grep命令使用速览（grep命令详解）

概述

grep命令用来根据指定的模式对目标文件进行逐行匹配检查，并打印匹配到的行。

用法

grep [opation] [pattern] file

pattern可以试特定的字符串或者正则表达式

-v 排除匹配结果
-i 忽略大小写
-w 只匹配指定的字符串
-o 只输出匹配的字符串
-n 输出行号
-E 同egrep命令

正则表达式的使用

grep中正则表达式有三种语法,basic, extended和perl类型，GUN版本的grep中，basic和extended在功能上没有区别，即提供了相同的功能，但是个别特殊字符的使用上会有不同，perl此处不做介绍。

可用的正则表达式元字符

A regular expression may be followed by one of several repetition operators:
       ?      The preceding item is optional and matched at most once.
       *      The preceding item will be matched zero or more times.
       +      The preceding item will be matched one or more times.
       {n}    The preceding item is matched exactly n times.
       {n,}   The preceding item is matched n or more times.
       {,m}   The preceding item is matched at most m times.  This is a GNU extension.
       {n,m}  The preceding item is matched at least n times, but not more than m times.

basic和extended使用上的注意项

In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).  
	Traditional egrep did not support the { meta-character, and some egrep implementations support \\{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.

   GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification.  For example, the command grep -E '{1' searches for the  two-character  string
   {1 instead of reporting a syntax error in the regular expression.  POSIX allows this behavior as an extension, but portable scripts should avoid it.

例子

[root@lab ~]# cat output.log 
pwpolicy root --minlen=6 --minquality=1 --notstrict --nochanges --notempty
pwpolicy roat --minlen=6 --minquality=1 --notstrict --nochanges --notempty
ROOT

[root@lab ~]# grep -o "root" output.log 
root

[root@lab ~]# grep -wn "root" output.log 
1:pwpolicy root --minlen=6 --minquality=1 --notstrict --nochanges --notempty

[root@lab ~]# grep  "[0-9]" output.log 
pwpolicy root --minlen=6 --minquality=1 --notstrict --nochanges --notempty
pwpolicy roat --minlen=6 --minquality=1 --notstrict --nochanges --notempty

[root@lab ~]# grep  -v "roat" output.log 
pwpolicy root --minlen=6 --minquality=1 --notstrict --nochanges --notempty
ROOT


[root@lab ~]# grep -i "root" output.log 
pwpolicy root --minlen=6 --minquality=1 --notstrict --nochanges --notempty
ROOT

[root@lab ~]# grep "^#34; output.log //打印空行