Grep命令主要用于从文件中查找指定的字符串。

首先建一个demo_file:

$ cat demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
This Line Has All Its First Character Of The Word With Upper Case.

Two lines above this line is empty.
And this is the last line.

例01:从单个文件中查找指定的字符串

$ grep "this" demo_file
this line is the 1st lower case line in this file.
Two lines above this line is empty.

例02:从多个文件中查找指定的字符串

$ cp demo_file demo_file1

$ grep "this" demo_*
demo_file:this line is the 1st lower case line in this file.
demo_file:Two lines above this line is empty.
demo_file:And this is the last line.
demo_file1:this line is the 1st lower case line in this file.
demo_file1:Two lines above this line is empty.
demo_file1:And this is the last line.

例03:忽略大小写使用 grep -i

$ grep -i "the" demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
This Line Has All Its First Character Of The Word With Upper Case.
And this is the last line.

例04:在文件中匹配正则表达式

如果你能在实际使用正则表达式，能使效率大大提高。在下面的例子中，匹配了所有以lines开头，以empty结尾的行。

$ grep "lines.*empty" demo_file
Two lines above this line is empty.

• ?         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