Grep - A powerful search utility
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The document provides an overview of the grep command, a text manipulation utility used for finding patterns in files or text. It details the syntax, options, exit statuses, and the use of regular expressions with examples. Additionally, the document covers advanced features, such as special characters and using grep with shell pipes.
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The grep command syntax
General syntax of grep command
grep [-options] pattern [filename]
Examples:
$ grep pattern filename
$ grep pattern file1 file2
$ grep -i desktop /etc/services
$ grep [yf] /etc/group
$ grep –vi tcp /etc/services
$ ip addr show | grep inet
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Regular expressions and grep
Syntax:
grep "REGEX" filename
Supports three different versions of regular
expression syntax: “basic” (BRE), “extended” (ERE)
and “perl” (PRCE)
bracket expression: [character_list ] matches any
single character in the list e.g. [01234], [A-D]
Example: grep ‟[A-Z][A-Z] [A-Z]‟ datafile
Character classes: predefined names of
characters lists within bracket expressions e.g.
[:alnum:], [:alpha:]
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Examples: repetition operators
'l..e' Matches lines containing an l, followed by
two characters, followed by an e
' *love' Matches lines with zero or more spaces,
of the preceding characters followed by the
pattern love [here, preceding character is space]
'o{5}„ Matches if line has 5 o‟s
'o{5,}„ at least 5 o‟s
'o{5,10}„ between 5 and 10 o‟s
$ grep ’5..’ datafile
Prints a line containing the number 5, followed
by a literal period and any single character
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The Backslash Characters
The ‘’ character, when followed by certain ordinary
characters, takes a special meaning:
‘b’ Match the empty string at the edge of a word.
‘B’ Match the empty string provided it‟s not at the
edge of a word.
‘<’ Match the empty string at the beginning of word.
‘>’ Match the empty string at the end of word.
‘w’ Match word constituent, it is a synonym for
„[_[:alnum:]]‟.
‘W’ Match non-word constituent, it is a synonym for
„[^_[:alnum:]]‟.
‘s’ Match whitespace, it is a synonym for „[[:space:]]‟.
‘S’ Match non-whitespace, it is a synonym for
„[^[:space:]]‟
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Grep - A powerful search utility
- 1. grep A powerful text search utility 1 Presented By Nirajan Pant MTech IT Kathmandu University 2/16/2014
- 2. 2 What is grep? A text manipulation program Used to find pattern in files or text global regular expression print (: g/RE/p) / general regular expression parser (grep) Other text manipulation commands – cut, tr, awk, sed grep family - grep, egrep, and fgrep Type man grep to find list of options 2/16/2014
- 3. 3 The grep command syntax General syntax of grep command grep [-options] pattern [filename] Examples: $ grep pattern filename $ grep pattern file1 file2 $ grep -i desktop /etc/services $ grep [yf] /etc/group $ grep –vi tcp /etc/services $ ip addr show | grep inet 2/16/2014
- 4. 4 grep and exit status If the pattern is found, grep returns an exit status of 0, indicating success if grep cannot find the pattern, it returns 1 as its exit status if the file cannot be found, grep returns an exit status of 2 Note: Other UNIX utilities such as sed and awk do not use the exit status to indicate the success or failure of locating a pattern; they report failure only if there is a syntax error in a command 2/16/2014
- 5. options 5 Option Description -b Display the block number at the beginning of each line. -c Display the number of matched lines. -h Display the matched lines, but do not display the filenames. -i Ignore case sensitivity. -l Display the filenames, but do not display the matched lines. -n Display the matched lines and their line numbers. -s Silent mode. -v Display all lines that do NOT match. -w Match whole word. 2/16/2014
- 6. 6 Examples: grep options $ grep -i desktop /etc/services $ grep –vi tcp /etc/services $ grep -v apple fruitlist.txt $ grep -l ’main’ *.c lists the names of all C files in the current directory whose contents mention „main‟. $ grep -r ’hello’ /home/gigi searches for „hello‟ in all files under the „/home/gigi‟ directory $ grep –w ’north’ datafile Only the line containing the word north is printed, not northwest $ grep -c "Error" logfile.txt 2/16/2014
- 7. 7 Regular expressions and grep Syntax: grep "REGEX" filename Supports three different versions of regular expression syntax: “basic” (BRE), “extended” (ERE) and “perl” (PRCE) bracket expression: [character_list ] matches any single character in the list e.g. [01234], [A-D] Example: grep ‟[A-Z][A-Z] [A-Z]‟ datafile Character classes: predefined names of characters lists within bracket expressions e.g. [:alnum:], [:alpha:] 2/16/2014
- 8. repetition operators: 8 . The period „.‟ matches any single character ? The preceding item is optional and 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. {,m} The preceding item is matched at most m times. This is a GNU extension {n,m} The preceding item is matched at least n times, but not more than m times. 2/16/2014
- 9. 9 Examples: repetition operators 'l..e' Matches lines containing an l, followed by two characters, followed by an e ' *love' Matches lines with zero or more spaces, of the preceding characters followed by the pattern love [here, preceding character is space] 'o{5}„ Matches if line has 5 o‟s 'o{5,}„ at least 5 o‟s 'o{5,10}„ between 5 and 10 o‟s $ grep ’5..’ datafile Prints a line containing the number 5, followed by a literal period and any single character 2/16/2014
- 10. 10 The Backslash Characters The ‘’ character, when followed by certain ordinary characters, takes a special meaning: ‘b’ Match the empty string at the edge of a word. ‘B’ Match the empty string provided it‟s not at the edge of a word. ‘<’ Match the empty string at the beginning of word. ‘>’ Match the empty string at the end of word. ‘w’ Match word constituent, it is a synonym for „[_[:alnum:]]‟. ‘W’ Match non-word constituent, it is a synonym for „[^_[:alnum:]]‟. ‘s’ Match whitespace, it is a synonym for „[[:space:]]‟. ‘S’ Match non-whitespace, it is a synonym for „[^[:space:]]‟ 2/16/2014
- 11. 11 More with grep Anchoring: caret ^ and the dollar sign $ matches beginning and end of a line respectively Alteration: alternate expressions may be joined by the infix operator | Concatenation: regular expressions may be concatenated Precedence: whole expression may be enclosed in parentheses to override the precedence rules and form a subexpression Basic vs Extended Regular Expressions: use the backslashed versions ?, +, {, |, (, and ) instead of ?, +, {, |, (, and ) Environment Variables: affects behavior of grep e.g. LC_ALL, LC_foo, and LANG 2/16/2014
- 12. 12 Examples: special characters grep '<c...h>' /usr/share/dict/words list all five-character English dictionary words starting with "c" and ending in "h" grep ‟<north‟ datafile grep ‟<north>‟ datafile grep Exception logfile.txt | grep -v ERROR grep ’^n’ file Prints all lines beginning with an n grep ’4$’ myfile Prints all lines ending with a 4 grep „bratb‟ datafile matches the separate word ‘rat’ grep „BratB‟ datafile matches „crate‟ but not ‘furry rat’ 2/16/2014
- 13. 13 grep with Shell Pipes Instead of taking its input from a file, grep often gets its input from a pipe. mpiuser@cp-master:~$ ls -l /home | grep '^d‘ drwx------ 2 root root drwxr-xr-x 27 mpiuser drwxr-xr-x 25 parlab 16384 Jan 28 13:13 lost+found mpiuser parlab 4096 Feb 11 11:18 mpiuser 4096 Feb 4 11:08 parlab drwxr-xr-x 27 parlab-user parcompute 4096 Feb 2 15:51 parlab-user mpiuser@cp-master:~$ cat /proc/cpuinfo | grep -i model model : 23 model name : Intel(R) Core(TM)2 Duo CPU model : 23 model name : Intel(R) Core(TM)2 Duo CPU E7400 @ 2.80GHz E7400 @ 2.80GHz 2/16/2014
- 14. 14 references http://www.computerhope.com/unix/ugrep.htm Christopher Negus and Christine Bresnahan. 2012. Linux Bible (8th ed.). Wiley Publishing, p.128-129, p.157 Alain Magloire et al. 1 January 2014. GNU Grep: Print lines matching a pattern (version 2.16) http://www.techonthenet.com/unix/basic/grep.php http://www.thegeekstuff.com/2009/03/15-practical-unixgrep-command-examples/ http://www.cs.gsu.edu/~cscyip/csc3320/grep.pdf 2/16/2014
- 15. 15 Any Questions Thank you !!! ? 2/16/2014