The grep and egrep of the Three Musketeers of Linux

Grep:the Three Musketeers of text processing on Linux

1 grep: Text filter (Pattern: pattern) tool; *(grep, Egrep, Fgrep)2sed:stream Editor, text Editing tool;3 awk:linux implementation gawk, Text Report Generator;

Grep:global search REgular expression and Print out of the line.

Function: Text Search tool, according to user-specified "mode" to match the target text line by row to check; print matching lines;Pattern: The filter condition written by the regular expression character and the text character;REGEXP: A pattern written by a class of special characters and text characters, some of which do not represent the literal meaning of a character, but are a function of control or wildcard;divided into two categories:Basic Regular Expressions: BREextended Regular expression: EREgrep-e, EgrepRegular expression engine

1 Options:2--color=Auto: Coloring the text to match to the display;3-V: Displays rows that cannot be matched to the pattern;4-I: ignoring character case;5-o: Show only the matching string;6-Q: Silent mode, do not output any information;7-A #:after, after # line8-B #: Before, front # line9-C #:context, front and back # linesTen  One-E: Use ere;

Basic regular Expression meta-characters:

Character Matching:

.: matches any single character;[]: matches any single character within the specified range[^]: matches any single character outside the specified range[:d Igit:], [: Lower:], [: Upper:], [: Alpha:], [: Alnum:], [:p UNCT:], [: Space:]number of matches: used after the number of characters to be specified, to specify the number of occurrences of the preceding character;*: Matches the preceding character any time;For example: grep "X*y"AbxyXayXxxxxxyGreedy mode. *: Any character of any length;\?: matches the preceding character 0 or 1 times, i.e. the preceding is optional;\+: Matches its preceding character at least 1 times;\{m\}: Matches the preceding character m times;\{m,n\}: Matches the preceding character at least m times, up to n times;\{0,n\}: Matches the preceding character up to n times;\{m,\}: Matches the preceding character at least m times;Location Anchoring:^: anchor at the beginning of the line, for the leftmost mode;$: End of line anchoring; for the rightmost side of the pattern;^pattern$: Used for pattern matching whole line;^$: Empty line;^[[:space:]]*$\< or \b: The first anchor of the word, the left side of the word pattern ;\> or \b: the ending anchor; for the right side of the word pattern ;\<pattern\>: matches the whole word; Group:\ (\): Bind one or more characters together and treat them as a whole;\ (xy\) *abNote: The contents of the pattern in the grouping brackets are recorded in internal variables by the regular expression engine, which are named: \1, \2, \3, ...\ 1: From the left side, the first opening parenthesis and matching the right parenthesis to match the pattern between the characters;\ (ab\+\ (xy\) *\):\1:ab\+\ (xy\) *\2:xyBack reference: references the pattern in the preceding grouping brackets to match the character (not the pattern itself) Practice Examples : 1. Display the line in the/proc/meminfo file that begins with the size S; (Requires: use two ways)

1 ' ^{s| S}' /proc/meminfo2'^s'  /proc/meminfo

2. Display the lines in the/etc/passwd file that do not end in/bin/bash;

1 ' /bin/bash$ '  /etc/passwd

3. Display the user name of the user with the largest ID number in the/etc/passwd file;

1 # sort-t:-k3-n/etc/passwd | Tail-1 | Cut-d:-f1

4, if the user root exists, display its default shell program;

1 # ID root &>/dev/null"^root\>" /etc/passwd | Cut-d:-f7

5. Find out the number of two or three digits in/etc/passwd;

1 " \<[0-9]\{2,3\}\> " /etc/passwd

6. Display a line in the/etc/rc.d/rc.sysinit file that begins with at least one white space character and that is followed by a non-whitespace character;

1 " ^[[:space:]]\+[^[:space:]] " /etc/rc.d/rc.sysinit

7. Find the line ending with ' LISTEN ' followed by 0, 1, or more whitespace characters in the result of the "Netstat-tan" command;

" listen[[:space:]]*$ "

8, add user bash, Testbash, basher and Nologin (its shell is/sbin/nologin), and then find the/etc/passwd file user name with the shell name of the line;

1 " ^\ ([[: alnum:]]\+\>\). *\1$ " /etc/passwd

Egrep and extended Regular expressions

Egrep = Grep-e

Extend the metacharacters of regular expressions:Character matching: . [] [^]number of matches: *?: 0 or 1 times;+: 1 or more times;{m}: matches m times;{m,n}: At least m, up to n times;Anchoring: ^ $\<, \b\>, \bGroup: ()Back reference: \1, \2, ...or:a|bC|cat:c or Cat Practice Examples : 1. Display the default shell and UID of the current system root, CentOS or User1 user;

1 ' ^ (root|centos|user1) \> ' /etc/passwd | cut-d:-f1,3,7

2, find the/etc/rc.d/init.d/functions file (CENTOS6) A word followed by a parenthesis line;

" ^[_[:alpha:]]+\ (\) " /etc/rc.d/init.d/functions

3, use echo output an absolute path, use Egrep to remove its base name;

1 " /MNT/SDC " " [^/]+/?$ " | cut-d"/" -f1

The grep and egrep of the Three Musketeers of Linux