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Grep is a frequently used command in Linux. Like the cut command, it is a member of the pipeline command. Its function is to analyze a line of data and extract the data we want from the analyzed data. It is equivalent to a search function. Of course, grep is much more powerful than cut. There are many search conditions for grep, and it can even cooperate with regular expressions to search.

Let's look at the usage of grep

$ grep [选项] ‘字符串’ 文件名

Note: In grep usage, the string is the string we want to search for; the file name is the data source, that is, the data we need to analyze. Because grep can accept data from standard input, grep is usually used as a pipeline command.

First, let's look at an example that does not use any options. To find the information of users with mail strings in /etc/passwd:

$ grep mail /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
//输出两行结果

Option 1

-v Here v is lowercase, which means outputting the data that does not match the string

-s Do not display error messages

-V The v here is uppercase, print the information of grep command

See the following example

$ grep –v mail /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
……
//显示了多行信息，唯独没有包含mail的那些行的数据。这个例子的结果和第一个例子的结果是相反的。

Next, let's look at the usage of -s

$ grep mail /etc/passwds
grep: /etc/passwds: No such file or directory  //显示错误信息，因为我们的系统中没有/etc/passwds这个文件
$ grep –s mail /etc/passwds
//这时输出的信息是空的，因为-s限制错误信息的输出

There is nothing much to say about the -V option. It just prints the information of the grep command used by our current system, including its version.

$ grep –V
grep (GNU grep) 2.5.1
Copyright 1988, 1992-1999, 2000, 2001 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Since grep has many options, one article would be too long, so we will introduce it in three articles. This article will first introduce its three options, and the other options will be introduced in Linux Extraction Command grep (II) and Linux Extraction Command grep (III) .