part of a word?
grep -w 'hello' *
searches only for instances of `hello' that are entire words; it
does not match `Othello'. For more control, use `\<' and `\>' to
match the start and end of words. For example:
grep 'hello\>' *
searches only for words ending in `hello', so it matches the word
`Othello'.
5. How do I output context around the matching lines?
grep -C 2 'hello' *
prints two lines of context around each matching line.
6. How do I force grep to print the name of the file?
Append `/dev/null':
grep 'eli' /etc/passwd /dev/null
7. Why do people use strange regular expressions on `ps' output?
ps -ef | grep '[c]ron'
If the pattern had been written without the square brackets, it
would have matched not only the `ps' output line for `cron', but
also the `ps' output line for `grep'.
8. Why does `grep' report "Binary file matches"?
If `grep' listed all matching "lines" from a binary file, it would
probably generate output that is not useful, and it might even
muck up your display. So GNU `grep' suppresses output from files
that appear to be binary files. To force GNU `grep' to output
lines even from files that appear to be binary, use the `-a' or
`--text' option.
9. Why doesn't `grep -lv' print nonmatching file names?
`grep -lv' lists the names of all files containing one or more
lines that do not match. To list the names of all files that
contain no matching lines, use the `-L' or `--files-without-match'
option.
10. I can do OR with `|', but what about AND?
grep 'paul' /etc/motd | grep 'franc,ois'
finds all lines that contain both `paul' and `franc,ois'.
11. How can I search in both standard input and in files?
Use the special file name `-':
cat /etc/passwd | grep 'alain' - /etc/motd
Bugs
Large repetition counts in the `{m,n}' 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.
Related commands:
egrep - Search file(s) for lines that
match an extended expression
fgrep - Search file(s) for lines that
match a fixed string
gawk - Find
and Replace text within file(s)
grep - Search file(s) for lines that
match a given pattern
tr -
Translate, squeeze, and/or delete characters