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grep: Search for a keyword

All commands in the grep family search a file or files for a pattern. The format used by grep is:

grep [ option ] ... expression [ file ] ...
egrep [ option ] ... expression ] [ file ] ...
fgrep [ option ] ... [ strings ] [ file ] ...

Each line found that matches the pattern is sent to standard output. grep patterns are limited regular expressions. egrep also grep -E patterns are full regular expressions. fgrep also grep -F patterns are fixed strings.

The options your instructor has used the most over the years are:

  • -c : Only the number of matching lines is printed
  • -i : not case sensitive.
  • -l : Names of the files with matching lines are listed
  • -n : Each line is preceded by the line number in the file
  • -x : on fgrep only lines matched in their entirety are printed
  • -v : All lines but those matching

Regular expressions are a class on their own. What follows are some of the symbols used in the grep group of tools:

  • x : Single character x matches on that character
  • \x : Matches on single character x even if x is a special symbol
  • ^ : Matches beginning of the line
  • $ : Matches the end of the line
  • . : Matches a single character
  • [s] : Matches any character in the string s
  • re* : Zero or more matches of the regular expression re
  • re+ : One or more matches of the regular expression re
  • re? : Matches zero or one occurrence of the regular expression re
  • (re) : Parentheses to enclose a regular expression

These special characters must be in quotes or the shell will interpret the special symbols instead of grep

*,+,?,|,(),[],$,\,^

Example usage:

cd ~
cat cli_workshop/data_files/operatingsystemlist

you will get this output:

OpenBSD     BSD             2016-03
FreeBSD     BSD             2016-04
NetBSD      BSD             2015-09
Dyson       Illumos         2015-07
SmartOS     Illumos         2016-07
OpenIndiana Illumos         2016-04
Debian      Linux           2016-06
Centos      Linux           2016-02
Gentoo      Linux           2016-07

Show all lines that have BSD:

grep --help

From reading the help menu you will see the -F flag which we will use:

dritchie@cliwkshp:~$ grep -F BSD cli_workshop/data_files/operatingsystemlist OpenBSD BSD 2016-03 FreeBSD BSD 2016-04 NetBSD BSD 2015-09


Show all lines that do not have BSD:

```bash
grep -F -v BSD cli_workshop/data_files/operatingsystemlist

you will see the following results:

Dyson       Illumos         2015-07
SmartOS     Illumos         2016-07
OpenIndiana Illumos         2016-04
Debian      Linux           2016-06
Centos      Linux           2016-02
Gentoo      Linux           2016-07

Show all occurrences of a single character, followed by an e, followed by one or more occurrences of an n:

grep -E '.en+' cli_workshop/data_files/operatingsystemlist

you will see the following results:

OpenBSD     BSD             2016-03
OpenIndiana Illumos         2016-04
Centos      Linux           2016-02
Gentoo      Linux           2016-07

Show all occurences of an l follow by zero or more l's

grep -E 'll*' cli_workshop/data_files/operatingsystemlist

you will see the following results:

Dyson       Illumos         2015-07
SmartOS     Illumos         2016-07
OpenIndiana Illumos         2016-04

Show all strings that have an n or an o in them.

grep -E '[no]' cli_workshop/data_files/operatingsystemlist

you will see the following results:

OpenBSD     BSD             2016-03
Dyson       Illumos         2015-07
SmartOS     Illumos         2016-07
OpenIndiana Illumos         2016-04
Debian      Linux           2016-06
Centos      Linux           2016-02
Gentoo      Linux           2016-07

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