Module 40 egrep | Illustrated Unix System V/Bsd

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Module 40
egrep

DESCRIPTION

The external egrep command is used to search ASCII text files for a given text pattern. All lines containing the pattern are written to the standard output. The pattern may be a word or string of characters. Full regular expressions may be used inside the pattern to specify certain sequences of characters . If egrep searches more than one file, each displayed line is preceded by the filename. If no filenames are given, egrep reads from the standard input. Thus you can use egrep in a pipe command.

The name egrep is a combination of editor command characters. It is from the editor command :g/RE/p, which translates to global Regular Expression print. Inside the editor this command would search the entire file for all lines matching the RE pattern . The -v option of egrep is from the editor command :v/RE/p, which searches the entire file for all lines except the ones containing the RE pattern . The e added to the front of egrep stands for expression grep. That is a grep that can search for full regular expression type strings.

The egrep command adds the feature options of the fgrep command and extends the pattern matching capabilities of the grep command. You can use full regular expressions to search for matches. These expressions can be stored in a file or used on the command line. Thus egrep is the most powerful of the grep commands but also requires the most system resources and runs the slowest in most cases. For flexibility and pattern matching power you lose speed.

NOTE:
The egrep command performs line oriented searches. Therefore, a phrase that spans more than one line is not matched. When deciding what string to use for the search, you should try to keep it as short as possible yet as unique as possible.

COMMAND FORMAT

Following is the general format of the egrep command.

 egrep [ -bchilnv ] [ -e pattern ] pattern file_list      egrep [ -bchilnv ] [ -e pattern ] -f file file_list


BSD (Berkeley)

 egrep [ -bclnsv ] [ -e pattern ] pattern file_list      egrep [ -bclnsv ] [ -e pattern ] -f file file_list

The form of the command uses the pattern provided on the command line to search the file_list for a match. In the second form multiple patterns are stored in file and egrep searches the file_list for each pattern listed.

Options

The following list describes the options and their arguments that may be used to control how egrep functions.

-b	Displays the block number in which the pattern was found before the line that contains the matching pattern .
-c	Displays only a total count of matching lines for each file processed .
-e - string	This allows you to specify a string that begins with a dash. Normally, any argument beginning with a dash is interpreted as an option, not a string or argument.
-f file	Read in the strings to search for from file . This allows you to create a file containing all of the strings you want egrep to search for in the file_list or standard input.
-h	Suppress the displaying of filenames which precede lines that match the specified patterns when multiple files are searched.
-i	Ignore the difference between uppercase and lowercase characters during comparisons.
-l	Displays only the names of the files containing the specified pattern . The lines containing the patterns are not displayed.
-n	Displays the line number before each line containing the pattern .
-v	Displays only the lines that do not match the pattern . The v command in the ex editor performs the same type of function. It is an exception search. Search for every line except the ones containing the given pattern .


BSD (Berkeley)
-h	Not supported.
-i	Not supported.
-s	Suppresses the displaying of diagnostic error messages for nonexistent and nonreadable files.

Arguments

The following list describes the arguments that may be passed to the egrep command.

pattern	Any combination of characters, numbers , and regular expression patterns. egrep searches for strings of text that match the pattern expression.
file_list	One or more files to search for the given pattern .

SEARCH PATTERNS (REGULAR EXPRESSIONS)

The following table contains each regular expression and the task that it performs when used inside a pattern . A regular expression consists of regular alphanumeric characters matching themselves and special characters matching certain patterns of text. Regular Expressions are often referred to as REs in UNIX terminology. Thus we use the RE notation for uniformity and brevity.


Alphanumeric RE	Description

c	Matches the character c .
string	Matches the set of characters string .

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