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linux grep命令的-P和-o选项的作用

# grep 命令的选项有哪些？

Usage: grep [OPTION]... PATTERN [FILE]...
Search for PATTERN in each FILE or standard input.
PATTERN is, by default, a basic regular expression (BRE).
Example: grep -i 'hello world' menu.h main.c

Regexp selection and interpretation:
-E, --extended-regexp PATTERN is an extended regular expression (ERE)
-F, --fixed-strings PATTERN is a set of newline-separated fixed strings
-G, --basic-regexp PATTERN is a basic regular expression (BRE)
-P, --perl-regexp PATTERN is a Perl regular expression
-e, --regexp=PATTERN use PATTERN for matching
-f, --file=FILE obtain PATTERN from FILE
-i, --ignore-case ignore case distinctions
-w, --word-regexp force PATTERN to match only whole words
-x, --line-regexp force PATTERN to match only whole lines
-z, --null-data a data line ends in 0 byte, not newline

Miscellaneous:
-s, --no-messages suppress error messages
-v, --invert-match select non-matching lines
-V, --version print version information and exit
--help display this help and exit
--mmap deprecated no-op; evokes a warning

Output control:
-m, --max-count=NUM stop after NUM matches
-b, --byte-offset print the byte offset with output lines
-n, --line-number print line number with output lines
--line-buffered flush output on every line
-H, --with-filename print the file name for each match
-h, --no-filename suppress the file name prefix on output
--label=LABEL use LABEL as the standard input file name prefix
-o, --only-matching show only the part of a line matching PATTERN
-q, --quiet, --silent suppress all normal output
--binary-files=TYPE assume that binary files are TYPE;
TYPE is 'binary', 'text', or 'without-match'
-a, --text equivalent to --binary-files=text
-I equivalent to --binary-files=without-match
-d, --directories=ACTION how to handle directories;
ACTION is 'read', 'recurse', or 'skip'
-D, --devices=ACTION how to handle devices, FIFOs and sockets;
ACTION is 'read' or 'skip'
-r, --recursive like --directories=recurse
-R, --dereference-recursive likewise, but follow all symlinks
--include=FILE_PATTERN search only files that match FILE_PATTERN
--exclude=FILE_PATTERN skip files and directories matching FILE_PATTERN
--exclude-from=FILE skip files matching any file pattern from FILE
--exclude-dir=PATTERN directories that match PATTERN will be skipped.
-L, --files-without-match print only names of FILEs containing no match
-l, --files-with-matches print only names of FILEs containing matches
-c, --count print only a count of matching lines per FILE
-T, --initial-tab make tabs line up (if needed)
-Z, --null print 0 byte after FILE name

Context control:
-B, --before-context=NUM print NUM lines of leading context
-A, --after-context=NUM print NUM lines of trailing context
-C, --context=NUM print NUM lines of output context
-NUM same as --context=NUM
--color[=WHEN],
--colour[=WHEN] use markers to highlight the matching strings;
WHEN is 'always', 'never', or 'auto'
-U, --binary do not strip CR characters at EOL (MSDOS/Windows)
-u, --unix-byte-offsets report offsets as if CRs were not there
(MSDOS/Windows)

'egrep' means 'grep -E'. 'fgrep' means 'grep -F'.
Direct invocation as either 'egrep' or 'fgrep' is deprecated.
When FILE is -, read standard input. With no FILE, read . if a command-line
-r is given, - otherwise. If fewer than two FILEs are given, assume -h.
Exit status is 0 if any line is selected, 1 otherwise;
if any error occurs and -q is not given, the exit status is 2.

Report bugs to: bug-grep@gnu.org
GNU Grep home page: <http://www.gnu.org/software/grep/>
General help using GNU software: <http://www.gnu.org/gethelp/>

# grep的-P选项作用
-P, --perl-regexp PATTERN is a Perl regular expression

-P选项表示可以使用perl语言的正则规则

# grep的-o选项作用
-o, --only-matching show only the part of a line matching PATTERN

-o选项表示只输出匹配的字串，而不是整行内容

示例的apkinfo.txt 文件内容：
Issuer: EMAILADDRESS=android11@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android22@android.com, CN=OsuLogin, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android33@android.com, CN=ServiceWifiResources, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android44@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android55@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android66@android.com, CN=OsuLogin, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android77@android.com, CN=ServiceWifiResources, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android88@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android99@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US

示例1：
期望结果：提取包含CN=Android的行
命令：cat apkinfo.txt | grep -P '(?<=CN=Android).*(?=)'
命令说明：此处使用的是-P选项，(?<=xxx).*(?=xxx) 这个属于perl语言的正则表达式，(?<=xxx)表示xxx作为开始位置，(?=xxx)表示xxx作为结束位置
输出结果：
Issuer: EMAILADDRESS=android11@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android44@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android55@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android88@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android99@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US

示例2：
期望结果：提取包含CN=xxx的行，并且只打印CN=xxx
命令：cat apkinfo.txt | grep -Po '(?<=android.com, ).*(?=, OU)'
命令说明：-P选项表示使用perl语言的正则表达式，-o选项表示只打印匹配的字串、不打印整行内容。"android.com, "为匹配的开始位置，", OU"为匹配的结束位置

输出结果：
CN=Android
CN=OsuLogin
CN=ServiceWifiResources
CN=Android
CN=Android
CN=OsuLogin
CN=ServiceWifiResources
CN=Android
CN=Android

如果去掉-o选项，那么是什么结果呢？
命令：cat apkinfo.txt | grep -P '(?<=android.com, ).*(?=, OU)'
输出结果：(输出了包含CN=xxx的整行内容)
Issuer: EMAILADDRESS=android11@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android22@android.com, CN=OsuLogin, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android33@android.com, CN=ServiceWifiResources, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android44@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android55@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android66@android.com, CN=OsuLogin, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android77@android.com, CN=ServiceWifiResources, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android88@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US
Issuer: EMAILADDRESS=android99@android.com, CN=Android, OU=Android, O=Android, L=Mountain View, ST=California, C=US

这就是-o参数的作用，-o选项表示只打印匹配的字串、不打印整行内容

示例3：
期望结果：只输出邮箱中的用户名信息
命令：cat apkinfo.txt | grep -Po '(?<=EMAILADDRESS=).*(?=@android.com)'
命令说明：使用-P和-o选项，-o选项表示只输出匹配的字串。(?<=EMAILADDRESS=)表示以EMAILADDRESS=作为开始位置，(?=@android.com)表示以@android.com作为结束位置
输出结果：
android11
android22
android33
android44
android55
android66
android77
android88
android99

https://blog.csdn.net/ahilll/article/details/82712694 Perl正则表达式超详细教程

环视锚定(断言)
"环视"锚定，即lookaround anchor，也称为"零宽断言"，它表示匹配的是位置，不是字符。

(?=...)：表示从左向右的顺序环视。例如(?=\d)表示当前字符的右边是一个数字时就满足条件
(?!...)：表示顺序环视的取反。如(?!\d)表示当前字符的右边不是一个数字时就满足条件
(?<=...)：表示从右向左的逆序环视。例如(?<=\d)表示当前字符的左边是一个数字时就满足条件
(?<!)...：表示逆序环视的取反。如(?<!\d)表示当前字符的左边不是一个数字时就满足条件
关于"环视"锚定，最需要注意的一点是匹配的结果不占用任何字符，它仅仅只是锚定位置。

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