GREP

Introduction#

Full name Global Regular Expression Print, originated from a Unix editor ed command g/re/p, where g (global) is global matching, re (regular expression) is regular expression, p (print) is print. Therefore, the grep tool is essentially used to globally match regular expressions and print the results.

man grep

grep, egrep, fgrep, rgrep - print lines that match patterns

Additionally, egrep, fgrep, and rgrep are respectively equivalent to grep -E, grep -F, and grep -r. These variants have been deprecated, but are still provided for backward compatibility. Moreover, if regular expressions are not used, using the -F option will be faster, because by default, even if no regular expression is used, grep will still treat the pattern as a regular expression.

Regular Expressions#

grep defaults to using POSIX regular expressions, divided into the following types based on different options:

1. Basic Regular Expressions (BRE): Default mode, POSIX standard, some metacharacters (such as ?, +, {}) require escaping with a backslash \.

2. Extended Regular Expressions (ERE): Used with grep -E or egrep, metacharacters like ?, +, {} can be used directly without escaping.

3. Perl-Style Regular Expressions (PCRE): Some grep implementations (such as GNU grep) provide the -P option to enable Perl-style regular expressions.

Common Examples#

Reverse Matching#

grep -v foo file

Ignore Case#

grep -i Foo file

Count Matching Lines#

grep -c foo file

Fixed String Matching#

grep -F foo file

Recursive Dir Search#

grep -r foo /path

Matching File Names#

grep -l foo *.txt

Non-Matching File Names#

grep -rL foo /path

With Next N Lines#

After

grep -A 3 foo file

With Previous N Lines#

Before

grep -B 3 foo file

With Context N Lines#

Context

grep -C 3 foo file

Match Multiple Words#

grep -E 'foo|bar' file

grep -e 'foo' -e 'bar' file

Exclude Files in Dir#

grep foo --exclude="*.log" /path

Match Files in Dir#

grep foo --include="*.txt" /path