grep(Global regular expression print)

    grep searches the input files for lines containing a match to a given pattern list. When it finds a match in a line, it copies the line to standard output (by default), or whatever other sort of output you have requested with options.

    Though grep expects to do the matching on text, it has no limits on input line length other than available memory, and it can match arbitrary characters within a line. If the final byte of an input file is not a newline, grep silently supplies one. Since newline is also a separator for the list of patterns, there is no way to match newline characters in a text.

Invoking grep

    The general synopsis of the grep command line is 

grep options pattern  input_file_name

There can be zero or more options. pattern will only be seen as such (and not as an input_file_name) if it wasn't already specified within options (by using the -e pattern or -f file options). There can be zero or more input_file_names.

grep options

Matching Control

  • -e pattern  , --regexp=pattern

    This can be used to specify multiple search patterns, or to protect a pattern beginning with a '-' (-e is specified by POSIX)

  • -f file  , --file=file

    Obtain patteerns from file, one per line.  (-f is specified by POSIX)

  • -i  , --ignore-case

    Ignore case distinctions. (-i is specified by POSIX)

  • -v , --invert-match

    Invert the sense of matching, to select non-matching lines.  (-v is specified by POSIX)

  • -w  , --word-regexp

    Select only those lines containing matches that form whole words.

  • -x  , --line-regexp

    Select only those matches that exactly match the whole line. For a eregular expression pattern, this is like parenthesizing the pattern and the surrounding it with '^' and '$'.

General Output Control

  • -c  , -count

    Suppress normal output, instead print a count of matching lines for each input file. With the -v option, count non-matching lines.

  • -o  , --only-matching

    Print only the matched(non-empty) parts of matching lines.

  • -q  , --quiet  , --silent

    Quiet, do not write anything to standard ouput. Exit immediately with zeron status if any match is found, even if an error was detected.

  • -Z  , --null

    Output a zero byte(the ASCII NUL character) intead 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.

Context Line Control

  • -A num  , --after-context=num

    Print num lines of trailing context after matching lines.

  • -B num  , --before-context=num

    Print num lines of leading context before matching lines.

  • -C num  , --context=num

    Print num lines of leading and trailing output context.

grep programs

  • -E

  • --extended-regexp

    Interpret the pattern as an extended regular expression (ERE). (-E is specified by POSIX.)

  • -F

  • --fixed-strings

    Interpret the pattern as a list of fixed strings (instead of regular expressions), separated by newlines, any of which is to be matched. (-F is specified by POSIX.)

  • -P

  • --perl-regexp

    Interpret the pattern as a Perl regular expression. This is highly experimental and ‘grep -P’ may warn of unimplemented features.

grep example

## Ex: Search multiple patterns using -e option[zbj@localhost ~]$ grep  -e 'root' -e 'zbj' /etc/passwdroot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/root:/sbin/nologinzbj:x:1000:1000::/home/zbj:/bin/bash## Ex: Ignore case using -i option[zbj@localhost ~]$ grep -i 'FTP' /etc/passwdftp:x:14:50:FTP User:/var/ftp:/sbin/nologin## Ex: Invert match using -v option[zbj@localhost ~]$ grep -v bash /etc/passwd | grep -v nologinsync:x:5:0:sync:/sbin:/bin/syncshutdown:x:6:0:shutdown:/sbin:/sbin/shutdownhalt:x:7:0:halt:/sbin:/sbin/halt## Ex: Count the number of matching patterns using -c option[zbj@localhost ~]$ grep -c 'nologin' /etc/passwd18## Ex: Display N number of lines before & after pattern matching[zbj@localhost ~]$ grep -B 3 'games' /etc/passwdhalt:x:7:0:halt:/sbin:/sbin/haltmail:x:8:12:mail:/var/spool/mail:/sbin/nologinoperator:x:11:0:operator:/root:/sbin/nologingames:x:12:100:games:/usr/games:/sbin/nologin[zbj@localhost ~]$ grep -A 3 'games' /etc/passwdgames:x:12:100:games:/usr/games:/sbin/nologinftp:x:14:50:FTP User:/var/ftp:/sbin/nologinnobody:x:99:99:Nobody:/:/sbin/nologinavahi-autoipd:x:170:170:Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin[zbj@localhost ~]$ grep -C 3 'games' /etc/passwdhalt:x:7:0:halt:/sbin:/sbin/haltmail:x:8:12:mail:/var/spool/mail:/sbin/nologinoperator:x:11:0:operator:/root:/sbin/nologingames:x:12:100:games:/usr/games:/sbin/nologinftp:x:14:50:FTP User:/var/ftp:/sbin/nologinnobody:x:99:99:Nobody:/:/sbin/nologinavahi-autoipd:x:170:170:Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin