How to Grep for Multiple Strings and Patterns

How to Grep for Multiple Strings and Patterns

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grep is a robust command-line instrument that lets you searches a number of enter recordsdata for traces that match an everyday expression and writes every matching line to plain output.

On this article, we’re going to point out you tips on how to use GNU grep to seek for a number of strings or patterns.

Grep A number of Patterns #

GNU grep helps three common expression syntaxes, Primary, Prolonged, and Perl-compatible. When no common expression kind is specified, grep interpret search patterns as primary common expressions.

To seek for a number of patterns, use the OR (alternation) operator.

The alternation operator | (pipe) lets you specify completely different attainable matches that may be literal strings or expression units. This operator has the bottom priority of all common expression operators.

The syntax for looking out a number of patterns utilizing the grep primary common expressions is as follows:

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grep 'pattern1|pattern2' file...

All the time enclose the common expression in single quotes to keep away from the interpretation and enlargement of the meta-characters by the shell.

When utilizing primary common expressions, the meta-characters are interpreted as literal characters. To maintain the particular meanings of the meta-characters, they should be escaped with a backslash (). For this reason we’re escaping the OR operator (|) with a slash.

To interpret the sample as an prolonged common expression, invoke grep the -E ( or --extended-regexp) choice. When utilizing prolonged common expression, don’t escape the | operator:

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grep -E 'pattern1|pattern2' file...

For extra details about tips on how to assemble common expressions, verify our article Grep regex.

Grep A number of Strings #

Literal strings are essentially the most primary patterns.

Within the following instance, we’re looking for all occurrences of the phrases deadly, error, and vital within the Nginx log error file:

grep 'fatal|error|critical' /var/log/nginx/error.log

If the string you are searching includes spaces, enclose it in double quotation marks.

Here is the same example using the extended regular expression, which eliminates the need to escape the operator |

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