How to use Grep command to find multiple strings

How to use the Grep command to find multiple strings

Hello everyone, I am Liang Xu!

Today we show you a very useful technique is to use grepa command to find more strings.

A brief introduction, the grepcommand can be understood as a powerful command line tool, you can use it to search for text matching regular expressions in one or more input files, and then use standard output for each matching text Write out the format.

Therefore, good promise to share with you how to use grepa variety of search mode commands and the use of grepsearch multiple strings:

Multi-mode Grep command

grepThe command supports three kinds of regular expression syntax: Basic , Extended and Perl-compatible . When the regular expression type is not specified, the grepcommand defaults the search mode to basic regular expression.

To search for multiple matching patterns, you can use the OR ( alternation ) operator. We can use the OR operator | ( pipe ) to specify different matches, which can be text strings or expression sets. It is worth noting that among all regular expression operators, this operator has the lowest priority.

Use grepthe command basic regular expression search multiple pattern matching syntax is as follows:

$ grep 'pattern1\|pattern2' filename

It should be noted here that the regular expression should always be enclosed in single quotation marks, because the content in single quotation marks is output as it is, and the content enclosed in single quotation marks will not be replaced regardless of whether it is a constant or a variable.

When using basic regular expressions, metacharacters are interpreted as literal characters. To retain the special meaning of metacharacters, they must be escaped with a backslash (\ ). This is why we need to escape the OR operator ( | ).

To interpret the pattern as an extended regular expression, call the grep -E(or --extended-regexp) option. When using extended regular expressions, there is no need to escape the OR operator ( | ):

$ grep -E 'pattern1|pattern2' file

Grep command to search multiple strings

Generally we think that text strings are the most basic pattern.

Next, we will use an example to search for all fatal , error, and critical strings that appear in a user's log error file . The syntax is as follows:

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

Also note that if the string to be searched contains spaces, you need to enclose it in double quotes.

Here is the same example using extended regular expressions, it does not require escape characters:

$ grep -E 'fatal|error|critical' /var/log/nginx/error.log

By default, grepcommands are case sensitive. To ignore case when searching, please call the grepadd -i(or --ignore-case) the option, for example:

$ grep -i 'fatal|error|critical' /var/log/nginx/error.log

When you want to search for a word, for example, you want to search for a word error, grepthe command output will contain all errorlines of the string that it contains in addition to the output errorline of the word, but also contains the output errorlessor antiterroristsother non- errorword lines, so It is extremely inconvenient.

Therefore, to return only the specified string is a line of whole-word or non-word character is enclosed by a line up, you can use the grepplus -w(or --word-regexp) options:

$ grep -w 'fatal|error|critical' /var/log/nginx/error.log

It is worth noting that word characters include letters, numeric characters (such as az, aZ, and 0-9), and underscore (_ ). All other characters are considered non-word characters.

to sum up

At work, we often need to use grepthe command to the search string, learned how to use grepsearch multiple strings this technique will have a chance to spend a. In fact, the grepcommand much more than this paper stresses function, if you grepfunction commands have any questions or would like to learn its other functions, please leave a message to tell me chanting!

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