linux - What is the difference between grep, pgrep, egrep, fgrep? - Super User

linux - What is the difference between grep, pgrep, egrep, fgrep? - Super User

grep is an acronym that stands for "Global Regular Expressions Print". grep is a program which scans a specified file or files line by line, returning lines that contain a pattern. A pattern is an expression that specifies a set of strings by interpreting characters as meta-characters. For example the asterisk meta character (*) is interpreted as meaning "zero or more of the preceding element". This enables users to type a short series of characters and meta characters into a grep command to have the computer show us what lines in which files match.

The standard grep command looks like:

grep <flags> '<regular expression>' <filename>  

grep prints the search results to the screen (stdout) and returns the following exit values:

0    A match was found.  1    No match was found.  >1   A syntax error was found or a file was inaccessible        (even if matches were found).  

Some common flags are: -c for counting the number of successful matches and not printing the actual matches, -i to make the search case insensitive, -n to print the line number before each match printout, -v to take the complement of the regular expression (i.e. return the lines which don't match), and -l to print the file names of files with lines which match the expression.

egrep

egrep is an acronym that stands for "Extended Global Regular Expressions Print".

The 'E' in egrep means treat the pattern as a regular expression. "Extended Regular Expressions" abbreviated 'ERE' is enabled in egrep. egrep (which is the same as grep -E) treats +, ?, |, (, and ) as meta-characters.

In basic regular expressions (with grep), the meta-characters ?, +, {, |, (, and ) lose their special meaning. If you want grep to treat these characters as meta-characters, escape them \?, \+, \{, \|, \(, and \).

For example, here grep uses basic regular expressions where the plus is treated literally, any line with a plus in it is returned.

grep "+" myfile.txt  

egrep on the other hand treats the "+" as a meta character and returns every line because plus is interpreted as "one or more times".

egrep "+" myfile.txt  

Here every line is returned because the + was treated by egrep as a meta character. normal grep would have searched only for lines with a literal +.

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