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This article follows up on Linux extraction command grep (I) and Linux extraction command grep (II) and continues to introduce the options of the grep command and its usage.

Option 3

-i is case insensitive to the search string

$ grep Mail /etc/passwd
# 大写的M检索的结果为空，因为默认是区分大小写的
$ grep Mail –i /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
# -i 使其不区分大小写

-w lowercase w. Forces matching of whole words

$ grep mail –w /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
# 这时只有一行数据

-x lowercase x. Forces the entire line to match

$ grep mail –x /etc/passwd
# 结果为空

-f specifies the file where the search string is located, and reads the content of each line of the file as the search string.

document/reg.txt

mail
nobody

Use this file as the one specified by -f

$ grep –f /reg.txt /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin

Of course, the content in the file can also be a regular expression

For example, change the content nobody to ^nobody (starting with nobody)

$ grep –f /reg.txt /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin

We see that there is one missing piece of data in the result.

-E specifies the search string as a regular expression pattern

$ grep mail /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
$ grep ‘(mail)’ /etc/passwd
# 结果为空 这里grep将’(mail)’作为一个字符串来进行检索，检索到的结果为空
$ grep –E ‘(mail)’ /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
# 使用-E 指定’(mail)’为正则表达式，所以检索出来两条数据

-e specifies a string as the search condition for the search content. The difference is that you can specify multiple strings.

$ grep mail nobody /etc/passwd   # 我们的原意是想在passwd中检索mail和nobody
grep: nobody: No such file or directory
/etc/passwd:mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
/etc/passwd:mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
# 但是我们看报错了，这时我们可以用-e来指定
$ grep –e mail –e nobody /etc/passwd
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin

Let's see the results are out. Isn't it very useful?

-G specifies that the search condition is a basic regular expression

-P specifies that the search condition is a Perl regular expression

Well, that's all about grep. If there is anything missing, please leave a message to supplement it. I hope this article is helpful to you.