grep, egrep, fgrep - print lines matching a pattern
SYNOPSIS
grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]
linux系统中grep命令是一种强大的文本搜索工具，它能使用正则表达式搜索文本，并把匹配的行打印出来，grep全称是Global Regular Expression Print
1. 常用选项:
-E, --extended-regexp: Interpret PATTERN as an extended regular expression. # 开启扩展（Extend）的正则表达式
-i, --ignore-case: Ignore case distinctions in both the PATTERN and the input files. # 忽略大小写
-v, --invert-match: Invert the sense of matching, to select non-matching lines. # 反过来，只打印没有匹配的，而匹配的反而不打印
-n, --line-number: Prefix each line of output with the 1-based line number within its input file. # 显示行号
-w, --word-regexp # 被匹配的文本只能是单词，而不能是单词中的某一部分，如文本中有liker，而我搜寻的只是like，就可以使用-w选项来避免匹配liker
Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word
constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the
underscore.
-c, --count: Suppress normal output; instead print a count of matching lines for each input file. # 显示总共有多少行被匹配到了，而不是显示被匹配到的内容，注意如果同时使用-cv选项是显示有多少行没有被匹配到。
-o, --only-matching: Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line. # 只显示被模式匹配到的字符串。
-A NUM, --after-context=NUM: Print NUM lines of trailing context after matching lines. # 显示匹配到的字符串所在的行及其后NUM行
-B NUM, --before-context=NUM: Print NUM lines of leading context before matching lines. # 显示匹配到的字符串所在的行及其前NUM行
-C NUM, -NUM, --context=NUM: Print NUM lines of output context. # 显示匹配到的字符串所在的行及其前后各NUM行
2. 模式部分:
基本正则表达式：
匹配字符
. : 任意一个字符
[abc] : 表示匹配[abc]中任意一个字符
[a-zA-Z] : 匹配a-z或A-Z之间任意一个字符
[^123] : 匹配123之外的任意一个字符